Published online 10 September 2009 | Nature | doi:10.1038/news.2009.899

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Italian quake analysis rumbles in

Satellite data pinpoints fault at heart of L'Aquila earthquake.

Radar images of earthquakeRadar images mark the faultRichard Walters/GRL

The earthquake that rocked the ancient city of L'Aquila, Italy, less than six months ago was caused by a fault not thought to be a major seismic hazard.

The Gran Sasso region near L'Aquila is criss-crossed with large, looming faults running through the mountainous terrain that their activity has created. But the first published analyses of the quake, which struck on 6 April and killed 307 people, suggest that the culprit was the Paganica fault, an undistinguished fracture in comparatively flat ground.

"It shows it is dangerous to work on the assumption that the faults associated with the largest topographic features are going to produce the largest events," says Richard Walters, who studies tectonics at the University of Oxford, UK.

Walters and his colleagues analysed radar images from the European Space Agency's Envisat satellite, taken before and after the earthquake. They used differences between the phases of radar waves reaching the Earth's surface in the two sets of data to spot changes in the area's topography.

Plotting interferograms, which show these changes as contours, and comparing them with models based on seismological data for the region, Walters's team saw a 25-centimetre dip on one side of the Paganica fault and a small rise on the other. The work is published in Geophysical Research Letters1.

The Paganica fault also stood out from Envisat radar data in a separate study by Simone Atzori from the National Institute of Geophysics and Volcanology (INGV) in Rome and his colleagues. This first assessment of the earthquake was also published recently in Geophysical Research Letters2.

The fault had drawn attention before the earthquake, says Laura Peruzza from the National Institute of Oceanography and Experimental Geophysics in Sgonico, Italy, who was not involved with the studies. She says that in an analysis she and her colleagues performed in 2006, Paganica appeared as the most dangerous fault in the region, and the L'Aquila region "jumped out of the map"3.

But, she adds, her study's methodology — which took into account the variable timing of recent geological disruptions, as well as their placement — is not accepted by regulatory authorities worldwide. The standard model assumes a constant rate of earthquakes.

Remaining strain

Walters's colleagues had previously pinpointed two particularly stressed regions in Italy, a land which has been stretching because of seismic activity for around three million years. One, he says, was the L'Aquila area, which is still in danger despite the pressure build-up relieved by the magnitude-6.3 April quake: "What has been released [by the L'Aquila earthquake] only makes up around 15% of the energy deficit."

Dramatic events such as large earthquakes are not the only way of releasing pressure, however; landslips or a long-lasting series of tremors called seismic swarms can let it out gently. And Atzori warns against reading too much into Walters's conclusions about the stress remaining in the region. "The calculation is made on the past few decades, but stress accumulates over many centuries," he says. The assessment of how much pressure has been released cannot be used to make predictions about when and where an earthquake might happen, he adds.

Another group, colleagues of Atzori from INGV, spotted a seismic swarm at the end of March this year and placed five extra GPS receptors in the region. These have provided excellent GPS data for the time of the earthquake, which have also been published4. "We were from a certain point of view lucky, and from another point of view clever," says Atzori.

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That seismic swarm might have been the precursor to the earthquake, so Atzori and colleagues are now closely monitoring another possible seismic swarm close to a nearby reservoir, Lake Campotosto. "We cannot say it will end in an earthquake," he says.

Analyses in the aftermath of the L'Aquila quake are a reminder that attention on the region should remain high, says Peruzza, but cannot be used to predict quakes in future. "It is impossible to predict exactly where the next fault will be active," she says. "The only thing to do is to live in safe buildings. Prediction is not the key to surviving." 

  • References

    1. Walters, R. J. et al. Geophys. Res. Lett. 36, L17312 (2009). | Article
    2. Atsori, S. et al. Geophys. Res. Lett. 36, L15305 (2009). | Article
    3. Pace, B., Peruzza, L., Lavecchia, G. & Boncio, P. Bull. Seismol. Soc. Am. 96, 107–132 (2006). | Article
    4. Anzidei, M. et al. Geophys. Res. Lett. 36, L17307 (2009). | Article
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