posted on 25 August 2016
from The Conversation
The Appenines region of central Italy has been struck by a deadly earthquake, with a magnitude of 6.2. The quake, which had an epicentre roughly 10km southeast of Norcia, Italy, occurred just over seven years after the 2009 L'Aquila earthquake that killed more than 300 people only 90km away.
The latest earthquake occurred at 3:36 am local time. The number of fatalities is unknown at time of writing but already exceeds 100. Buildings have collapsed in nearby Amatrice and residents are reportedly trapped in rubble.
General tectonic setting of Italy, showing seismicity over the past 10 years from the USGS catalogue. USGS
This earthquake is no surprise. Italy is prone to earthquakes; it sits above the boundary of the African and European plates. The oceanic crust of the African plate is subducting (sinking) under Italy, creating iconic natural features such as the volcano at Mount Vesuvius. These plates are converging at a rate of around 5mm each year.
Both the L'Aquila and Norcia earthquakes were located below the central Appenines, which form the mountainous spine of Italy.
The Earth's crust under the Appenines of central and western Italy is extending; eastern central Italy is moving to the north east relative to Rome. As a result, this region experiences normal faulting: where one part of the earth subsides relative to another as the crust is stretched.
The fault systems in the central Appenines are short and structurally complex, so the earthquakes are not large by global standards, the largest almost invariably hover around magnitude 6.8 to 7.0. But because the quakes are shallow and structurally complex, and because many of the local towns and cities contain vulnerable buildings, strong shaking from these earthquakes has the potential to inflict major damage and loss of life in urban areas.
This region also seems to be particularly prone to earthquake clustering, whereby periods of relative quiet are interrupted by several strong earthquakes over weeks to decades.
A history of quakes
Both Norcia and L'Aquila feature prominently at either end of a zone of large Appenine earthquakes. This zone has produced many strong earthquakes. The latest Norcia earthquake occurred only around 90km northwest of the L'Aquila earthquake and very close to the epicentre of the 1979 Norcia earthquake, which had a magnitude of 5.9.
But the area's earthquake history can be traced back over seven centuries. During this period, this region has been hit by at least six earthquakes that have caused very strong to severe shaking. Amatrice, so badly damaged in the most recent quake, was severely damaged in 1639. A few decades later, in 1703, roughly 10,000 people were killed in Norcia, Montereale, L'Aquila and the surrounding Appenine region in three magnitude 6.2-6.7 earthquakes.
Parts of Norcia were subsequently built upon the surface rupture created in the 1703 earthquake. Another earthquake in 1997 killed 11 people.
In this most recent event, an estimated 13,000 people would have experienced severe ground shaking, probably lasting 10-20 seconds.
The estimated damage of this latest earthquake will almost inevitably exceed US$100 million, and may top US$1 billion. Amatrice appears to be among the populated areas that were most severely affected.
What lies ahead?
The region now faces a prolonged and energetic aftershock sequence; over the first 2.5 hours following the mainshock, at least four earthquakes of around magnitude 4.5 were recorded in the region by the US Geological Survey. More than 10,000 aftershocks were recorded following the L'Aquila earthquake in 2009.
We note that within the region, there is excellent and continuously improving scientific information about the hazard. But the knowledge of the hazard has not always translated well into measures that directly reduce economic loss and fatalities in earthquakes.
Following the L'Aquila earthquake, six scientists were convicted of manslaughter for failing to inform the public adequately of the earthquake risk. Although the charges were subsequently dropped, this marked a major development in the way blame is apportioned after large natural events, particularly with regard to effective hazard communication.
Numerous vulnerable buildings remain, and the recovery process is commonly plagued by long disruptions and inadequate government funding to recover rapidly. Both the 2009 L'Aquila earthquake and this most recent quake highlight just how important it is to translate hazard assessments into improving the resilience of infrastructure to strong shaking. The focus should remain on linking science, engineering and policy, this is often the biggest challenge globally.
Brendan Duffy, Lecturer in Applied Geoscience, University of Melbourne; Mark Quigley, Associate professor, University of Melbourne, and Mike Sandiford, Chair of Geology & Redmond Barry Distinguished Professor, University of Melbourne
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