EDITED BY : Sonia Calvari, Alessandro Bonaccorso, Clive Oppenheimer and Letizia Spampinato PUBLISHED IN : Frontiers in Earth Science

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# EXPLORING VOLCANIC PAROXYSMAL EXPLOSIVE ACTIVITY FROM MAGMA SOURCE TO GROUND AND ATMOSPHERE

Topic Editors:

Sonia Calvari, Istituto Nazionale di Geofisica e Vulcanologia, Italy Alessandro Bonaccorso, Istituto Nazionale di Geofisica e Vulcanologia, Italy Clive Oppenheimer, University of Cambridge, United Kingdom Letizia Spampinato, Istituto Nazionale di Geofisica e Vulcanologia, Italy

Image: Il Nido dell'Etna.

Photo of the 5 January 2012 paroxysm occurred at Etna volcano, kindly donated by "Il nido dell'Etna" hotel in Linguaglossa, showing the lava fountain at the base of the image (in red) and the eruptive column and plume rising from the New SE Crater and reaching an estimated height of more than 9 km above sea level. The view is from Linguaglossa, North-East base of the volcano.

Paroxysmal explosive activity is one of the most spectacular natural phenomena, which is recognized as having strong impact not only at a local scale but whose effects can also reach far areas and, indeed, can significantly affect the atmosphere, and the environment in the overall. The most devastating and recent example occurred in 2010, when the Icelandic Eyiafjallajökull volcano erupted disrupting air traffic all over Europe and the North Atlantic for weeks. Between 2008 and 2013, the long-lasting eruption of Chaitén volcano in Chile produced plumes 14-20 km high reaching the coast of Argentina and causing ash fallout as far as 800 km from the vent, and the continuously erupting volcanoes of the Kamchatka Peninsula and of the Aleutian arc have caused often treats to air traffic. The eruption of Pinatubo (Philippines) in 1991 had a strong impact all over the globe, causing significant and measurable atmospheric perturbation and impacting the world temperature. More recently, Mount Etna in Italy displayed tens of paroxysmal explosive episodes affecting the air traffic, viability, settlements, environment, and economics.

Over time, several studies have been devoted to understanding what drives paroxysmal explosive activity. Owning to the treating characteristics, so far great efforts have been made trying to detect precursory signals, parameterize the phenomena, apply conceptual and experimental models, and assess the associated hazards. Published papers have used (i) geophysical data aimed at constraining the source region (depth, size, and position), (ii) gas chemistry and mineral geochemistry and petrology to identify the driving force of explosions and characterize the nature of the involved magmas, (iii) volcanology data and observations as well as ground-based and satellite remote sensing to quantify the volumes of erupted products and track the eruptive process, and (iv) laboratory experiments and plume models to characterize the rheology of the erupted products and forecast the impact of the eruptive clouds on the environment, climate, and the whole planet. In this book, we present a collection of ten papers written by 67 authors spanning from seismicity and ground deformation to geochemistry, volcanology and other geophysical techniques applied to the characterization of paroxysms at several active volcanoes.

Citation: Calvari, S., Bonaccorso, A., Oppenheimer, C., Spampinato, L., eds. (2019). Exploring Volcanic Paroxysmal Explosive Activity From Magma Source to Ground and Atmosphere. Lausanne: Frontiers Media. doi: 10.3389/978-2-88963-133-9

# Table of Contents

*06 Editorial: Exploring Volcanic Paroxysmal Explosive Activity From Magma Source to Ground and Atmosphere*

Sonia Calvari, Alessandro Bonaccorso, Clive Oppenheimer and Letizia Spampinato

*09 An Overview of the Dynamics of the Volcanic Paroxysmal Explosive Activity, and Related Seismicity, at Andesitic and Dacitic Volcanoes (1960–2010)*

Vyacheslav M. Zobin

*25 Coupled Short- and Medium-Term Geophysical Signals at Etna Volcano: Using Deformation and Strain to Infer Magmatic Processes From 2009 to 2017*

Marco Aloisi, Alessandro Bonaccorso, Flavio Cannavò and Gilda M. Currenti

*38 Mapping Volcanic Deposits of the 2011–2015 Etna Eruptive Events Using Satellite Remote Sensing*

Gaetana Ganci, Annalisa Cappello, Giuseppe Bilotta, Alexis Herault, Vito Zago and Ciro Del Negro

*50 Understanding the SO2 Degassing Budget of Mt Etna's Paroxysms: First Clues From the December 2015 Sequence*

Roberto D'Aleo, Marcello Bitetto, Dario Delle Donne, Mauro Coltelli, Diego Coppola, Brendan McCormick Kilbride, Emilio Pecora, Maurizio Ripepe, Lois Claire Salem, Giancarlo Tamburello and Alessandro Aiuppa

*73 Mass Eruption Rates of Tephra Plumes During the 2011–2015 Lava Fountain Paroxysms at Mt. Etna From Doppler Radar Retrievals*

Valentin Freret-Lorgeril, Franck Donnadieu, Simona Scollo, Ariel Provost, Patrick Fréville, Yannick Guéhenneux, Claude Hervier, Michele Prestifilippo and Mauro Coltelli

*89 First Volcanic Plume Measurements by an Elastic/Raman Lidar Close to the Etna Summit Craters*

Antonella Boselli, Simona Scollo, Giuseppe Leto, Ricardo Zanmar Sanchez, Alessia Sannino, Xuan Wang, Mauro Coltelli and Nicola Spinelli

*101 Tephra From the 3 March 2015 Sustained Column Related to Explosive Lava Fountain Activity at Volcán Villarrica (Chile)* Jorge E. Romero, Franco Vera, Margherita Polacci, Daniele Morgavi,

Fabio Arzilli, Mohammad Ayaz Alam, Jorge E. Bustillos, Alicia Guevara, Jeffrey B. Johnson, José L. Palma, Mike Burton, Evelyn Cuenca and Werner Keller

*115 Magma Budget From Lava and Tephra Volumes Erupted During the 25-26 October 2013 Lava Fountain at Mt Etna*

Daniele Andronico, Boris Behncke, Emanuela De Beni, Antonino Cristaldi, Simona Scollo, Manuela Lopez and Maria D. Lo Castro

*131 Assessing Impact to Infrastructures Due to Tephra Fallout From Öræfajökull Volcano (Iceland) by Using a Scenario-Based Approach and a Numerical Model*

Sara Barsotti, Dario Ingi Di Rienzo, Thorvaldur Thordarson, Bogi Brynjar Björnsson and Sigrún Karlsdóttir

## *148 Paroxysmal Explosions, Lava Fountains and Ash Plumes at Etna Volcano: Eruptive Processes and Hazard Implications*

Sonia Calvari, Flavio Cannavò, Alessandro Bonaccorso, Letizia Spampinato and Alessandra G. Pellegrino

# Editorial: Exploring Volcanic Paroxysmal Explosive Activity From Magma Source to Ground and Atmosphere

Sonia Calvari 1 \*, Alessandro Bonaccorso1 , Clive Oppenheimer 2 and Letizia Spampinato1

*1 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo - Sezione di Catania, Catania, Italy, 2 Department of Geography, University of Cambridge, Cambridge, United Kingdom*

Keywords: explosive activity, paroxysm, ash plume, volcanoes, Etna volcano, geophysical data, gas geochemistry, satellite remote sensing

#### **Editorial on the Research Topic**

#### **Exploring Volcanic Paroxysmal Explosive Activity From Magma Source to Ground and Atmosphere**

Volcanic paroxysmal explosive activity has enormous potential destructive power and usually causes widespread damages to the Society (NAS–National Academies of Sciences Engineering Medicine, 2017). Serious problems can occur even during explosive activity related to modest eruptions, such as the case of the 2010 Eyiafjallajokull eruption in Iceland that paralyzed the air traffic in the European continent and in the North Atlantic. In general, a crucial issue, and also an intriguing challenge, is to evaluate the state of the preparatory phase leading to an eruption. It is important to understand the characteristics of the volcano state both for the long-term preparatory phases, usually anticipating the strongest eruptions, and also for the medium- to short-term phases preceding the more frequent and usually less powerful eruptions, but with more immediate consequences.

The first paper of the collection gives an overview of the long-term dynamics of the volcanic paroxysmal activity at andesitic and dacitic volcanoes during 1960–2010 (Zobin). In this study two groups of eruptions with VEI 5–6 and VEI 3–4 were considered. The main tool used was the seismic monitoring of the volcanoes. The eruptions of the first group are characterized by long periods of quiescence (longer than 120 years) and precursory volcano tectonic seismic swarms. The second group develops in more individual styles, each typical of a certain volcano. The study suggests that the eruptions with higher VEI are related to a plugged magmatic conduit, whereas eruptions with lower VEI are usually associated to open conduits.

As a specific case of volcano producing frequent explosive eruptions with low VEI, the second paper of this Research Topic considers the Etna eruptive activity during 2009–2017 by using ground deformation and strain data (Aloisi et al.). Etna volcano was characterized over this period by an incredible lively eruptive activity. This comprised 44 lava fountain episodes from the New South East Crater, two sequences of lava fountains from the Voragine crater, as well as some periods of summit effusive activity with a more prolonged supply of lava flows. The authors produced a complete representation of the different sources that characterized the different periods both in the medium-term (i.e., the preparatory phases showing inflation and the eruptive phases showing deflation) and in the short-term (i.e., the fast discharge associated with eruptive events).

Ganci et al. investigated the middle-term behavior of Mt. Etna. Detection of the thermal anomalies allowed retrieval of radiative power time-series and associated volumes, and thus characterization of each of the paroxysms in terms of intensity and magnitude. Topographic data

Edited and reviewed by: *Valerio Acocella, Roma Tre University, Italy*

> \*Correspondence: *Sonia Calvari<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Volcanology, a section of the journal Frontiers in Earth Science*

Received: *09 August 2019* Accepted: *19 August 2019* Published: *03 September 2019*

#### Citation:

*Calvari S, Bonaccorso A, Oppenheimer C and Spampinato L (2019) Editorial: Exploring Volcanic Paroxysmal Explosive Activity From Magma Source to Ground and Atmosphere. Front. Earth Sci. 7:227. doi: 10.3389/feart.2019.00227*

**6**

derived from satellite imagery gave the total volume of products erupted from 2005 to 2015. Overall, the integration of thermal and topographic data highlighted that the 2011–2012 lava fountains had higher intensity than those of 2013–2015 and that the total volumes erupted from 2005 to 2015 resulted being below those typically erupted by Mt. Etna in a decadal time scale.

Recent eruptive activity from Etna offered a great opportunity to measure the SO2 gas flux from ground-based and satellite instruments prior to, during and after the paroxysmal sequences (D'Aleo et al.), allowing to detect the switch from an active crater to another on the basis of increased and decreased SO2 flux. Based on these measurements, the degassed magma volume was estimated, leading to inferring on the fraction of erupted magma compared to the amount emplaced into the shallow feeding system. Another important parameter essential for hazard assessment is Mass Eruption Rate (MER), which has been estimated for 47 paroxysmal episodes using a Doppler Radar installed on Etna (Freret-Lorgeril et al.). The Lidar detection of the volcanic plume, volcanic ash concentration in atmosphere, and characterization of optical properties of volcanic particles, represent the distal characterization of the paroxysmal activity, essential for modeling volcanic ash clouds and their impact (Boselli et al.).

Mapping of the erupted products and measurement and retrieval of the main eruptive parameters are crucial to understand the eruptive dynamics and investigate into the volcano shallow feeder system. Nowadays, volcanology can take advantage from different measurement techniques and methodologies spanning from direct field observations to ground- and satellite-based remote sensing. Indeed, the opportunity to inspect volcanic phenomena by multidisciplinary approaches allows getting overall view of the volcanic scenarios. The study of the tephra fallout produced by the 3 March 2015 lava fountain at Villarrica basaltic-andesitic volcano by field surveys, laboratory analysis, and satellite imagery provided characterization and parameterization of the paroxysm (Romero et al.). The integration of the key constraints shaded light in the mechanism that triggered the paroxysmal explosion in a volcano that commonly shows persistent activity associated to lava lake-like dynamics. At Mt. Etna, the multidisciplinary approach permitted the study of the recent paroxysmal activity at the volcano summit at both short- and long-time scale. Andronico et al. focused on the 25–26 October 2013 lava fountain. Using field observations and ground- and satellite-based imagery and photogrammetry, they mapped the lava flow field and estimated the volume of the pyroclastic cone. Mapping of the fallout deposit in field and laboratory analysis gave grain size distribution, composition, and total erupted mass.

The hazards posed by explosive activity is a function of the size of the eruption and of the elevation reached by the eruptive column and ash plume, that may impact the atmosphere or even the stratosphere and affect aviation, infrastructures, viability, climate, and health. The Volcanic Explosivity Index (VEI; Newhall and Self, 1982) is a way to estimate the size of an explosive eruption, with a VEI 6 posing severe hazard all over the country and a VEI 2–4 having only local effects. The last two papers of this collection evaluate the hazard posed by two end-member eruptions, such as the VEI 6 potential for Öræfajökull volcano in Iceland (Barsotti et al.) and the VEI 2–4 of Etna volcano in Sicily (Calvari et al.). Barsotti et al. use a numerical model to assess the impact on infrastructures in Iceland caused by a rare but potentially highly destructive activity occurring at the Öræfajökull volcano, which experienced this event only once during the last 1100 years. Etna instead has experienced hundreds of mild VEI 2–4 eruptions during the last decades, and Calvari et al. statistically analyze several of these events to propose an empirical law that allows them to evaluate the maximum elevation of an ash plume as soon as the lava fountain height has reached the peak steady value, measured using the images recorded by the monitoring thermal camera network.

Most of the papers comprised in this collection pertain to Mt. Etna, which has been identified as Decade Volcano by the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI), thus confirming itself as an ideal volcano laboratory (Bonaccorso et al., 2004). The empirical laws thus far proposed for predicting the extension and impact of the eruptive clouds (Calvari et al.) are thus applicable just to this volcano, and in the future a greater effort has to be dedicated to multidisciplinary studies involving more volcanoes and cross-analyses of their data (e.g., Zobin) in order to obtain general statements, laws and/or formulas that could be applicable to those volcanoes which are less monitored. More peculiar and still unknown processes could arise from in depth studies of other basaltic volcanoes from the Earth and other planets, increasing our ability to understand and predict their behavior. A growing attention is dedicated to the use of UAV (unmanned aerial vehicles; e.g., Neri et al., 2017; Favalli et al., 2018), remote-sensing (Boselli et al.; Calvari et al.; D'Aleo et al.; Freret-Lorgeril et al.) and satellite measurements (Ganci et al.) for their safety issues, low cost and broad perspective, that result in an important integration of field studies (Andronico et al.; Romero et al.), numerical simulations (Barsotti et al.), and laboratory experiments (e.g., Dellino et al., 2010). It is a general feeling that the innovative methods will grow even more in the future, possibly coupled with virtual reality, allowing results and simulations of volcanic processes impossible to imagine nowadays.

## AUTHOR CONTRIBUTIONS

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

## REFERENCES

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Calvari, Bonaccorso, Oppenheimer and Spampinato. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# An Overview of the Dynamics of the Volcanic Paroxysmal Explosive Activity, and Related Seismicity, at Andesitic and Dacitic Volcanoes (1960–2010)

#### Vyacheslav M. Zobin\*

Centro Universitario de Estudios Vulcanológicos, Universidad de Colima, Colima, Mexico

#### Edited by:

Alessandro Bonaccorso, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Italy

#### Reviewed by:

Sonia Calvari, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Italy Luca D'Auria, Instituto Volcanológico de Canarias, INVOLCAN, Spain Paul Davis, UCLA Department of Earth, Planetary, and Space Sciences, United States

\*Correspondence:

Vyacheslav M. Zobin<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Volcanology, a section of the journal Frontiers in Earth Science

Received: 07 September 2017 Accepted: 13 April 2018 Published: 03 May 2018

#### Citation:

Zobin VM (2018) An Overview of the Dynamics of the Volcanic Paroxysmal Explosive Activity, and Related Seismicity, at Andesitic and Dacitic Volcanoes (1960–2010). Front. Earth Sci. 6:46. doi: 10.3389/feart.2018.00046 Understanding volcanic paroxysmal explosive activity requires the knowledge of many associated processes. An overview of the dynamics of paroxysmal explosive eruptions (PEEs) at andesitic and dacitic volcanoes occurring between 1960 and 2010 is presented here. This overview is based mainly on a description of the pre-eruptive and eruptive events, as well as on the related seismic measurements. The selected eruptions are grouped according to their Volcanic Explosivity Index (VEI). A first group includes three eruptions of VEI 5-6 (Mount St. Helens, 1980; El Chichón, 1982; Pinatubo, 1991) and a second group includes three eruptions of VEI 3 (Usu volcano, 1977; Soufriere Hills Volcano (SHV), 1996, and Volcán de Colima, 2005). The PEEs of the first group have similarity in their developments that allows to propose a 5-stage scheme of their dynamics process. Between these stages are: long (more than 120 years) period of quiescence (stage 1), preliminary volcano-tectonic (VT) earthquake swarm (stage 2), period of phreatic explosions (stage 3) and then, PEE appearance (stage 4). It was shown also that the PEEs of this group during their Plinian stage "triggered" the earthquake sequences beneath the volcanic structures with the maximum magnitude of earthquakes proportional to the volume of ejecta of PEEs (stage 5). Three discussed PEEs of the second group with lower VEI developed in more individual styles, not keeping within any general scheme. Among these, one PEE (SHV) may be considered as partly following in development to the PEEs of the first group, having stages 1, 3, and 4. The PEEs of Usu volcano and of Volcán de Colima had no preliminary long-term stages of quiescence. The PEE at Usu volcano came just at the end of the preceding short swarm of VT earthquakes. At Volcán de Colima, no preceding swarm of VT occurred. This absence of any regularity in development of lower VEI eruptions may refer, among other reasons, to different conditions of opening of the magmatic conduit during these eruptions.

Keywords: volcano seismology, eruption dynamics, magma ascent dynamics, seismicity and tectonics, volcanoes andesitic and dacitic

**9**

#### INTRODUCTION

Understanding volcanic paroxysmal explosive activity requires the knowledge of many associated processes. Paroxysmal volcanic eruption is defined in "Geological dictionary" (Paffengoltz, 1978) as "particularly violent eruption of the volcano, which is the most dangerous and tense stage of this eruptive cycle, at which the whole cavity of the crater is opened". An overview on the dynamics of six paroxysmal explosive eruptions (PEEs) at andesitic and dacitic volcanoes, occurring between 1960 and 2010, is presented here. The selected eruptions are grouped according to their Volcanic Explosivity Index (VEI, Newhall and Self, 1982).

The paroxysmal explosive eruption (PEE) of the first group with VEI 5-6 (height of plume more than 25 km; volume of ejecta 109 -1011 m3 ) can significantly destroy the volcano and affect the atmosphere, and the environment in the overall. For example, as a result of the paroxysmal 27 August 1883 explosion of Krakatau volcano, the northern two-thirds of the Krakatau Island, which is situated between Java and Sumatra Islands (**Figure 1**), collapsed beneath the sea, generating series of devasting pyroclastic flows, which traveled for the runout distances up to 40 km across the Sunda Strait, and immense tsunamis that ravaged adjacent coastlines. The explosions and tsunamis killed 36,417 persons, devastating 165 villages. The ash cloud rose 60 km above the volcano. Millions of tons of dust were hurled into the atmosphere and stratosphere around the world, for many years producing exotic colors in the sky, halos around the sun and moon, and a spectacular array of anomalous sunsets and sunrises. Acoustic waves from this explosion had traveled around the Earth seven times (Winchester, 2003; http://volcano.si.edu/volcano.cfm?vn= 262000).

The paroxysmal explosive eruption of the second group with VEI 3-4 (height of plume from 3 to 25 km; volume of ejecta 107 - 109 m3 ) may destroy the summit of volcano, or lava dome in the crater, or crater sectors by Vulcanian explosions, emplacing pyroclastic current densities (PCDs) that may affect the nearest settlements and threaten aviation. The 4–5 November 2010 PEEs at Merapi volcano, Java Island (**Figure 1**), represent an example of this type. The explosion sequence destroyed the large lava dome, and the summit crater had enlarged to a diameter of 300– 400 m. Ash column that ascended to 17 km altitude was produced emplacing a pyroclastic flow that traveled for runout distance of about 16 km. The PCDs and related surges of 4–5 November destroyed numerous villages over broad area of the upper slopes of the volcano. More than 300,000 habitants were evacuated; 386 deaths were recorded. Residents in the towns up to 240 km away reported that "heavy gray ash" blanketed trees, cars, and roads (Surono et al., 2012).

In this review article, we present the case studies of PEEs of andesitic and dacitic volcanoes that occurred during 50 years, from 1960 to 2010, to show the development and effects of PEEs and to highlight any regularity in their dynamics during the eruptive cycles.

Here we selected six PEEs of andesitic and dacitic volcanoes with VEI 3-6 (http://volcano.si.edu/) that occurred during this period. The first group of PEEs includes three VEI 5-6 paroxysmal explosive events of andesitic and dacitic type recorded during the period of study (Mount St. Helens, 1980; El Chichón, 1982; and Pinatubo, 1991). The forth event that occurred during this period, the VEI 5 1963 Agung eruption, had no detailed observations for discussion. Three events of the second group (VEI 3) are selected from the long list of eruptions as the only having the detailed observations and seismic monitoring comparable with the information obtained for the three events of the first group (Usu, 1977; Soufriere Hills volcano, 1996; and Volcán de Colima, 2005).

The case descriptions are represented by two groups of information. The Research Topic of this Special Issue is concerned with the Exploring Volcanic Paroxysmal Explosive Activity from Magma Source to Ground and Atmosphere. According to this definition of the topic, "from magma source to ground and atmosphere," the first group of information includes the geological, visual, and satellite observations.

For a long time, including the period of our reviewed eruptions, the only tool that allowed reconstructing in details the dynamics of eruptions was seismic monitoring of the volcanoes. So we discuss the seismic information within the second group of information. We use the catalogs of seismic events recorded during the eruptive cycles of selected volcanoes together with the descriptions of these events to reconstruct the main stages in the development of their paroxysmal eruptive activity. Earthquake location catalogs are not an exact representation of the true earthquake locations. The most important source of systematic errors in earthquake locations is the inherent dependence of earthquake locations on the assumed seismic velocity structure of the Earth's crust, the number of seismic stations and configuration of the seismic network. Within this article, we consider the accuracy of earthquake locations by the local seismic networks around volcanoes as ±1-3 km for epicenters and ± 2–4 km for hypocenters.

The magnitudes of earthquakes used in the article were taken from the original catalogs.

#### DYNAMICS OF PAROXYSMAL EXPLOSIVE ERUPTIONS OF THE FIRST GROUP (VEI 5-6)

In this section, three paroxysmal explosive eruptions of the first group (VEI 5-6) at volcanoes Mount St. Helens, El Chichon, and Pinatubo, seismically monitored during eruptive activity, are discussed. The main data about these eruptions are collected in **Table 1**.

#### Mount St. Helens, Cascades, USA, 1980

Mount St. Helens (MSH) is a stratovolcano of 2,950 m of height. It is situated within the Cascade Range in the state of Washington, U.S.A. The eruptive history of MSH began about 40,000 years ago with dacitic volcanism. The products of eruptive activity during the past 2,500 years were represented mainly by dacite-andesite-dacites. The volcano had been apparently dormant since 1857 (Mullineaux and Crandell, 1981).

FIGURE 1 | Index map with positions of volcanoes discussed in the article. Open triangles, volcanoes mentioned in Introduction; filled triangles, volcanoes discussed in the article. MSH, Mount St. Helens volcano; VdeC, Volcán de Colima; SHV, Soufriere Hills volcano. The geographical map is taken from the site http://www.lib. utexas.edu/maps/world.html.

1, Simkin and Siebert, 1994; 2, Christiansen and Peterson, 1981; 3, Carey and Sigurdsson, 1986; 4, Macías et al., 2003; 5, Scott et al., 1996; 6, Newhall et al., 1996.

#### Paroxysmal Eruption of 18 May 1980, Its Prelude and Its Effects

The eruption cycle of re-awakening of Mt. St. Helens volcano, that included the paroxysmal explosive eruption of 18 May 1980, began on 16 March 1980 with a preliminary swarm of volcanotectonic earthquakes (**Figure 2**). The first phreatic eruption from its summit crater was observed on March 27, and was followed then by a sequence of small phreatic explosive eruptions up to the climactic eruption on the morning of 18 May (Mullineaux and Crandell, 1981).

According to a series of photos taken during the first minutes of the eruption (Voight, 1981), the eruption began with a largescale collapse of the volcanic edifice which was followed by a directed blast eruption. The northward-directed lateral blast of the bulging north flank of the volcano erupted about 2 km3 of cold and juvenile material and devastated an area of nearly 600 km2 . These events triggered a 9-hr magmatic eruption that drove a vertical Plinian column more than 20 km high. A new crater, with an area of 1.5 × 3.0 km2 and of depth of about 500 m, formed in the northern flank and summit of the volcano. The

height of the volcano was reduced by about 400 m (Christiansen and Peterson, 1981).

The local effects of the 18 May eruption were observed within the 600-km2 devastated area. Fifty-seven people were killed and missing. Autopsies showed that most of the people killed in the eruption likely died from asphyxiation after inhaling hot ash (Bernstein et al., 1986). Civil infrastructures such as highways, railways, airports bridges, buildings, etc., immediately north of the volcano were almost completely destroyed by the lateral blast and debris avalanche (Schuster, 1981).

The 18 May eruption sent large amounts of dust and volcanic ash into the atmosphere. The plume spread quickly to the east, bringing darkness, and reduced visibility for the northern states of the USA (Robock, 1982). The dust cloud produced cooling of surface air temperatures during the daytime and warming at night. However, the amount of sulfur injected into the stratosphere was low, and thus had no impact on climate (Robock, 1981).

#### Seismic History of the Mt. St. Helens's PEE

**Figure 2** shows temporal variations in daily number of volcano-tectonic seismic events, recorded by the regional Pacific Northwest Seismic Network (PNSN, https://pnsn.org/ pnsn-data-products/earthquake-catalogs), and their maximum magnitudes during two stages of activity: pre-eruptive and before the great 18 May PEE. The 1980 swarm of volcanotectonic shallow earthquakes began on 16 March with small events; the first significant ML 4.2 event was recorded beneath Mount St. Helens on 20 March. The number and magnitude of earthquakes sharply increased starting 25 March with peak number on 27 March, when the first phreatic explosion occurred. After the beginning of the eruption, the number of earthquakes gradually decreased during one month; then their number was more or less stable up to 18 May. The magnitudes of volcanotectonic earthquakes of this period were rather high; seven April earthquakes were in the range of ML between 5.0 and 5.2. During period from 1 to 16 May, the maximum magnitudes of earthquakes decreased, six of them were in the range of ML between 4.3 and 4.7.

The pre-eruptive 93 earthquakes, occurring before the 27 March explosions, were located within the area of about 10 × 20 km2 along the NW-SE structure, including MSH in its NW part (**Figure 3A**). These seismic events formed two clusters: a shallow one, at the depth range of 0 to 5 km, and a deep one, at the depth range from 18 to 25 km (**Figure 3B**). During the active period, from 27 March and until the PEE of 18 May, totally 627 located earthquakes occurred around the volcanic edifice, mainly within the depth range from 0 to 10 km; the deeper pre-eruption cluster disappeared.

The largest earthquake with ML 5.7 occurred on 18 May at 08:32:11.41 on the background of lower-level seismic activity, just before the great directed blast of the volcano that initiated at 08:32:21, about 10 s after the earthquake-trigger (Voight, 1981). This seismic event was located at the depth of 2.8 km beneath the NW flank of MSH (**Figure 3**). Study of its Pwave focal mechanism (Zobin, 1992) and its comparison with the CMT solution (Dziewonski et al., 1988) showed that this event represented initial left-lateral strike-slip along the NE-SW oriented fault that in a few seconds was transformed in a normal fault triggering a sector failure of volcanic edifice.

As mentioned by Christiansen and Peterson (1981), the Plinian phase of paroxysmal eruption continued for about 9 h. On the background of decreasing intensity of the Plinian phase, a 4-h sequence of 21 magnitude 4.0 and 4.5 events was recorded (**Figure 3**). The earthquake foci repeated mainly the distribution of the foci recorded during the stage before the 18 May PEE.

#### El Chichon Volcano, México, 1982

El Chichón Volcano (1,100 m a.s.l.) is located in the state of Chiapas, southern Mexico. All the magmas, produced during the past 370,000 years, are K-alkaline andesites. Before the 1982 eruption, the volcanic structure consisted of two nested andesitic lava domes inside the crater that have been repeatedly reactivated during the Holocene (Layer et al., 2009). The last dated Plinian eruption occurred in 1320–1433 A.D. (Macías et al., 2003), then the volcano was inactive.

1Here and later in the article, local time for all volcanic events was used.

FIGURE 3 | Epicenters (A) and hypocenters (B) of earthquakes located during March to May 1980 at MSH. The earthquakes located before 27 March, when the first explosion occurred, are shown by circles (1); the earthquakes located from 27 March until the 18 May PEE are shown by crosses (2); the sequence of the magnitude 4.0–4.5 earthquakes that occurred during last 4 h of the Plinian stage is shown by diamonds (3). The location of the ML 5.7 earthquake of 18 May is shown with a star (4). The position of volcano MSH is shown by the 1,500 m contour. Data from the PNSN catalog.

#### Paroxysmal Eruptions of 3–4 April 1982, Their Prelude and Their Effects

The eruption cycle of re-awakening of El Chichón volcano, that included the PEEs of 3–4 April 1982, began on 1 March 1982 with a swarm of volcano-tectonic earthquakes (Havskov et al., 1983). Near mid-night (at 23:32) of 28 March El Chichón exploded abruptly and vigorously; the eruption then continued until ∼ 06:00 the next morning. This powerful explosive eruption fed a 20-km high eruption column and produced heavy ashfalls. During next 6 days (29 March−3 April), the eruptive activity continued intermittently at much-reduced level, consisting of occasional small phreatic explosions. Then, in the evening of 3 April at 19:35 and early morning of 4 April at 05:22, the El Chichón's two paroxysmal eruptions (VEI 5) occurred. The Plinian eruption columns of the explosions that occurred during 3 April and 4 April were 24 km high, with volume of 0.8 × 109 m3 , and 22 km high, with volume of 0.8 × 109 m3 , respectively. While of shorter duration (each lasting <2 h), both these eruptions were more powerful than the 28 March eruption. In addition to producing voluminous ashfalls, each eruption also generated pyroclastic flows and surges that swept all flanks of the volcano. As a result of this sequence of great explosions, a nearly complete excavation of El Chichón summit dome was performed, and the 1-km-wide crater formed in its place (Carey and Sigurdsson, 1986; Tilling, 2009).

Local effects of this eruption were tragic. Pyroclastic flows and surges obliterated some villages within a roughly 7 km radius of the volcano, killing about 2,000 people, and ashfalls downwind posed socio-economic hardships for many thousands of inhabitants of the states of Chiapas and Tabasco. The unexpected and vigorous eruption of 28 March caused hasty, confused evacuation of most villagers in the area. Eruptive activity greatly diminished during the next 5 days, and then the most powerful and lethal eruptions occurred 3–4 April tragically, after many evacuees were allowed by authorities to return home (Tilling, 2009).

The stratospheric cloud, created by the eruption of El Chichón, was about 2.5 km wide and reached an altitude of 25 km; over 20 million tons of sulfur gases were emitted during this event. The stratospheric cloud quickly spread out, encircling the globe in about three week's time. A maximum drop of 0.2◦C in the global temperature was measured within two months of the eruption (Vignola, 1999). A significant 10% ozone decrease was recorded in the north temperate regions of North America, Europe, and Asia within the atmospheric layers of 8–16 and 16–24 km during four seasons after the eruption (Angell, 1998).

#### Seismic History of the El Chichon's PEEs

The eruption was preceded by a 28-day seismic swarm of volcano-tectonic earthquakes (**Figures 4**, **5**), which began on 1 March 1982. Seismic activity was fortuitously recorded by the 14-station seismic network of Comisión Federal de Electricidad, installed by UNAM during 1979–1980 around the Chichoasén hydroelectric plant (∼ 50 km south of the volcano's summit; Havskov et al., 1983; Jiménez et al., 1999). Totally 270 events with magnitude Mc ≥ 2 of the 1 March - 4 April swarm were located (**Figure 5)**.

The swarm began with a sharp increase in the number of earthquakes during the first 6 days of activity. Two maximum magnitude events (Mc 4) of the swarm were recorded on 3 and 6 March. Then a gradual decrease in the seismic activity in number and magnitudes of events was observed during the next 2 weeks. The activity increased again beginning from 19 March. The majority (∼80%) of earthquakes, preceding the first explosion of 28 March, occurred within an area of 10 × 20 km2 having the volcano in the center of this area, ranging within depths from 0 to 25 km (**Figure 5B**).

Just after the explosion of 28 March seismic activity disappeared but 2 days prior to the PEEs of 3–4 April, the number of earthquakes with Mc ≥ 2 began to increase. After the first explosion of 28 March and before and during the two PEE events of 3 and 4 April, the earthquake foci had filled a vertical N-S

oriented volume beneath the volcano at the depth interval from 0 to 8 km (**Figure 5B**).

The PEE of 4 April (05:35) was followed by a sequence of five intermediate-size (Mc 3.7–3.8) earthquakes that occurred beginning from 07:53 during 5 h on the background of decreasing intensity of the Plinian activity of the volcano (**Figure 5**). The earthquake foci were distributed around and beneath the focal volume filled during the stage before the 3–4 April PEEs.

#### Pinatubo Volcano, Luzon Island, Philippines, 1991

Mount Pinatubo is a composite volcano of 1,745 m elevation and is situated in the Luzon volcanic arc (**Figure 1**). Pre-1991 eruption, Mount Pinatubo represented a dacite-andesite dome complex and stratovolcano that was surrounded by an extensive apron of pyroclastic-flow and lahar deposits from large explosive dacitic eruptions. Large, late-stage dome, filling the crater before the 1991 eruption, was left by the penultimate Plinian eruption that occurred about 500 year before the recent eruption (Newhall et al., 1996).

#### Paroxysmal Eruption of 15 June 1991, Its Prelude and Effects

The eruption cycle of re-awakening of Pinatubo volcano, which included the paroxysmal explosive eruption of 15 June 1991, began in March 1991 (**Figure 6A**). Citizens of small communities on the lower northwest flank of Mount Pinatubo felt earthquakes beginning from 15 March, 1991. On 2 April, 1991, during or just after phreatic explosions, vigorous steam vents opened on the upper north slope of the volcano.

From 2 April to late May, steam jetted to heights of 300 to 800 meters. Sometimes these jets were accompanied by ash emissions that reached heights of 1,500 to 3,000 meters.  A new lava dome began to grow in the crater from 9 June. Then a series of strong explosive eruptions began on 12 June, 1991. These culminated in the violent climactic Plinian eruption (VEI 6) of 15 June at 13:42 (L.T.) which caused

the collapse of the volcano summit and lasted ∼9 h. This eruption produced about 10 km3 of porous, pumiceous deposits, and formed a caldera of 2.5 km in diameter (Sabit et al., 1996; Wolfe and Hoblitt, 1996).

Local effects of this paroxysmal eruption were significant. Pyroclastic flows that were emplaced from the Plinian column during the 15 June paroxysmal eruption traveled as much as 12 to 16 km from the vent in all sectors, impacted directly an area of almost 400 km2 , and profoundly altered the landscape (Scott et al., 1996). The successive forecasting of the eruptive events and their effects enabled Philippine civil leaders to organize massive evacuations, including the evacuation of Clark Air Base (U.S. Air Force), which was located just east of the volcano. Those evacuations saved thousands of lives. Nonetheless, the climactic eruption coincided with a typhoon on June 15 that caused 200 to 300 deaths and extensive property damage, owing to an extraordinarily broad distribution of heavy, wet tephra-fall deposits (Wolfe and Hoblitt, 1996).

From about 13:40 to about 22:30 on 15 June, Mount Pinatubo produced the largest stratospheric SO2 cloud ever observed by modern instruments, and the major stratospheric aerosol event since Krakatau exploded in 1883. The giant umbrella cloud covered an area of 300,000 km2 at 19:40 and was sustained and growing during the whole 9 h climax, eventually reaching >1,100 km in diameter. The altitude of umbrella cloud was estimated as 25 km at the eastern edge and 34 km at its center for the 15:40 image. The stratospheric cloud was observed by the TOMS satellites to encircle the Earth in about 22 days (Self et al., 1996). After the 1991 Pinatubo eruption, the globally averaged stratospheric temperature rose by about 2◦C for about 2 years (Robock, 2000). It was noted also that the sulfuric aerosols, released before and during the eruption, circled the Earth within 3 weeks and remained in the atmosphere for 3 years, reflecting enough sunlight to cool the entire planet by half a degree Celsius during that time (Wendel and Kumar, 2016).

#### Seismic History of the Pinatubo PEE

As it was noted, the citizens of small communities on the lower northwest flank of Mount Pinatubo felt earthquakes beginning on 15 March, 1991 but the local seismic observations began by the USGS seismologists only on 7 May (**Figure 6A**). Therefore, the earthquake swarm prior to the 2 April initial re-awakening of the volcano was not instrumentally recorded. At the same time, no events with magnitude ≥ 4.5 were located by the worldwide seismic survey of USGS (https://earthquake.usgs.gov) in the region of the volcano during March-May.

**Figures 6A**, **7** show the development of the seismic activity beginning from 7 May, when the local network was installed, and until 12 June, 1991, when the seismic network was destroyed by large explosions. Volcano-tectonic seismic events were recorded within two clusters (**Figure 7A**): at a distance of about 5 km NW of the summit (I) and just beneath the summit (II). The first cluster of seismic events developed mainly within the depth range from 7 to 3 km while the second group of events was shallower, forming a vertical column within the depth range from 4 km b.s.l. to 1 km a.s.l. This shift of earthquake foci from cluster I to cluster II supposedly indicated the migration of seismicity to the surface beneath the volcano along the fault shown with a dashed line in **Figure 7A**.

The events from the distant zone I dominated from 21 May to 5 June. Then seismicity of zone II beneath the volcano began to increase its activity, and the intensive swarm of low magnitude (from 0 to 1; largest magnitude 1.5) events was recorded there during 6 to 7 June (**Figure 6A**). This swarm culminated in the 7 June lava dome extrusion. Just after the beginning of the lava dome growth, the number of earthquakes sharply decreased, and the strong 12 June explosion occurred on the background of low-level seismicity for both seismic zones.

During the culminated stage of great explosions, which occurred from June 12 to 15, the local seismic network was out of service, and the only information about seismic activity was obtained from the worldwide seismic survey of USGS (https:// earthquake.usgs.gov) at the level of magnitudes ≥ 4.5. This suggests that the 9-h climactic sequence of explosions of 15 June that began at 13:42 was accompanied by a swarm of earthquakes "triggered" by the explosion (**Figure 6B**).

During the 9-h climactic sequence of explosions, totally 25 earthquakes of magnitude range from 4.5 to 5.7 were recorded. They were located within Luzon island to the north and northeast of the volcano (note that during March−14 June no seismic events of this magnitude range were recorded there). Epicenters of five of these earthquakes, located within the area of the seismic swarms occurring 7 May−12 June, are shown in **Figure 7A**. The precision of the depths estimations of these events is very low, and for all of them the USGS survey took the depths simply as 10 km that means "shallower than 33 km" (https://earthquake. usgs.gov/earthquakes/eventpage/terms.php). Therefore, they are not shown in the distribution of hypocenters of **Figure 7B**.

The comparative 9-h development of the eruption plume and of the seismic activity is shown in **Figure 6B**. The first earthquakes of this sequence, with magnitude mb 5.1, occurred at 15:39, about 2 h after the beginning of the explosion sequence (**Figure 6B**). The number of events and magnitudes of the swarm reached a maximum on the background of decrease in the plume height 5–6 h after the beginning of the explosion. Earthquake with magnitude Mw 5.7 was recorded at 18:41; earthquake with magnitude Mw 5.6 was recorded at 19:15.

#### DYNAMICS OF PAROXYSMAL EXPLOSIVE ERUPTIONS OF THE SECOND GROUP (VEI 3-4)

In this section, three PEEs of the second group (VEI 3-4) of volcanoes Usu, Soufriere Hills and Volcán de Colima are discussed. They were selected from the database of the Global Volcanism Program (http://volcano.si.edu/) as the most detailed

described with the field observations and having continuous seismic monitoring. The main data about these eruptions are collected in **Table 2**.

depth, and profile of volcanic edifice are shown. The fault is shown with a

#### Usu Volcano, Hokkaido, Japan, 1977

Usu Volcano is located on the southern rim of the Toya Caldera, Hokkaido (**Figure 1**). It is a truncated stratovolcano with the highest peak of about 700 m above sea level (a.s.l.) crowned with a small caldera, 1.8 km across and about 500 m deep (**Figure 8B**). Within the summit caldera there are two lava domes, Ko-Usu and Oo-Usu. The historic activities commenced in 1663 with a Plinian eruption of rhyolitic pumice. Characteristic eruptions of dacitic magma including nuee ardente and lava dome or cryptodome building have been repeated every 30 to 50 years. The eruptions

dashed line.

1, Katsui et al., 1978; 2, Simkin and Siebert, 1994; 3, Katsui et al., 1985; 4, GVP, http://volcano.si.edu/volcano.cfm?vn=360050; 5, Robertson et al., 1998; 6, GVP, http://volcano.si. edu/volcano.cfm?vn=341040; 7, Savov et al., 2008.

occurred both from the summit and the northeastern base of Usu Volcano (Niida et al., 1980; Yokoyama et al., 1981; Katsui et al., 1985).

based on data modified from Yokoyama et al. (1981).

#### Paroxysmal Eruption of 7 August 1977, Its Prelude and Effects

The eruption cycle of Usu volcano, which included the paroxysmal explosive eruption of 7 August 1977, began on 6 August 1977 after 32 years of quiescence (**Table 2**). A major pumice eruption of hypersthene dacite occurred from the summit after 32-h preceding earthquake swarm. Large eruptions lasted for a week from August 7 to 14, opening craters 1–4, each about 100 m across.

The first explosion in this sequence was paroxysmal. An ashladen gray cloud rose from the southeastern base of Ko-Usu beginning from 09:12 of 7 August without notable explosion sounds. The eruption column grew rapidly, and at 10:30 rose up to a maximum height of 12 km in elevation. The eruption lasted about 2.5 h and ceased at 11:40. A new crater (Crater I), about 100 m across, opened at the southeastern base of the Ko-Usu lava dome (**Figure 8B;** Katsui et al., 1978, 1985).

Large volume of pumice and ash was ejected and transported to the southeast of the volcano by the westerly wind. The pumice and ash showers caused destruction of cultivated fields and forests around the volcano. The alarm was issued by the Japan Meteorological Agency before the beginning of eruption (Yokoyama et al., 1981), and no one was killed or injured seriously by the eruption activity, though some damage was caused not only to cultivated lands and forests by the ejecta, but also to some buildings, underground constructions and others by the subsequent crustal movements (Katsui et al., 1978).

#### Seismic History of the Usu PEE

The first earthquake of the premonitory swarm of volcanotectonic earthquakes occurred at 01:09 on 6 August (**Figure 8A**). Beginning from 03:30, the earthquakes were strong enough to be felt in the whole area around the Usu Volcano, up to about 8 km from the volcano, but no damage was caused. Rapid increase in frequency of earthquakes occurred about 01:00 on 7 August and continued to the time of beginning of PEE eruption (**Figure 8A;** Suzuki and Kasahara, 1979; Katsui et al., 1985).

The earthquake swarm consisted of about 3,000 small (magnitude 1-2) volcano-tectonic earthquakes that were located beneath the summit crater at a depth from 0 to 2 km (**Figure 8B**). The new eruptive Crater 1 (shown as a cross in **Figure 8B**) was situated within the epicentral zone of earthquakes. The maximum magnitude M 3.7 earthquake was observed half an hour before the beginning of eruption (Yokohama and Seino, 2000).

During the PEE, volcano-tectonic earthquakes disappeared but volcanic tremor accompanied the total process of the volcanic column growth. **Figure 9** shows the temporal variations in the height of the eruption column together with the variation in the amplitudes of tremor observed at the seismic station SSU (2.5 km from Crater 1). There is a good correlation between these parameters. The amplitude of tremors reached maximum values when the eruption cloud reached its maximum height (Suzuki and Kasahara, 1979).

#### Soufriére Hills Volcano, Montserrat, 1996

Andesitic stratovolcano Soufriére Hills (SHV) with the height of 915 m is situated in the southern part of Montserrat Island, West Indies (**Figure 1**) and represents a Peleean lava dome complex constructed largely of hornblende andesite. The SHV edifice consists of four andesite lava domes and a fan of associated pyroclastic flow deposits. Its summit crater was likely formed c. 4,000 years ago.

#### Paroxysmal Eruption of 17 September 1996, Its Prelude and Effects

The eruption cycle of Soufriere Hills volcano (SHV), which included the paroxysmal explosive eruption of 17 September 1996, began with phreatic explosion on 18 July 1995 after 450 years of dormancy without any preliminary seismic activity beneath the volcano (http://volcano.si.edu/volcano.cfm? vn=360050). Several steam vents formed a NNW line across the Castle Peak Dome. Phreatic explosions with associated steam and ash columns rose to 3 km in height. The largest explosion occurred on 21 August. These explosions continued for four months and gave a way to continuous eruption of juvenile andesitic magma in the form of lava dome and lava flows that began around 15 November 1995 and continued until the PEE of 17 September 1996 (Young et al., 1998).

On 17 September 1996, the SHV started a 9-h period of dome collapse. This collapse began at 11:30 and was followed by pyroclastic flows generation involving totally 11.7 × 106m3 (Dense Rock Equivalent, DRE) in which 40% of the dome collapsed. After 2.5 h of quiescence, a paroxysmal explosive eruption occurred at 23:42. Explosive activity continued for about 50 min. A large crater-like depression 210 m deep formed in the dome open to the east. The height of plume reached 11-15 km, the volume of ejecta was estimated as 3.2 × 106 m3 (DRE). Pumice and lithic lapilli fell widely across southern Montserrat and houses were destroyed in the nearest village (Robertson et al., 1998).

The image of 4-band Advanced Very High Resolution Radiometer (AVHRR) multispectral data, provided from the NOAA polar-orbiting satellite series, shows that the cloud, elongating from Montserrat about 1 h after the 17 September PEE, was 175 km long E-W and 75 km wide N-S. The image, taken later in about 5 h, shows that with a distance from the island the cloud quickly became very diffuse (http://www.geo.mtu.edu/ volcanoes/west.indies/soufriere/govt/images).

#### Seismic History of the Soufriere Hills Volcano PEE

We use the catalog of volcano-tectonic earthquakes prepared by (Aspinall et al., 1998; Miller et al., 1998). This catalog included about 600 volcano-tectonic earthquakes with magnitudes from −0.5 to 2.4 that were located from 28 July 1995 (10 days after beginning of the eruption on 18 July) to the end of February 1997. Gardner and White (2002) wrote that the 18 July vent opened without notable accompanying seismicity.

The pre-PEE July 1995-September 1996 volcanotectonic earthquakes occurred within three epicentral zones (**Figure 10A**): beneath St. George Hill (SGH), that is situated about 5 km to the NW from the volcano (I), along the structure extending to the NE of the volcano (II), and beneath Soufriére Hills volcano (III).

As noted before, the eruption developed into two main stages: phreatic and magmatic, separated by a long (about 1 year) stage of the construction of lava dome and emplacement of associated lava flows. We describe here the variations in seismic activity associated with the stage of phreatic explosions that reached its peak on 21 August 1995 and the stage of occurrence of magmatic PEE that occurred on 17 September 1996.

Increase in seismic activity before the phreatic explosion of 21 August began with a 2-day swarm of earthquakes along the NE extent of the SHV (Zone II) from 5 to 6 August, then a new two-day swarm was recorded beneath SGH (Zone I) that continued from 12 to 13 August (**Figures 10A, 11A,C**). These seismic events occurred at the depth range from 6 to 3 km b.s.l. Beginning from 20 August the seismic foci migrated upward to the surface filling the volume III beneath the volcano. During 2 days, 20 and 21 August, about 140 earthquakes with magnitudes between −0.5 and 0.4 were recorded. The seismic events recorded

beneath the volcano clustered within a vertical column shape volume extending from 3 km b.s.l. to the summit of the volcano.

The explosion occurred at 08:03 of 21 August (B. Voight, personal communication, 2002) during the maximum level of seismic activity within zone III beneath the volcano. Just after the explosion, the number of seismic events sharply decreased.

The magmatic PEE of this SHV's eruptive cycle occurred 1 year later, on 17 September 1996. The seismic activity that preceded this paroxysmal explosion from 10 August is shown in **Figures 10B**, **11B,D**. The zones I and II, that were active before the 21 August 1995 phreatic explosion, now were calm. The earthquake foci were clustered mainly beneath the SHV (zone III) forming a vertical column rising from the depth of 3 km b.s.l. to the summit of the volcano.

The swarm of earthquakes had no clear beginning (**Figure 10B**); it occurred on the background of continuous seismic activity due to the process of partial collapsing of the dome that began in July 1996. Nevertheless, we can mark the gradual increase in earthquake numbers beginning from 29 August; therefore, the swarm continued for about 23–24 days. The earthquakes were of low magnitudes between 0 and 1.9; the largest event with magnitude 1.9 occurred on 17 September at 10:01, just before the 9-h crater collapse preceding the PEE. The explosion occurred during the apparent decrease in number of seismic events. After the PEE event, the number of earthquakes sharply decreased.

#### Volcán de Colima, México, 2005

Andesitic stratovolcano Volcán de Colima (height of 3,860 m) is the most active in Mexico (**Figure 1**). Eruptions of Volcán de Colima during the last 480 years involved a wide spectrum of eruption styles, including small phreatic explosions, major blocklava emissions, and large explosive events including Plinian, sub-Plinian and Vulcanian eruptions (Bretón González et al., 2002). The 1997–2011 cycle, discussed below, was preceded 3 years earlier by a short episode on 13–21 July 1994 culminated in the phreatic explosion of 21 July. The explosion destroyed the 1991 crater dome and produced avalanches and ash fall.

#### Paroxysmal Eruption of 5 June 2005, Its Prelude and Effects

The eruption cycle of Volcán de Colima, which included the 2004–2005 episode with paroxysmal explosive eruption of 5 June 2005, began on 22 November 1997 with 1-year sequence of volcano-tectonic earthquake swarms. Beginning from 20 November 1998, sequence of four endogenous episodes of lava dome construction and three episodes of lava domes destructed by Vulcanian explosions, was recorded. The most intense Vulcanian explosions (VEI 3) were recorded during the 2004–2005 episode of lava dome construction-destruction. The extrusions of andesitic magma, which occurred during September-November 2004, had formed a new lava dome filling the active crater. The March 2005 explosions destroyed the 2004 lava dome. The repetitive building of new small-size domes was then observed during the April to July, 2005 explosive activity; each of them was destroyed by the following explosions. Finally,

1996 (B) at Soufriére Hills volcano. I, II, and III are the earthquake sequences that occurred beneath St. George Hill (SGH) (I); within the NE extent of the volcano (II), and beneath the volcano (III). In (A), the moment of phreatic explosion of 21 August is shown by double arrow. In (B), the largest preliminary earthquake is shown by single arrow; the moment of PEE of 17 September is shown by double arrow. Data from the catalog of MVO.

a crater ∼ 260 m across and ∼ 30 m deep was left (Zobin et al., 2010). A photo, taken on 16 June from the southern side of the volcano (Zobin et al., 2010), shows the erosion channel at the summit, which was formed by pyroclastic flows related to the recent explosions. Many small impact craters formed on the floor of the crater.

The explosion, which occurred on 5 June 2005 at 14:20, was the largest in this sequence and may be considered as the paroxysmal event of the 2004–2005 episode as well as the total 1997–2011 cycle. This PEE blasted an ash plume which reached 9 km a.s.l. and had a length of 222 km. The time duration of the PEE, considering the time of explosion column growth and development, was not estimated but it continued during less than half of hour. The eruption column collapse was followed by emplacement of pyroclastic density currents with runout of 5.1 km and volume of 2.3 × 105 m3 . Fall of ash was observed in the nearest villages. The local airport canceled all flights for a day. After the explosion of 5 June, evacuation was carried out of the village of Juan Barragan located nearest to the volcano (Zobin et al., 2016; http://volcano.si.edu/volcano.cfm?vn=341040).

#### Seismic History of the Volcán de Colima PEE

The 1998–2013 sequence of the lava dome constructionsdestructions was preceded by a swarm of volcano-tectonic earthquakes only before the first 1998–1999 episode. The third episode of the cycle that developed during September 2004 to

Data from the catalog of MVO.

December 2005 (http://volcano.si.edu/volcano.cfm?vn=341040) was not preceded by any volcano-tectonic seismicity. As it can be seen in **Figures 12**, **13**, the seismic activity before and during the episode was produced by numerous rockfalls, pyroclastic flows, and small explosions. Their daily number sharply increased during the lava dome extrusion occurring from the end of September 2004 to the beginning of November 2004 and stayed at a level of 200–300 rockfall events and 50–60 small explosions. Before the beginning of and during the sequence of the March-April 2005 large explosions, the number of the seismic signals of rockfalls and small explosions was at low level, between 1 to 5 daily events. The number of the seismic events of both types began to increase until 10-20 daily earthquakes in the middle of May. The beginning of June, before the 5 June PEE, was characterized by a sharp decrease of small explosions but also increase in the number of rockfalls (**Figure 12**). Another type of the seismic activity before the PEE was represented by microearthquakes that sharply increased in their number about 12 h before the PEE (**Figure 13B**). These microearthquakes, clearly seen on the seismogram recorded at a distance of 1.6 km from the crater (**Figure 13A**), were identified as micro-explosions and micro-rockfalls (Zobin et al., 2010).

## DISCUSSION

Now let us discuss the dynamics of paroxysmal explosive activity of andesitic and dacitic volcanoes presented in this article. At first, we consider the dynamics of PEEs of the first group (VEI

FIGURE 12 | The number of daily earthquakes ascribed to rockfalls and pyroclastic flows (solid line) and to small explosions (dashed line) at Volcán de Colima during September 2004–July 2005. The September-November 2004 period of the lava dome construction and the period of the March–July 2005 large explosions are labeled. Arrow shows the moment of the 5 June PEE. Modified from (GVP, http://volcano.si.edu/volcano.cfm?vn=341040).

5-6) that occurred at the volcanoes of Mount St. Helens, El Chichon, and Pinatubo. The following main stages of their dynamics may be formulated:

1. Stage of quiescence. It may continue from 120 years (MSH) till 500–550 years (Pinatubo, El Chichón).

(A), and variations in cumulative number of microearthquakes during 3 to 5 June before the 5 June PEE (B). Arrows in (A,B) indicate the moment of PEE. Insert in (B) shows the photo of the 5 June explosion taken by Televisa (http:// noticieros.televisa.com). Data from catalog of Colima Volcano Observatory.

column with collapse of the edifice summit (El Chichón and Pinatubo).

5. Stage of co-PEE seismo-reaction. Swarms of earthquakes appeared during decreasing intensity of the Plinian activity of PEEs. At MSH, a sequence of 21 magnitude 4.0–4.5 events was recorded during 4 h; at El Chichon, a 5-h sequence of five intermediate-size (Mc 3.7–3.8) earthquakes was recorded; at Pinatubo, a sequence of 25 earthquakes of magnitude range from 4.5 to 5.7 was recorded during 7 h.

Let us discuss the nature and significance of these stages in development of eruption process. The stage of re-awakening, represented by the swarms of volcano-tectonic (VT) earthquakes, may indicate the input of new magma in the system of magma reservoirs beneath the volcano. This process covers areas significantly exceeding the area of volcanic structures and occurs within the depths between 10 and 25 km and the surface.

When magma reaches the volcanic edifice, the volcanic conduit filling begins. The foci of VT earthquakes cluster within a vertical volume just beneath the crater indicating magma ascent. The process of volatile exsolution from the melt as well as gas expansion during the stage of phreatic explosions promotes stress accumulation within the volcanic structure for initiation of the PEE.

During the PEEs, a great volume of volcanic rocks ejects, liberating space in the interiors beneath the volcano. It causes collapse of the upper magmatic system and, as a consequence, the swarms of "triggered" earthquakes whose magnitude may reach up to 5.7. The observations, obtained at our three PEEs of the first group (See **Table 1** and text), show that the maximum magnitude of "triggered" earthquakes, Mmax, has a tendency of log-linear dependence on the volume of ejecta, V, during PEEs (**Figure 14**).

The appearance of these "triggered" seismic events during the PEEs indicates the partial emptying of the magma reservoirs, possibly also associated with the reactivation of regional tectonic structures. The areas of activation of post-eruption seismicity allow locating these magma reservoirs (Zobin, 2017).

In the case of MSH, the 50 km long regional structure within Cascade Range was activated to the north and to the south of the volcano, in addition to outward dipping fault zones merging below the volcano (Scandone and Malone, 1985). The depths of earthquakes reached 20 km (Weaver et al., 1981). In the case of El Chichón, the earthquake foci filled the depth interval from 0 to 25 km beneath the volcano (Jiménez et al., 1999). In the case of Pinatubo, the zone beneath the volcano spread to the west within volcanic edifice and occupied all beneath-volcano space filling it up to the depths of 25 km, about 20 km deeper than before the 15 June explosion. The seismicity also showed a clustering along outward dipping fault zones merging just below the volcanic edifice (Mori et al., 1996).

The dynamics of three selected PEEs of the second group (VEI 3), Usu volcano, SHV and Volcán de Colima, was out of the above-mentioned scheme. The dynamics of the PEE development at SHV was the only that partly kept within the main phases of the PEEs of the first group. At first, it was reawakening volcano having stage 1 of quiescence during 450 years

before the 1995-1996 cycle of eruption. The stage 2 was absent; no preliminary volcano-tectonic activity was recorded. At the same time, as was noted by Aspinall et al. (1998), the spatial separation of some of the hypocenters into two clusters, one under SGH and another under the SHV, recorded during a phreatic stage (stage 3 for SHV) was reminiscent of pre-eruption activity (Stage 2) recorded in 1991 at Pinatubo volcano (compare **Figures 7**, **11**). Stage 5 was absent; probably due to low ejecta volume (two-three orders of volume less than ejected during the PEEs of the first group; see **Tables 1**, **2**), no "triggered" earthquakes were recorded during the 50-min plume development.

The appearance of PEEs at Usu volcano and Volcán de Colima was out of the proposed stage schemes. The preliminary VT swarm at Usu volcano was short, and lasted only for 32 h. No phreatic explosion stage was observed before the PEE. Probably due to low ejecta volume, no "triggered" earthquakes were recorded during the 2-h plume development, only volcanic tremor accompanied the PEE (See **Figure 9**).

The PEE appearance at Volcán de Colima was developed as standard lava dome construction-destruction. No preliminary VT swarm was recorded, only seismic signals associated with rockfalls and small explosions suggested such a process. The PEE simply "cleaned" the crater, removing the lava dome, without any destruction of the crater.

This absence of any regularity in the development of lower VEI eruptions of the second group may refer, among other reasons, to different conditions of opening of the magmatic conduit during these eruptions. All three PEEs of the first group were preceded by a stage of quiescence that continued from 120 years (MSH) until 500–550 years (Pinatubo, El Chichón). This indicates the similar plugged conditions of their conduits and, as a result, the similar 5-stage development of eruptions. Among the PEEs of the second group, only SHV, having a stage of quiescence that continued for about 370 years, may be considered as a plugged conduit volcano. As we see, the build-up of its eruption was characterized by at least three of five stages noted for the PEEs of the first group. The two other PEEs, at Usu and Volcán de Colima, were supposedly developed in the conditions of open or semiplugged conduit, as suggested by their recent pre-PEE activity.

Therefore, this study confirms that eruptions with higher VEI are more likely to be related to a plugged magmatic conduit, whereas eruptions with lower VEI are usually more associated to open conduit conditions.

## CONCLUSIONS

Review of the dynamics of PEEs development at andesitic and dacitic volcanoes during 1960–2010 was made for two groups of PEEs, of VEI 5-6 and VEI 3-4. The first group included three eruptions of VEI 5-6 and the second group included three eruptions of VEI 3. The PEEs of the first group have similarity in their development: long (more than 120 years) period of quiescence, preliminary VT swarm, period of phreatic explosions and then, PEE appearance. It was shown that the PEEs during their Plinian stage "triggered" the earthquake activity beneath the volcanic structures with the maximum magnitude of earthquakes proportional to the volume of ejecta of PEEs. For these PEEs of the first group, a 5-stage scheme of development process is proposed. The three discussed PEEs of the second group developed in more individual styles, probably resulting from the different conditions of opening or plugging of the magmatic systems. Therefore, this study confirms that eruptions with higher VEI are more likely to be related to a plugged magmatic conduit, whereas eruptions with lower VEI are usually more associated to open conduit conditions.

## AUTHOR CONTRIBUTIONS

The author confirms being the sole contributor of this work and approved it for publication.

## ACKNOWLEDGMENTS

The comments of V. Acocella, A. Bonaccorso, S. Calvari, and two reviewers helped to improve the manuscript. N. Stalboun helped to improve our English grammar. The catalogs of volcanotectonic earthquakes were provided by Z. Jiménez (El Chichon volcano), J. Mori (Pinatubo volcano), and W. Aspinall (Soufriere Hills Volcano) and taken from the WEBsite https://pnsn.org/ pnsn-data-products/earthquake-catalogs (Mount St. Helens) and https://earthquake.usgs.gov (Pinatubo volcano).

## REFERENCES

Eruptions and Lahars of Mount Pinatubo, Philippines, eds C. G. Newhall and R. S. Punongbayan (Seattle, WA: University of Washington Press), 165–196.

of the 2005 Vulcanian explosions at Volcán de Colima, México. Bull. Volcanol. 78:27. doi: 10.1007/s00445-016-1019-2

Zobin, V. M., Melnik, O. E., Gonzalez, M., Macedo, O., and Bretón, M. (2010). Swarms of micro-earthquakes associated with the 2005 Vulcanian explosion sequence at Volcán de Colima, México. Geophys. J. Int. 182, 808–828. doi: 10.1111/j.1365-246X.2010.04647.x

**Conflict of Interest Statement:** The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The reviewer, SC, and handling editor declared their shared affiliation.

Copyright © 2018 Zobin. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Coupled Short- and Medium-Term Geophysical Signals at Etna Volcano: Using Deformation and Strain to Infer Magmatic Processes From 2009 to 2017

Marco Aloisi, Alessandro Bonaccorso\*, Flavio Cannavò and Gilda M. Currenti

Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania-Osservatorio Etneo, Catania, Italy

In active volcanoes a main challenge is to identify and characterize the dynamics of magmatic sources from deformation and strain data. This task is of primary importance in frequently eruptive volcanoes, such as Etna. After the main flank eruption of 2008–2009 and until 2017, Etna volcano was characterized by a lively eruptive activity of different phases. These comprised 44 lava fountain episodes from the New South East Crater (NSEC), two sequences of close episodes of lava fountains from the Voragine crater (VOR), as well as some periods of summit effusive activity with a more prolonged supply of lava flows. Several studies have described and modeled single lava fountains episodes of the NSEC and VOR, in particular through high precision data from borehole strain-meters. In this study, we broaden the analysis also considering the medium-term volcano recharging/discharging periods preceding/accompanying the different eruptive phases during 2009–2017 by constraining the source positions through deformation recorded by the permanent GPS network. Together with the modeling deduced from the strain-meter data we produce a more complete representation of the different sources that characterized the different periods both in the medium-term (i.e., the preparatory phases showing inflation and the eruptive phases showing deflation) and in the short-term (i.e., the fast discharge associated with eruptive events). Our modeling explains the pathway of magma from the intermediate-shallow plumbing system to the surface and highlights a clear separation between the inflation and the deflation source depths, coherently with petrological constraints on the spatio-temporal evolution of magma transfer and storage.

Keywords: deformation and strain, volcano sources, inflation/deflation phases, eruptions, Etna volcano

## INTRODUCTION

The monitoring and study of ground deformation pursue the goal to provide a clear contribution to understanding of magma ascent processes both during the preparatory and final phases leading to the eruptions in the various volcanoes that are monitored throughout the world. Magma during ascent can usually be stored in reservoirs that form, through the gradual accumulation of magma, sources of internal overpressure. In turn, these gradually deform the volcanic edifice, which responds with an inflation. Then, during the eruptive phases the volcano releases the accumulated

#### Edited by:

Roberto Sulpizio, Università degli studi di Bari Aldo Moro, Italy

#### Reviewed by:

Marco Viccaro, Università degli Studi di Catania, Italy Geoff Kilgour, GNS Science, New Zealand

> \*Correspondence: Alessandro Bonaccorso<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Volcanology, a section of the journal Frontiers in Earth Science

Received: 27 March 2018 Accepted: 20 July 2018 Published: 10 August 2018

#### Citation:

Aloisi M, Bonaccorso A, Cannavò F and Currenti GM (2018) Coupled Short- and Medium-Term Geophysical Signals at Etna Volcano: Using Deformation and Strain to Infer Magmatic Processes From 2009 to 2017. Front. Earth Sci. 6:109. doi: 10.3389/feart.2018.00109

**25**

pressures and its edifice returns to equilibrium showing a deflation. Through the recording of the deformation patterns, the geodetic techniques allow us to constrain the position and the shape of the magmatic sources (for a review Dzurisin, 2003, 2007 and citations herein).

Two general problems characterize the modeling of the deformation signals recorded in the last decades: (i) eruptions are usually not frequent phenomena in the same volcano, and in most of the active volcanoes it is necessary to wait long intervals (from several years to tens-hundreds of years) between subsequent eruptions; (ii) the smaller the eruptions, i.e., if they involve reduced erupted volumes, the more difficult it is to detect appreciable deformation variations because of the resolution of the current instrumentation. As a result, in the various volcanoes worldwide, the few strongest eruptions are usually better studied.

Etna has been frequently erupting in the last 40 years, showing a great variety of eruptive styles. Since 1971, there have been eighteen major flank eruptions, numerous summit eruptions, periods of permanent Strombolian activity and hundreds of explosive events generating lava fountains (for a review of the Etna activity see Branca and Del Carlo, 2004; Allard et al., 2006). In the 30-year period (1980–2010), the ground deformation monitoring systems on Etna enabled investigating and modeling the storage sources of numerous effusive flank eruptions. All these major eruptions shared the same deformation dynamics: a medium-term recharging period (lasting months to years) that precedes the eruption producing the inflation of the volcano. The recharge phase ends with the propagation of the final intrusions (i.e., the feeder dikes) that produce marked and rapid deformation. The eruptive phase then begins, characterizing the discharge phase causing the volcano deflation. During 1980–2010 the deformation sources associated with these cycles of charging/discharging mediumterm phases were inferred at depths ranging between 0 and 9 km b.s.l. (e.g., Bonforte et al., 2008; Aloisi et al., 2011), a zone that is defined as the shallow-intermediate plumbing system. Results of the modeling of the sources for the main eruptions, as obtained from ground deformation studies, are reported in Bonaccorso and Davis (2004), Bonforte et al. (2008), Aloisi et al. (2011), Bruno et al. (2012), and Patanè et al. (2013). The studies mainly investigated the pressurization phases recorded during the inflation preceding the eruption onset. For the period from 2002 to 2006 Palano et al. (2017) reviewed the inflation /deflation preceding /accompanying the three main flank eruptions 2002–2003, 2004 and 2006 by using GPS data in a common modeling approach. However, the heterogeneity of the models and approaches used by the various Authors in the previous studies poses serious problems when a comparison of the inferred source is needed to investigate the long-period/multi-eruption dynamics. Indeed, the identification of different sources and mechanical parameters of the medium does not guarantee a comparable class of solutions, which should be required to avoid bias and to compare the positions, dimensions and volumes changes of the sources with consistency.

The co-eruptive deformation associated with lava fountains has usually been very small and difficult to detect by classic geodetic measurements such as GPS. However, by the end of 2014, four high precision borehole strain-meters have been installed. These instruments were capable of detecting clear strain changes associated with the lava fountaining activity, providing a tool to infer the source feeding the associated explosive activity (i.e., Bonaccorso et al., 2013, 2016).

In this paper, we provide a homogenous and consistent overview of the different active sources in the period 2009– 2017. We did this by re-analyzing the medium-term recharging phases that preceded the different eruptions and the mediumterm discharge periods characterized by cycles of several lava fountains or prolonged effusive phases. Finally, for some significant periods, we constrained the positions of the pressure sources and their volume changes that produced the deformation patterns recorded by the permanent GPS network. We used a uniform modeling approach to obtain comparable solutions of the inferred sources. This allowed a more complete and robust representation of the different sources that acted during the different recharging (i.e., medium-term preparatory phases) and discharging (i.e., the eruptive phases) phases. We highlight and discuss how the inferred sources are in agreement with a recent conceptual model of the Etnean plumbing system, made up of different magma storage zones, which were constrained by petrological and geochemistry studies (i.e., Kahl et al., 2015; Cannata et al., 2018). To the best of our knowledge, this is the first study where results from GPS and borehole strain-meter networks are used together to investigate medium- and short-term deformation sources at Etna volcano.

#### Etna 2009–2017 Activity

In the last decade, the activity of Mt. Etna did not produce major flank eruptions but instead has been characterized by tens of lava fountain events. A lava fountain is a powerful gas emission ejecting lava fragments to heights ranging from tens to hundreds of meters (Wilson and Head, 1981; Wolff and Sumner, 2000). In the Hawaiian cases the lava fountains typically range from ∼ 10 to 100 m in height, and occasionally reach more than 500 m. Most of the material returns to the surface to form pyroclastic cones, rootless flows or lava ponds (Head and Wilson, 1987). Lava fountains at Etna are often different from the Hawaiian style for their more explosive power and for the formation of a several kilometers high, sustained eruptive column causing widespread ash fallout (e.g., Calvari et al., 2011; Bonaccorso et al., 2014). It is for this reason that they have often been referred as "paroxysmal" (e.g., Bonaccorso et al., 2011a,b, 2013; Gambino et al., 2016) to highlight their greater power when compared to the Hawaiian lava fountains.

After the last flank eruption of 2008–2009, there were 52 lava fountains from January 2011 to May 2016, erupted from Etna's main craters (**Figure 1**). 45 episodes (44 from January 2011 to December 2013, and one episode in December 2014) led to the formation of the New South East crater (NSEC), next to the existing South East crater (SEC) SEC crater (inset in **Figure 1**). Moreover, two sequences of 4 and 3 events took place at the Voragine crater (VOR) on 3–5 December 2016 and

FIGURE 1 | Etna map and multidisciplinary stations used in this study (GPS and borehole dilatometer networks). In the upper-left inset, the detail of the summit craters is shown. They are: Voragine (VOR), NE crater (NEC), Bocca Nuova (BN), SE crater (SEC), New SE crater (NSEC).

18–21 May 2016, respectively. Generally, all of the 2011–2013 lava fountain episodes from the NSEC showed generally similar main characteristics, with the height of the lava fountain reaching 300–1,000 m, ash columns reaching 5–9 km, and associated 4–6 km long lava flows descending the eastern flank of the volcano (i.e., Calvari et al., 2011; Behncke et al., 2014; De Beni et al., 2015). The average total DRE (density rock equivalent) volume of magma emitted during each of the 2011–2013 NSEC lava fountains, including both pyroclastic products and lava flows, was ∼ 2.5 × 106 m3 per event (De Beni et al., 2015). After the 2011–2013 NSEC lava fountains, the eruptive activity switched to moderate lava effusion from the NSEC toward NE during January–April and July–August 2014. On 28 December 2014, a further lava fountain episode occurred at the NSEC (Gambino et al., 2016).

VOR is part of the main central crater and, in the last decades, has been much less active than the NSEC. Before 2015, the two main eruptive episodes were a sub-plinian explosion on 22 July, 1998 (Bonaccorso, 2006) and a powerful lava fountain on 4 September, 1999 (Calvari et al., 2002; Harris and Neri, 2002). After these two episodes, activity through the VOR was relatively quiet for several years until the sudden and powerful sequence on 3-5 December, 2015, with a further eruption in May, 2016. The lava fountains of 3–5 December 2015 rank among the most violent to have occurred at Etna in the last two decades (Aloisi et al., 2017; Bonaccorso and Calvari, 2017). A summary of 2009–2017 eruptive activity is shown in **Table 1**.

The 2011–2016 lava fountains were characterized by a marked explosivity feeding a gas thrust region of several hundreds of meters. This sustained a several kilometer-high eruptive column expanding well beyond the lava fountain portion and causing widespread ash fallout (e.g., Calvari et al., 2011; Bonaccorso et al., 2014; Bonaccorso and Calvari, 2017). The short lived activity of lava fountains, although spectacular, emitted limited volumes of magma (∼ 2.5 × 106 m3 per event) compared to the prolonged effusive flank eruptions, which usually emit from 1.0 × 107 to 2.0 × 108 m3 . However, during 2011–2014, the high frequency of the NSEC lava fountain episodes erupted a total volume (pyroclastics products plus effusive flows) of ∼150 × 106 m3 (De Beni et al., 2015). Over this 2011–2014 time interval, this total erupted volume gives an average magma eruption rate similar (even higher) to the long-term output rate in the last decades through the usual effusive eruptions that is 25 × 106 m3 per year (Harris et al., 2011; Bonaccorso Calvari and Calvari, 2013). In other words, the 2011–2014 NSEC activity was characterized by an unusually high frequency of explosive episodes compared to Etna's activity in past decades, although the average magma erupted rate in this interval is close to the long-term output rate of the volcano mainly emitted through major effusive eruptions.

## Strain and Deformation Monitoring Networks

The first permanent GPS stations were installed on Mt. Etna during November 2000 and, since then, the network has been further developed (Palano et al., 2010). At present, the volcanic edifice is monitored by 39 permanent stations (**Figure 1**). In this paper, we consider the data recorded by the GPS network from June 2009, immediately after the end of the 2008 flank eruption, to November 2017. Daily raw observations were processed using the GAMIT/GLOBK software (Herring et al., 2010) adopting the methodology described in Gonzalez and Palano (2014). Using the GLRED module of GLOBK, precise baselines of the whole network have been computed on a daily based solution. To further investigate the dynamics of fast ground displacements during the strongest eruptions, we considered also GPS data solutions at high rate (1 Hz). In particular, the high-rate GPS data were collected from the network dual-frequency GPS receivers and processed through Geodetics R RTD software package by epoch-by-epoch algorithm (Bock et al., 2001; Nikolaidis et al., 2001) to obtain a solution of position every second for all the measured points. Then, the solutions were post-processed to improve their reliability, eliminating the outliers by using an interquartile range (IQR) filter and filtered for the multipath noise (due to multi-path ground reflections of GPS signal) following the technique by Nikolaidis et al. (2001) and implemented in Cannavò et al. (2015). The remaining solutions were used

TABLE 1 | Eruptive periods from June 2009 to April 2017 with the type of eruptive activity and the detected inflation or deflation phases modeled in this study.

to estimate the baseline variation between pair stations in a time window of 3 min, as this is the minimum interval to obtain a satisfactory signal-to-noise ratio in the baseline time series.

In November 2011, two high precision borehole strainmeters, installed at a depth of 180 m in the western flank (DRUV and DEGI, **Figure 1**), began operating. Another two sensors were installed in November 2014 at a depth of 120 m in the upper heights of the volcano (DMSC and DPDN, in **Figure 1**). During 2015–2016, DPDN had several interruptions due to technical problems at the station. A description of the installations and implementation of the network is reported in Bonaccorso et al. (2016). The strain-meters are all Sacks-Evertson instruments, also called dilatometers, which measure the volumetric strain (change in volume/original volume) of the surrounding rock with a nominal precision up to 10−11 in a wide frequency range (10−7 - 102 Hz). This type of strainmeter is considered the most sensitive instrument used for monitoring deformation in geophysics. A detailed description of the design and capabilities of these instruments is given in Roelloffs and Linde (2007). The high sensitivity of the instrument is also affected by strain changes induced by environmental effects; however, installation at depths greater than 100 m reduces the noise generated in near surface rocks (e.g., by thermo-elastic effects). GPS and borehole strain-meters measurements are complementary techniques. The GPS network has a lower precision of ∼10−6 (resolution of millimeters over kilometer distances) but on Etna it has a denser coverage and is able to detect the cumulated medium-tem inflation /deflation phases. The borehole strain-meters have a better precision and can detect the effect of small and brief transient phenomena such as the lava fountains, whose associated strain are in the range of 10−7 -10−8 , difficult to be detected with GPS.

## RESULTS-STRAIN AND DEFORMATION MODELING

## Co-eruptive Short-Term Deformation and Inferred Sources

Despite the long-term drift characterizing the instruments during the first years of setup, clear strain changes were revealed during the short duration of the lava fountain episodes at NSEC (Bonaccorso et al., 2013, 2014). Soon after their installation, the borehole dilatometers detected the short-term variations produced by the lava fountains with high accuracy (**Figure 2**). All the recorded lava fountains showed the common aspect that the duration of the strain change coincides with the duration (usually a few hours) of the lava fountain. The strain change starts when the lava fountain begins and stops when the lava fountain finishes (**Figure 2**). It is interesting to note that strain suddenly changes only during the lava fountain, which suddenly empties the feeding reservoir, implying that the strain is sensitive to the magma output and the source emptying (e.g., Bonaccorso et al., 2013, 2016). Similar conclusions were also obtained from strain changes recorded during the frequent Vulcanian

eruptions at Sakurajima volcano, Japan (Iguchi et al., 2008). The strain changes recorded during the 2011–2013 lava fountains erupted from the NSEC allowed constraining the decompression source at a shallow level (shallow plumbing system). Bonaccorso et al. (2013) applied a numerical model by using the finite element method (FEM) to evaluate the elastic response of the volcano edifice to a deflating source. The Authors inverted the average strain changes at the two working strain-meters (DEGI and DRUV), together with the average tilts recorded at radial distances of 2.2, 4, and 6–8 km. The best model reproducing the recorded strain and tilt changes was an ellipsoidal source constrained at sea level, with an aspect ratio of 0.5 (i.e., more elongated vertically), that underwent a change in volume of ∼2 × 106 m3 . Bonaccorso et al. (2013) also estimated ∼0.5 × 106 m3 as the additional volume of erupted magma due to the compressibility of residing magma within the source, therefore inferring a mean volume for each lava fountain episode coherent with the one deduced by surveys and aerophotogrammetry (De Beni et al., 2015).

Short-term strain changes were also recorded during the two powerful December 2015 and May 2016 sequences at VOR. Preliminary data analysis and modeling show that the strain changes at the three stations (DRUV, DEGI, and DMSC) are caused by the activation of a deeper portion of the magmatic plumbing system (Bonaccorso and Calvari, 2017). **Figure 3** shows a comparison between the strain signals recorded during the lava fountains of 28 December 2014 at NSEC and 3 December 2015 at VOR, respectively. As mentioned, the source of the NSEC was inferred at 0 km (b.s.l.) in agreement with all the episodes from the NSEC (Bonaccorso et al., 2016). All the strainmeters recorded negative changes temporally coincident with the duration of the lava fountain event. On the contrary, a clear positive change during the VOR episode was recorded at DMSC. In the case of a deflating pressure source, the volumetric strain reverses its sign moving radially from the source, as it goes from compressional (positive) to extensional (negative) regime. This transition from positive to negative depends on the source depth. Therefore, the negative/positive pattern of the strain variations at the stations may enable to constrain the source depth. Generally, the deeper the source, the higher the radial distance at which the strain changes from positive to negative (Roelloffs and Linde, 2007). During the NSEC events, no positive strain changes were recorded at any of the stations, indicating a shallow source producing a narrow compressional pattern close to the summit area. Conversely, during the VOR episode the positive change recorded at DMSC is a clear indication of the activation of a deeper deflating source, whose associated strain pattern produces a compressional regime at higher radial distance from the source (Bonaccorso and Calvari, 2017).

Very weak deformation was recorded in the GPS time series during the NSEC events; instead, concurrent changes in the baseline variations, computed from high frequency GPS data analysis, were observed during the VOR activity (**Figure 4**). Coherently with the high frequency solutions, for the first episode, the GPS daily-based solutions showed displacements, up

fountains. A clear reverse in sign at the near summit station DMSC is detected, implying a deeper source position for the 2015 case (right).

to 5 mm, recorded at the entire network and showed a coherent radial pattern of deflation (Aloisi et al., 2017). The time series of high frequency solutions at the summit stations allowed us to follow the entire dynamic of the fountain in detail (**Figure 4**). In particular, the comparison between the mean baseline variation recorded by the GPS for the summit baselines crossing the crater area and the strain time series recorded by strain-meters shows a synchronous beginning of variation with the onset of the fountain event (**Figure 4**).

## Medium-Term Recharging /Discharging Phases Inferred by GPS Data

We investigated the deflation-inflation cycles observed between June 2009 and November 2017, presenting an analytical solution to model the recorded GPS data. The variation of the area (**Figure 5**), recorded at an intermediate altitude triangle (EMEG-EMCN-ESLN), represents the average deformation of the volcano edifice well, clearly showing deflation-inflation cycles (e.g., Aloisi et al., 2011; Bruno et al., 2012; Patanè et al., 2013). The area of the triangle increases during periods of magma ascent, causing inflation. It decreases during and after eruptive periods, when magma outpours from the magmatic accumulation zones. As mentioned, the analyzed time interval has been studied in previous works. In particular, Aloisi et al. (2011) analyzed the 2009–2010 time period; Patanè et al. (2013) investigated the year 2011; Spampinato et al. (2015) and Bruno et al. (2016) analyzed some ground deformation phases observed during 2012–2013; Cannata et al. (2015) and Greco et al. (2016) analyzed some phases recorded during the 2013; Viccaro et al. (2016) and Gambino et al. (2016) investigated the 2014 event; finally, Aloisi et al. (2017); Bonaccorso and Calvari (2017) and Cannata et al. (2018) studied the 2015–2017 activity. Different approaches were used by the Authors to model the observed deformation patterns. With the aim of obtaining comparable solutions among the sources in terms of the involved volumes and their locations, we decided to re-analyze some significant phases in the considered time window, using a uniform approach, i.e., to apply the same methodology to the different phases. Different inflation and deflation phases were selected from the areal dilation time series (**Figure 5**). In particular, assuming that any change in the area dilatation at the surface means that the deformation source is changing in position, pressurization or volume, we selected time spans characterized by coherent crustal deformation patterns, during which a homogeneous volcanic source is reasonably acting inside the volcano. We chose to investigate the following intervals, representing a selection of some of the more significant observed variations (**Tables 1**, **2**): (I) a first phase of lasting inflation from 14 June 2009 to 31 December 2010; (II) a second phase of short inflation observed from 20 May 2011 to 16 July 2011; (III) the following third phase showing a deflation from 16 July 2011 to 17 October 2011; (IV) a fourth phase showing a new inflation of the volcano edifice from 26 April 2012 to 15 February 2013; (V) the following fifth phase of deflation from 15 February 2013 to 30 April 2013; (VI) a sixth phase of inflation from 3 May 2013 to 23 October 2013; (VII) and (VIII) two following phases of deflation from 20 January 2014 to 1

FIGURE 4 | Recorded strain and GPS signals during the lava fountains at NSEC and VOR caters. The 23 November 2013 lava fountain was one of the most powerful events of the 44 recorded from NSEC in 2011–2013. (A) Mean baseline variation calculated in percentile for the summit baselines crossing the crater area and (B) strain detected at DRUV station during the lava fountain of 23 November 2013 at NSEC. (C) Mean baseline variation calculated in percentile for the summit baselines crossing the crater area and (D) strain detected at DRUV station during the lava fountain of 3 December 2015 at VOR crater. The GPS did not detect changes during the lava fountains at NSEC, while a clear negative change (i.e., line contraction) was detected during the VOR more powerful event. Instead the strain-meter recorded small negative changes, i.e., expansion of the medium surrounding the station, during the NSEC lava fountains but recorded a bigger change during the VOR event.

April 2014 and from 09 July 2014 to 15 August 2014; (IX) a ninth phase representing the inflation recorded from 16 May 2015 to 27 November 2017. The last chosen phase is interrupted by short sub-periods of deflation related to the VOR explosive activity. As mentioned in the previous paragraph, we also detailed each lava fountain occurring at the VOR crater on Mount Etna, during the first days of December 2015. Moreover, to better understand this eruptive phase, it was useful to study the short cycles of inflation that preceded the December 2015 (from 18 May to 1 December 2015, phase IXa) and the May 2016 (from 7 December 2015 to 16 May 2016, phase IXb) VOR explosive sequences.

For each different deformation phase, we computed the baseline variations recorded at the GPS network in the corresponding time interval. Baseline estimations are not affected by possible biases in the choice of a local reference frame, and moreover they show lower levels of noise than the single GPS components. Therefore, with large GPS networks and for weak deformation, directly inverting baseline variations can represent a robust method to constrain a model for the recorded data, avoiding added uncertainties into the source modeling. We used the entire GPS network except the stations located on the lower eastern flank of the volcano (ERIP, EBAG, ELEO, EPOZ, ELIN, TABLE 2 | Final optimal solutions for each modeled period.

Estimated center coordinates (Xc, Yc, Zc) and volume change of the source are reported. We estimated uncertainties of ∼ 300 [m] for the horizontal components, of ∼ 700 [m] for the vertical component and, finally, of ∼ 1.9 ×107 [m3 ] for the volume change.

ETEC, EBDA, ELAC; see **Figure 1**) because it is well known that the eastern flank of Mt. Etna is affected by an almost constant ESE-ward sliding motion (for a review see, e.g., Aloisi et al.,

2011; Murray et al., 2018). This large displacement is most likely not directly linked to the magma recharging/discharging processes; therefore, it is appropriate to exclude this area of the GPS network when the volcano plumbing system is investigated. For all the phases, we performed an analytical inversion of GPS baselines, describing the pressure source with a finite spherical magma body using the analytical solution of McTigue (1987). We chose this source model among other analytical models available in the literature since it showed a good trade-off between the number of degrees of freedom and the obtained data fit. This deformation source is described by five parameters: the coordinates "x", "y," and "z" of the sphere center, the radius "r" and the overpressure "P" on the source wall. It can be used to infer the centroid of the pressurizing/depressurizing sources. Model parameters were estimated for each phase, performing an inversion by means of the Pattern Search technique (Lewis and Torczon, 1999) together with a local Genetic Algorithm Search (Goldberg, 1989), finally followed by a non-linear least squares to better refine the solution. The uncertainty of each model parameter was estimated adopting a Jackknife re-sampling method (Efron, 1982). We included the effects of the topography using the method of Williams and Wadge (2000). The medium was assumed to be homogeneous and isotropic with a Young modulus of 75 GPa and a Poisson ratio of 0.25 (e.g., Aloisi et al., 2011). The volume change 1V was estimated by means of the formulation described in Tiampo et al. (2000), using a shear modulus of 30 GPa. It is noteworthy that this value is an overestimation for a hot volcanic region and that a more correct evaluation of the shear modulus needs more attention, for example using a non-elastic rheology that is beyond the scope of this work. The modeled and recorded deformation pattern for two typical phases of inflation and deflation are shown in **Figure 6**. Final optimal solutions for each phase are shown in **Figure 7**, **Table 2**.

By using a uniform approach for all the selected periods (i.e., same kind of representative source, inversion method, and rheology parameters) we provided highly comparable solutions avoiding the bias that may often affect the solutions of different approaches. Under these conditions, the result highlights a clear separation between the inflation and deflation source depths. This is a new insight into the reconstruction of the magma movement and storage within the plumbing system of Mt. Etna volcano, as discussed in depth below.

## DISCUSSION AND CONCLUSION

Medium- and short- term deformation sources at Etna volcano have been investigated with the aid of GPS and strain-meter data. These two kinds of deformation measurements have proven complementary to each other. The high-resolution and accuracy of the strain data have enabled us to follow rapid transient deformation related to the short-term discharge of magma through each fountaining event. GPS daily time series have been used to detect medium-term deformation processes related to the recharging of the magmatic plumbing system. Medium-term processes are difficult to recognize in the strain data because of the instrumental drift, while short-term processes are hidden in the GPS data due to the lower signal-to-noise ratio. For the most powerful eruptive events however, the processing of high rate GPS data has shown a general agreement in the temporal variations with respect to strain changes. The comparison of both datasets enabled us to assess their intrinsic advantages and drawbacks.

At Etna basaltic magmas and bubbles/liquid mixture rise for their positive buoyancy in the 22–2 km b.s.l. and, mainly related to their amount of H2O, they can stall and form magmatic storage at different depths (Corsaro and Pompilio, 2004) that can cause the volcano deformation.

As previously found by many Authors (see Aloisi et al., 2011 for an overview), the pressure sources modeled here are located along the western border of the high velocity body (HVB), delineated using seismic tomographies (e.g., Patanè et al., 2006) (**Figure 8**). Furthermore, there is a general agreement that this border zone is coincident with the pathway that the magma takes toward the surface (e.g., Bonforte et al., 2008). Along this pathway, the magma can store and differentiate at various

FIGURE 7 | The upper panel shows the areal location of pressure sources modeled for each phase (inflation in red and deflation in blue). The lower panel shows a cross-section highlighting the calculated depth of each phase. For comparison, the trace of the high vp body estimated by Aloisi et al. (2002) is also shown.

depths, forming a multilevel plumbing system characterized by several magmatic environments, where different types of olivines can form (e.g., Kahl et al., 2015; Cannata et al., 2018). Thanks to the uniform approach of modeling applied here, for the inflation/deflation phases modeled by GPS data in the investigated period, our results highlight a very clear separation in depth between the inflation and the deflation sources (**Figure 7**). The centroid of the inflation sources is located at ∼ 6.5 km (b.s.l), the centroid of the deflation sources is located at ∼ 4.5 km (b.s.l.).

With regards to the frequent short-term explosive lava fountains, for the NSEC events the strain changes constrained a shallower source located at a depth of 0 km b.s.l. (Bonaccorso et al., 2013). Regarding the four events that involved the VOR crater during the first days of December 2015, both Aloisi et al. (2017) and Cannata et al. (2018) found that the recorded deformation pattern is compatible with a deflation source located under the crater, at a depth of ∼5 km b.s.l. Comparing with data recorded for the NSEC lava fountains, this result suggests that eruptions through VOR are sourced from a deeper portion of the magmatic plumbing system. This evidence agrees with petrological data that show the activation of magmatic environments deeper than those generally observed for other recent Etnean eruptions, involving deep basic magmas brought to shallow crustal levels in very short time scales (Cannata et al., 2018). Particularly, the olivine populations and

FIGURE 8 | Sketch section of Etna's plumbing system. (1) deep magma supply. (2) intermediate storage zone inferred by the deformation sources modeled for the recharging-discharging phases investigated in this study; (2a) is the zone comprising the sources of the recharging phases (inflation) whose centroid is indicated with the lower star; (2b) is the zone comprising the sources of the eruptive discharging phases (deflation) whose centroid is indicated with the upper star. (3) Shallow storage feeding the lava fountain episodes of NSEC inferred by strain changes recorded by borehole strain-meters. The positions of the centroid of the modeled sources 2a, 2b and 3 are coincident with the different levels of the magmatic environments constrained by Cannata et al. (2018) by petrological data. In the section the seismic velocity estimated by Aloisi et al. (2002) is also shown.

the associated magmatic storage zones, although comparable with those recognized in the products emitted from the NSEC crater, show a different distribution. The most abundant olivine samples Fo70–73 (expressed in Fo%), attributable to the shallow portion of the plumbing system, decrease from 63% in the case of NSEC events (Giuffrida and Viccaro, 2017) to 37% in the case of the VOR events. Concurrently, the olivine populations associated with deeper zoning patterns increase significantly. Therefore, the NSEC events were predominantly supplied by the shallow magma storage with main cycles of near regular recharging /discharging of this source that fed the lava fountains. As anticipated, this accords with the previous source modeling obtained by the analysis of the strain changes (Bonaccorso et al., 2013). For the lava fountains of NSEC, several geochemical and geophysical studies (Aiuppa et al., 2010; Bonaccorso et al., 2011a,b; Calvari et al., 2011; Ganci et al., 2012; Corsaro et al., 2017) supported the interpretation that these events were violent releases of bubble-rich magma layers previously decoupled from the melt and trapped at shallow depths close to sea level. The observed behavior for the NSEC lava fountains is a cyclic process of charge/discharge. This interpretation supports the "foam collapse" model proposed by Jaupart and Vergniolle (1989), as derived from a combination of theoretical approaches and laboratory experiments. The strain data confirm the validity of this eruptive model indicating a depressurizing source located at about sea level that is incapable of accumulating larger magma volumes and triggers frequent events with a nearly similar amount of magma stored (Bonaccorso et al., 2013).

Instead, the VOR events were sustained by different storage zones of the magmatic plumbing system with a higher contribution from deeper zones, which implicate a more complex dynamics that is more difficult to be modeled with simple shaped sources. Therefore, we believe that VOR eruption needs further in-depth studies.

In a general framework, these zones of the plumbing system identified by different deformation sources perfectly agree with three of the magmatic environments found in Cannata et al. (2018), as inferred by petrological data (**Figure 8**). In particular, the Authors found a magmatic environment at 290–230 MPa (∼ 7-10 km b.s.l.), where the composition of the olivine Fo78 (expressed in Fo%) forms. This environment corresponds to the depth level that we found for the medium-term inflation phases. Another level at 160–120 MPa (about 3–5 km b.s.l.) referred to crystals with Fo75–76 cores is related to the depth found for the medium-term deflation phases and the periods of lava fountains. And, finally, a shallower level close to the sea level with Fo70– 73 is related to the NSEC activity. In **Figure 8** it is reported the scheme with the comparison of the position of the three magmatic environments inferred by petrological data and the geodetic sources inferred for the inflation and deflation phases (by GPS data) and for the NSEC lava fountain events (by high precision strain-meters).

It is noteworthy that the depth levels found for the intermediate storage, where the inflation/deflation processes are observed, and the shallow one are approximately coincident with the levels of two main geological discontinuities separating different main units in the Etnean crust (**Figure 8**). These levels are (i) the separation between the Hyblean carbonate platform and the upper flyschoid units of the Apennine-Maghrebide chain, found at about 6 km b.s.l. (Lentini, 1982); (ii) the discontinuity between the Apennine-Maghrebide chain and the volcanic cover, located at about sea level (Monaco et al., 2011). Moreover, a persistent level of magma ponding at the depth of the deeper discontinuity (∼ 6 km depth b.s.l.) is also supported by melt inclusion studied for previous eruptions (Métrich et al., 2004). Therefore, we believe that the depth of about 6 km (b.s.l.) represents a critical level for the magma ascending toward the surface. Along the western border of the HVB, where magma is believed to ascend, at this depth level, tomographic studies (e.g., Aloisi et al., 2002) estimated density values in the range 2,300–2,600 kg/m3 . At about 10 km (b.s.l.), the estimated values are about 2,700–2,900 kg/m3 . Therefore, in the depth range 5.5– 7.5 km (b.s.l.), where inflation processes occur, basaltic magma (with a density of ∼2,800 km/m3 ) should pond for negative buoyancy. Likely, the structural discontinuity between the Hyblean platform and Apennine — Maghrebide chain, facilitates magma ponding. In this intermediate storage, due to the almost continuous magma input from depth (Bonaccorso Calvari and Calvari, 2013), the magma can accumulate, producing the observed inflation phases. During ponding, differentiation processes are also observed (Cannata et al., 2018). When the value of the magma overpressure is enough to break the medium and to reach the surface, the apical portion of this intermediate storage (∼ 4.5 km depth b.s.l., **Figure 8**) ascends, producing the observed deflation phases.

This interpretation on the vertical shift of the inflation and deflation sources would mainly characterize the phases preceding (inflation) and accompanying (deflation) the eruptions with a more explosive behavior. This aspect is also confirmed by the results of previous studies. During the 2009–2010 inflation period, Aloisi et al. (2011) observed a progressive deepening of the deformation source over time, ascribed by the Authors to the role of the volatiles in preparing a more explosive eruption. Confirmation of this hypothesis can be found in Palano et al. (2017) where the Authors found that the 2002–03 flank eruption, characterized by a strong explosive activity, show a shift between the inflation and deflation sources. In particular, the Authors found that for the 2002–2003 eruption the deflation source was about 2 km shallower than the inflation source. Instead, for the less explosive 2004–2005 and 2006 flank eruptions, they observed the separation between inflation/deflation sources was not so marked. This separation between the centroids of the inflation/deflations sources is particularly clear in our results because shown for more cases of eruptive phases modeled by using a uniform approach.

## AUTHOR CONTRIBUTIONS

AB conceived the study objectives. AB, GC, and MA coordinated and shared the writing of the paper and the research it is based on, static GPS data modeling was performed by MA. High frequency GPS data treatment was taken care of by FC. All authors contributed to the discussion of the results.

## ACKNOWLEDGMENTS

We would like to thank the two Reviewers and the Editor in Chief V. Acocella for their constructive comments, which helped us to substantially improve the manuscript. We are particularly

## REFERENCES

indebted to the technical staff of Ground Deformation Group of INGV-OE who ensure the regular working of the permanent GPS and borehole strain-meter monitoring networks. We thank S. Conway for correcting and improving the English of the paper.

revealed by recent inflation/deflation cycles. J. Geophys. Res. 113:B05406. doi: 10.1029/2007JB005334

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Aloisi, Bonaccorso, Cannavò and Currenti. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Mapping Volcanic Deposits of the 2011–2015 Etna Eruptive Events Using Satellite Remote Sensing

Gaetana Ganci 1 \*, Annalisa Cappello1 , Giuseppe Bilotta1 , Alexis Herault 1,2, Vito Zago1,3 and Ciro Del Negro1

1 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Catania, Italy, 2 Conservatoire National des Arts et Métiers, Paris, France, 3 Dipartimento di Ingegneria Elettrica Elettronica e Informatica, Università di Catania, Catania, Italy

#### Edited by:

Clive Oppenheimer, University of Cambridge, United Kingdom

#### Reviewed by:

Diego Coppola, Università degli Studi di Torino, Italy Klemen Zaksek, Universität Hamburg, Germany Laura Becerril Carretero, Instituto de Ciencias de la Tierra Jaume Almera (ICTJA), Spain

> \*Correspondence: Gaetana Ganci<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Volcanology, a section of the journal Frontiers in Earth Science

Received: 14 March 2018 Accepted: 04 June 2018 Published: 21 June 2018

#### Citation:

Ganci G, Cappello A, Bilotta G, Herault A, Zago V and Del Negro C (2018) Mapping Volcanic Deposits of the 2011–2015 Etna Eruptive Events Using Satellite Remote Sensing. Front. Earth Sci. 6:83. doi: 10.3389/feart.2018.00083 Estimates of lava volumes provide important data on the lava flooding history and evolution of a volcano. For mapping volcanic deposits, including lava flows, the advancement of satellite remote sensing techniques offers a great potential. Here we characterize the eruptive events occurred at Mt Etna between January 2011 and December 2015 leading to the emplacement of numerous lava flows and to the formation of a new pyroclastic cone (NSEC) on the eastern flank of the South East Crater. The HOTSAT system is used to analyze remote sensing data acquired by the SEVIRI sensor in order to detect the thermal anomalies from active lava flows and calculate the associated radiative power. The time-series analysis of SEVIRI data provides an estimation of event magnitude and intensity of the effusive material erupted during each event. The cumulative volume estimated from SEVIRI images from 2011 to 2015 adds up to ∼106 millions of cubic meters of lava, with a time-averaged rate of ∼0.68 m3 s −1 . This estimate is independently supported and bounded using a topographic approach, i.e., by subtracting the last topography of Etna updated to 2005 from a 2015 digital elevation model (DEM), produced using tri-stereo Pléiades satellite images acquired on December 18, 2015. The total volume of products erupted from 2005 to 2015, calculated from topography difference by integration of the thickness distribution over the area covered, is about 287 × 106 m3 , of which ∼55 × 106 m3 is the volume of the NSEC cone. This 10-year volume is below the typical erupted volumes on decadal scale at Mt Etna, interrupting its stable and resilient output trend.

Keywords: Etna volcano, satellite remote sensing, SEVIRI, Pléiades imagery, digital elevation model

## INTRODUCTION

Mapping the volcanic deposits constitute a critical component for constraining magma supply, understanding magma plumbing system, and controlling lava flow morphological parameters, and represents an important indication of the amount of magma the volcanic system is capable of transporting during an eruptive event (Wadge et al., 1975; Stevens et al., 1999). In particular quantifying the emitted lava volumes gives insights into emplacement processes and may aid computer simulations of lava flow paths for hazard assessment studies, providing both input data for predictive lava flow modeling, and tools for iterative validation and/or refining of parameters (Del Negro et al., 2013; Cappello et al., 2016). However, the calculation of the amounts of lava erupted is still source of uncertainty, and problems occur when comparing volume values that are measured using different approaches (Harris et al., 2007).

Generally, volcanic products can be mapped following two different approaches. The simplest method is based on field measurements by multiplying the covered area, which can be determined with a good precision, by an estimated average thickness that remains the major source of uncertainty (Behncke et al., 2014). The second approach requires accurate mapping of the pre and post-eruption Digital Elevation Models (DEMs). This topographic approach is advantageous because deposit thickness is mapped throughout the flow field, resulting in detailed threedimensional measurements of the morphology and yielding a highly accurate volume estimate. The method is limited by the availability of topographic data of the surface underneath the volcanic deposit of interest, and such measurements are difficult, if not impossible, to be made retrospectively (Del Negro et al., 2016).

The recent availability of satellite images gives new opportunities for mapping volcanic products. Indeed high spatial resolution data acquired in stereo or tri-stereo configuration (e.g., Pléiades) can be used to frequently update the topography in volcanic areas and thus provides a new tool for measuring volcanic deposits by differencing successive topographies. In addition, high temporal resolution data provided by geostationary infrared remote sensing (e.g., SEVIRI) allow to estimate mean eruptive rates associated to the lava flow and to compute the related volumes.

Here we employed both Pléiades data to map the eruptive products and SEVIRI data to characterize the eruptive episodes occurred at Mt Etna between January 11th, 2011 and December 8th, 2015.

Mt Etna is one of the most active volcanoes in the world, showing a persistent activity at the summit craters and frequent effusive eruptions from eruptive vents located on the volcano flanks (Cappello et al., 2013). Today five craters are active on the summit of Mt Etna (Neri et al., 2017): the North-East Crater (NEC), the Voragine (VOR), the Bocca Nuova (BN), the South-East Crater (SEC), and the New South-East Crater (NSEC). NEC, VOR, and BN formed inside the old Central Crater in 1911, 1945 and 1968, respectively. SEC, which has been the most active summit crater over recent decades, was built up during the 1971 eruption. Finally, the NSEC has formed in 2007 as a pit crater at the South-east base of the SEC (Acocella et al., 2016), but rapidly grew during the lava fountains occurred since January 2011 (Vicari et al., 2011). After the last flank eruption in 2008– 2009 (Ganci et al., 2012b), the volcanic activity of Etna has shown a more explosive behavior, with an increasing number of paroxysmal events at the summit craters (Bonaccorso and Calvari, 2013; Cappello et al., 2013). Indeed, from 2011 to 2015, the SEC area has been the source of 53 eruptive episodes, most of them characterized by the emission of lava fountains, pyroclastic material, and short-lasting lava flows that mostly spread within the Valle del Bove (VdB).

In the following we estimated the effusion rate and hence the lava flow volume from SEVIRI data and computed some morphometric parameters (area, maximum length, thickness, and volume) from the three-dimensional distribution of the volcanic deposits obtained by DEM difference using a new 2015 digital topography of Mt Etna derived from Pléiades data. The volcanic deposits obtained from DEM difference provide an independent maximum bound to the SEVIRI-derived lava flow volume and possibly an estimate of loose materials.

## MATERIALS AND METHODS

## Thermal-Infrared Satellite Data Sources and Analyses