L’Aquila earthquake, April 2009

The L’Aquila Earthquake, Italy

Research published in GRL: Walters et al. 2009“The 2009 L’Aquila Earthquake (Central Italy): an InSAR source mechanism and implications for seismic hazard”

Introduction and Location

L'Aquila fig 1
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Figure 1: Map of Italy centered around the L’Aquila region

COMET researchers have used satellite data to pinpoint the fault responsible for the recent L’Aquila earthquake, and have used their results to evaluate seismic hazard in the region.

On the 6th April 2009 a magnitude 6.3 earthquake struck L’Aquila, the capital city of the Abruzzo region, central Italy (figure 1). The earthquake killed around 300 people and made tens of thousands of people homeless. L’Aquila is a mediaeval city, nestled in the Apennines, and it has a long history of earthquakes that have repeatedly damaged the city.

Earthquake and Tectonic Setting

L'Aquila fig 2
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Figure 2: Google Earth view of the L’Aquila region with main faults marked

For the last ~3 million years, tectonic processes have been stretching peninsular Italy in a NE-SW direction, currently at a rate of a few millimeters each year. This stretching or extension is expressed as normal faulting earthquakes (the blue focal mechanisms in figure 1) generally located in the areas of high ground; the Apennines. The L’Aquila earthquake is typical of these earthquakes. The earthquake was initially thought to have occurred on one of the well known faults that bound large mountains in the region, for example the L’Aquila fault (figure 2) that bounds the Gran Sasso range; the highest mountains in peninsular Italy. However, COMET scientists, working in collaboration with researchers at UCL, University of Leeds and INGV, Italy, quickly realised that the responsible fault was actually the Paganica fault, a fault that does not bound a large mountain range.

L'Aquila fig 3
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Figure 3: Earthquake interferogram reveals surface displacement

Intereferometry and Modelling

The scientists determined the exact location of the earthquake fault by using radar data from the European Space Agency’s environmental satellite ENVISAT. They used this data to produce interferograms of the area (e.g. figure 3) which show how the ground surface has been warped by the earthquake. Each of the multicoloured interference fringes in an interferogram represents a contour of ground motion towards or away from the satellite. Figure 3 shows that the area SW of the fault moved away from the satellite by around 25 cm, and the area NE of the fault moved towards the satellite by around 8 cm. The COMET+ scientists then modelled these interferograms to find the precise location and geometry of the fault. Their models show that the fault was roughly 12 km long, dipped to the SW and slipped by around 0.6-0.8 m.

L'Aquila fig 4
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Figure 4: Stress change on faults around L’Aquila

Fault Stress Changes

The result of the earthquake is an overall reduction in the stress within the Earth’s crust that has built up over time due to the extension of the Italian Apennines. It is possible to measure the change in stress on the other faults surrounding Paganica to examine which ones have been brought closer or further away from failure. The COMET scientists used software developed at the USGS to measure this, and found that a significant increase in stress is applied to similar faults to the north-west (see figure 4). This has important implications for seismic hazard in the region as the faults bound a large reservoir, “Lago di Campotosto”.