The verdict in the L’ Aquila case

The seismological community is still in a state of shock today after yesterday’s verdict of manslaughter against six scientists in Italy. In understanding this case, it’s important to be aware of the full facts.

In the spring of 2009, a series of small but highly perceptible earthquakes was being felt in the central Italian city of L’ Aquila and the nearby villages. As there was much public concern about whether these were leading to something worse, a meeting was called at which six scientists and one government official (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 occurs and dies away again without any large event. Seismologists call this a swarm, and swarms are actually common in the UK, the last prominent one being in Manchester in 2002. So were the L’ Aquila earthquakes foreshocks or a swarm?

The only way to try and answer this is to look at historical precedents, since there is no measurable property that distinguishes a foreshock from a common small earthquake. The panel noted two things. Firstly, swarms are not uncommon in Italy. This is a fact. Second, large earthquakes in Italy usually do not have foreshocks. This is also a fact. Hence, on the balance of probabilities, the L’ Aquila events were more likely to be a swarm. I can’t think that any seismologist would disagree with this.

But – they were careful to add – 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 problems started straight after the meeting. The civil defence official, who was not a seismologist, gave a TV interview, in which he said that the small earthquakes were acting as a safety valve by releasing energy; therefore a large earthquake was now unlikely; therefore residents could relax and have a nice glass of wine.

This was wrong. 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 later, 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 tiny fraction of the energy available.

It was this falsely reassuring statement that formed the basis of much of the prosecution’s case. But why then prosecute the seismologists, who gave advice according to the best science as they saw it?

In dealing with natural hazards, one seldom deals with certainties. I remember some years ago, being asked by a journalist whether an earthquake that had just occurred in central Russia would cause a tsunami. I was able to reply, with complete confidence, that as a tsunami requires water, and the earthquake was about as far from the sea as it is possible to get anywhere on Earth, that there would certainly be no tsunami.

But that is a rare case. Usually one is dealing with probabilities. Imagine a game where you must roll two dice, and you win if you roll anything other than two sixes. If you ask my opinion, I will say that the game is heavily in your favour: there is a 35 in 36 chance that you will win. You roll the dice and two sixes come up. Was my advice wrong? Of course not.

The fear of many scientists, and not just seismologists, is that the verdict in the L’ Aquila trial will inevitably compromise the relationship between scientists concerned with natural hazards, and the public and state. What would happen in Britain the next time a swarm of earthquakes causes alarm? Do I advise people that for a swarm in Britain to develop into a magnitude 6 earthquake would be unprecedented and extremely unlikely? Or should I keep silent in case the extremely unlikely event occurs and I end up in a law court?

I would like to think it couldn’t happen in Britain. But then I would not have imagined it could have happened in Italy either.

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