Workers check the damages at the church of San Bernardino, in L'Aquila, after the earthquake that hit the city in April 2009. Credit: Anatoli Zhdanov/UPI Photo/Alamy.
Seismologists debate whether it’s possible to recognize when small earthquakes are really foreshocks preceding a stronger event. Now a team from Italy and Germany has re-analysed the seismic activity that preceded the magnitude-6.1 earthquake that hit L’Aquila on 6 April 2009, killing more than 300 people, to check whether the amplitude and energy of small foreshocks showed different patterns from the normal background seismicity that occurs all year.
Their conclusion does not suggest that the April 2009 event could have been predicted, but it identifies a new parameter that could help study seismic sequences and identify potential that a large earthquake is imminent.
“Our biggest problem is that we will never be able to be on the source when an earthquake strikes”, says lead author, Matteo Picozzi, from the University of Naples. But in recent years, sophisticated networks have been developed that improve scientists’ ability to observe phenomena in the Earth's crust by accessing large volumes of data at the European level. “For the L’Aquila sequence we had the advantage of looking at a very big dataset, unlike previous studies, and we knew in advance what was the seismicity background and what were the foreshocks,” he says.
Using free data from RAMONES, an online platform that extracts, analyses and stores data on earthquakes in Central Italy, Picozzi and his colleagues studied1 thousands of small earthquakes (with magnitude from 1.8 to 3.5) that happened in the L’Aquila area between January 2005 and December 2009, both leading up to and immediately after the main shock. They focussed in particular on two parameters: the seismic moment, that is essentially a measure of an earthquake’s size, and the radiated energy, that measures the dynamic and friction properties of the rupture. They introduced a parameter of their own design, called Energy Index (EI), that measures the relationship between seismic moment and radiated energy, and how it changes over time. They found that for the week before the 6 April 2009 earthquake the measure of EI deviates significantly from the background seismicity, signalling the onset of the activation phase of the main shock.
The authors suggest that in the future EI may be combined with other parameters for a nearly real-time monitoring, to intercept the preparatory phases of earthquakes and improve hazard estimates.
“Several authors have investigated the spatial and temporal patterns of the 2009 L’Aquila earthquake in central Italy, but the study of Picozzi and colleagues has examined the seismological features of a well-defined foreshock sequence prior to the magnitude- 6.1 earthquake in a novel manner”, says Zoe Mildon, an expert on earthquake dynamics at the University of Plymouth. “For EI to be considered as a forecasting method for large earthquakes, it would be essential to repeat this study for earthquake swarms or clusters of microseismicity that do not lead to mainshocks, and for mainshocks that do not have such a clearly defined foreshock sequence”, she adds. “A better understanding of the possible underlying physical mechanisms of these seismological observations would help clarify the causal link between foreshocks and mainshocks.”
