Comment on the L'Aquila verdict by Dr Roger Musson

BGS has received a number of requests for interviews following the announcement that six Italian scientists and one government official have been charged with manslaughter following the L’Aquila earthquake in 2009 in Italy.

The geological community is concerned after yesterday's verdict, that outcome of the L' Aquila trial will inevitably compromise the relationship between scientists concerned with natural hazards, and the public and state.

The background to this is that in the spring of 2009, a series of small but highly perceptible earthquakes were being felt in the central Italian city of L' Aquila and the nearby villages. As there was much public concern about whether these earthquakes were building up  to something worse, a meeting was called at which six scientists and one government official from in the civil protection agency were summoned to give advice. The six scientists were people at the top of the engineering seismology community in Italy, people of the highest international standing.

There were two possibilities to deliberate. Firstly, it sometimes happens that a large earthquake is preceded by a series of smaller ones. The term for these events is foreshocks. Secondly, it sometimes happens that a series of intense small earthquakes occur and die away again without any large event. Seismologists call this a swarm. There is no measurable property that distinguishes a foreshock from a common small earthquake. The panel noted two things, based on historical experience. Firstly, swarms are not uncommon in Italy. Second, large earthquakes in Italy usually do not have foreshocks. Hence, on the balance of probabilities, the L' Aquila events were more likely to be a swarm and would die away without any large event.

They were careful to add that L' Aquila is in a well-known area of high seismic hazard, and it was always possible that a strong earthquake could strike at any moment.

The offical from the Dept of Civil Protection, who was not a seismologist, gave a TV interview, in which he stated that the small earthquakes were acting as a safety valve by releasing energy; consequently a large earthquake was now unlikely; therefore residents could relax and have a nice glass of wine.

There is no basis for such a statement, and this is why: the energy involved in a 6.3 magnitude earthquake like the one that struck L' Aquila a few days after the scientists met,  is about 50,000 times more than any of the small earthquakes. Therefore the sequence that had been felt up to that date had barely skimmed off the slightest fraction of the energy available.

The statements made in the TV interview were taken by many inhabitants of L’ Aquila as a sign that they had nothing more to fear, an assumption that proved incorrect when the earthquake struck a few days later.

The verdict of this trial is not about the science of predicting earthquakes, but about the communication of risk to the general public. 

Roger Musson