DONET in Operation
DONET is a submarine cabled real-time seafloor observation infrastructure which was designed to realize precise earthquakes and tsunamis monitoring on seafloor in the long period of time. The density of observatories is comparable to the earthquake observatory network on land and the tsunami monitoring is an unprecedented capability. The main purpose of DONET is to monitor the hypocentral region of Tonankai earthquake that is predicted to occur with a probability of more than 70% within the next 30 years according to the report published by the Earthquake Research Committee. DONET consists of an approximately 300km length of backbone cable system, 5 science nodes, and 20 observatories. Its installation on 20 stations at Kumanonada started in 2006 and has been completed in July 2011. In August 2011, the seismic data has started to be provided to the Japan Meteorological Agency and the National Research Institute for Earth Science and Disaster Prevention, where the data will be used for the earthquake early warning.
As shown in the figure below, DONET recorded distinct tsunami signals of the 2011 Tohoku earthquake with pressure gauges. The maximum amplitude at the stations is about 0.2-0.3 m in the period band between 100 to 10000 sec. These signals were found 15 min before the arrivals at the nearest site, Owase city, Mie prefecture. This shows that the offshore DONET data are very useful to quickly estimate water heights in near shore areas for disaster mitigation. The pressure gauge data are also useful for analyzing micro-tsunami, geodetic deformation, tide, water temperature, and related ocean phenomena.
In addition, the hypocenters of earthquakes that occurred around the Nankai trough have been determined by using the data obtained from DONET. Intensive seismic activity off the Kii Peninsula was found. The earthquakes are mainly distributed in three clusters, of which locations well overlap with the aftershock distribution of the 2004 off the Kii Peninsula earthquakes (M_JMA=6.9 and 7.4). Thus, it is considered that most earthquakes in the present activity are aftershocks of the 2004 earthquakes.