EXPERT REACTION: Magnitude 6.2 'quake hits central Italy

A magnitude 6.2 earthquake at a shallow depth of 10km in south east of Norcia (in southeastern Umbria) in central Italy occurred at 1:36 (UTC) on 24 Aug 2016. This earthquake has happened as a result of a normal fault in Central Apennines mountain, which is one of the most complex tectonic areas in the world due to collision of Eurasia and Africa tectonic plates. It is estimated that these tectonic plates move 20 to 30mm per year toward each other.  In April 2009, L’Aquila Earthquake with the magnitude of Mw 6.3 in the region killed more than 250 people. Let’s not forget that in the recent history, 1915 Avezzano earthquake with magnitude of 6.7 in central Italy killed more than 30,000 people.

Vast areas of central Italy particularly in Umbria region comprise of shallow bedrock varying between 10 to 20m below the ground surface. Although limestone can be found as the bedrock material, Marl (weak sedimentary rock) is the most common types of bedrock in Umbria, while overlying layers are mainly silty clay soils, which are occasionally soft deposits. Areas with heavy construction on Marl in Umbria region should be checked now since there is a high chance of sliding and softening of foundation after this earthquake.

The earthquake affected areas are hilly with several open valleys and large intra-mountain basins. Landslides have induced significant amount of damage in Umbria, to people, private and public buildings, and road and railway network. Occurrence of earthquake induced landslides for the Perugia Municipality, which is the second largest in the region, is very likely and transport network and houses near slopes and hilly areas should be inspected as a matter of priority. 

The moment magnitude 6.2 earthquake occurred at 3:36 am local time with an epicentre approximately 10km SE of Norcia, Italy. The size of the fault that ruptured is probably around 10 x 10 km. The maximum amount of slip on the fault will probably be about 1 meter. A surface rupture or other form of surface deformation is possible. Strong aftershocks will affect this region over the coming days to months. The largest aftershock from the 2009 L’Aquila earthquake was a magnitude 5.5. In some cases these aftershocks can cause more damage than the preceding larger earthquake. For every magnitude 5 aftershock, 10 magnitude 4s are expected. There will be thousands of small aftershocks throughout the region from this event, but most will go unfelt.

This earthquake is no surprise. This is one of the most seismically active parts of Italy as clearly identified in many seismic hazard maps. During the past seven centuries, Norcia has been hit by six earthquakes that have caused very strong to severe shaking. Roughly 10,000 people were killed in Norica, Montereale, L’Aquila and the encompassing Appenine region in three magnitude 6.2-6.7 earthquakes in 1703. Parts of Norcia were subsequently built upon the surface rupture created in the 1703 earthquake. Another earthquake in 1997 caused 11 fatalities. In this most recent event, an estimated 13,000 people would have experienced severe ground shaking that would have persisted for probably 10-20 seconds. The estimated damage will almost inevitably exceed $100M USD, and may exceed $1B.

This part of Italy is experiencing crustal extension; eastern central Italy is moving to the NE relative to Rome. As a result, this region experiences normal faulting earthquakes as the land is torn apart. The fault systems are short and structurally complex, so the earthquakes are not overly large by global standards (almost always < magnitude 6.8 to 7). But because the earthquakes 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. Furthermore, this region seems to be particularly prone to earthquake clustering, whereby periods of relative quiescence are interrupted by several strong earthquakes over timescales lasting from weeks to decades.

My experience in the region is that there is excellent and continuously improving scientific information about the hazard here. But the knowledge of the hazard has not always translated well into measures that directly reduce future economic loss and fatalities in earthquakes. Numerous vulnerable buildings remain, and the recovery process is commonly plagued by long disruptions and inadequate government funding to recover rapidly. For example, when I visited L’Aquila in 2013, large parts of the city were still cordoned off, numerous buildings had not been repaired or demolished, and recovery had stagnated.

Both the 2009 L’Aquila earthquake, and this most recent earthquake, highlight just how important it is to translate hazard assessments into improving the resilience of infrastructure to strong shaking. The focus should remain on the science-engineering-policy interface; this is often the most challenging linkage to improve upon globally.

The magnitude 6.2  Norcia earthquake occurred in the Central Apennines, a mountainous area of high earthquake hazard in central Italy. The focus of the earthquake was only 10 km deep and this would have contributed to strong shaking at the surface. At least six magnitude 4 or greater aftershocks were recorded in the first few hours; aftershocks are likely to continue for several weeks. The earthquake happened about 50 km away from the location of the 1997 Umbria-Marche seismic sequence which included eight magnitude 5 or larger earthquakes, one of which badly damaged the Basilica of St Francis in Assisi.

This earthquake occurred in an area rated a high earthquake hazard region of Italy. Buildings should be designed and built to withstand this level of shaking without collapse. That they don't is typical of the attitude to the hazard in Italy and Australia where the risk of being killed in a vehicle accident is much higher.

The following information about how this earthquake relates to previous earthquakes is taken directly from the USGS website:

http://earthquake.usgs.gov/earthquakes/eventpage/us10006g7d#executive

The central Apennine region has experienced several significant earthquakes in recorded history. In September 1997, a Mw 6.0 earthquake 50 km north-northwest of the August 24, 2016 event killed 11, injured over 100 and destroyed approximately 80,000 homes in the Marche and Umbria regions. This 1997 event was part of a series of earthquakes known as the Umbria-Marche seismic sequence, which included eight events of magnitude greater than M5.0 in a two-month period between September and November of that year, including the events that substantially damaged the Basilica of St Francis in Assisi. In April 2009, a Mw 6.3 earthquake 45 km to the south-southeast of the August 24, 2016 event, near the town of L’Aquila, killed at least 295, injured over 1,000 and left 55,000 or more homeless. The L’Aquila earthquake resulted in significant landsliding in the local area, and was also followed by a vigorous aftershock sequence, including 5 other events of M 5.0 or larger.  The preliminary location of the 2016 earthquake appears to be in a gap between the aftershock sequences of the 1997 and 2009 events. The largest instrumentally recorded earthquake within 100 km of the 2016 event was the January 13, 1915 M6.7 earthquake, which occurred 68 km to the south-southwest near Avezzano. The 1915 earthquake killed approximately 32,000 people.