Of the many devastating pictures to come out of central Italy after last week’s deadly earthquake, the clock tower of Amatrice standing defiantly amid the rubble of the town has become an iconic image.




Unlikely survivors

Nepal’s Dharawara tower in 2013, before it was destroyed in the 2015 earthquake. KATHMANDU NEPAL FEB 2013, CC BY-SA


Similar towers are relatively commonplace in Italy and part of the country’s charm. The town of San Gimignano, about 200km from the centre of the Amatrice earthquake, has 14 towers that date as far back as the 12th century – and have consequently survived many earthquakes big and small. Other towers can be seen in Alba in northern Italy.

Further afield, a memorable image of the Izmit earthquake in Turkey in 1999 was of the tower of the Golcuk Mosque standing forlornly among the ruins.

Photos from the 1906 San Francisco earthquake show a slender tower and an array of chimneys standing in the rubble of the city.

In many instances, however, the towers fall, as happened to the Dharahara tower during the magnitude-7.8 Nepal earthquake in April 2015.

Why do some of these slender icons survive repeated earthquakes and others fall? An article in The Economist suggested that the clock tower was better constructed than the surrounding buildings, pointing out that it even survived better than a modern school and hospital. The L’Aquila experience suggests that this is probably one part of the story.


The Dharawara tower collapsed during the 2015 earthquake in Nepal. It is being rebuilt. EPA/NARENDRA SHRESTHA



On shaky ground

It is very likely that the clock tower’s survival was influenced by the relationship between the frequency of the earthquake waves and the natural resonance of the building. To understand why, we have to consider how earthquakes interact with buildings.

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The same is true of buildings with different heights. A building is effectively an upside-down pendulum and taller buildings have longer natural periods of oscillation (swinging back and forth).



Low-frequency (long period) earthquake waves are amplified in sediment and form a greater part of the seismic energy radiated by larger earthquakes, such as the Tohuku earthquake in Japan and the Nepal quake that felled the Dharahara tower.




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The concept is beautifully demonstrated in a video by Robert Butler of the University of Oregon.


A resonant problem

Of course, real structures and real earthquakes are far more complex. Real structures have many natural frequencies, and earthquakes vibrate across a spread (or spectrum) of frequencies.



This process is equally important in other regions. The magnitude-6.8 Myanmar earthquake on August 24 damaged many historic temples in the Irrawaddy Valley, but none appears to have collapsed. These high-but-squat structures are susceptible to high-frequency shaking, whereas the passage of earthquake waves through alluvium is likely to have amplified mainly low-frequency earthquake waves.

Many of Myanmar’s iconic temples were damaged during the recent quake, but not destroyed. EPA/HEIN HTET


Notably, much of the damage to the temples seems to have occurred as a result of the collapse of recent cheap “restorations”.

Building practices are extremely important in mitigating the effect of shaking on buildings. Modern buildings are commonly fitted with devices to reduce the effects of resonance. Engineered solutions are available to retrospectively enhance the performance of unreinforced masonry buildings, with little impact on their aesthetics.




This feature originally appeared in The Conversation.





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