Predicting L'Aquila Earthquake: is it right to blame the scientists?
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Hello, and welcome to More or Less on the BBC World Service, your weekly ration of numerical nutrition.
I'm Jan Wong. This week we'll be reporting on a rogue Indian statistic which has fuelled a full-scale political row.
While in his address to the Punjab University, Rahul said seven out of ten youth in Punjab were drug addicts.
But first, this week six Italian scientists and one ex-government official were sentenced to jail for manslaughter. Why? Because they made overly reassuring statements before this happened.
A big earthquake in Italy. There's widespread damage to a medieval town where many are dead.
The earthquake which hit the Italian town of L'Aquila in March 2009 killed 309 people. Perhaps we shouldn't be surprised that amid the grief and anger which followed, some in Italy wanted to apportion blame. But is it right to blame the scientists? Well, at its heart, this is a story about numbers and probability. So let's start with an obvious question. Is it actually possible to predict earthquakes? The short answer is no. That's Ian Mayne, Professor of Seismology and Rock Physics from the University of Edinburgh, Scotland, and also a member of a committee on earthquake forecasting following the tragedy in Lachida.
In another sense, we can forecast that they are almost certain to happen with a very low probability. maybe once in 100, once in 1,000 years, depending on the size of the earthquake in a given zone. And that's a hazard that's there all of the time. A fault can go at any time, but we know it's going to happen.
And in the case of this earthquake in Italy, it was preceded by a whole series of smaller tremors. I assume that that means you can change the estimate of probability that a major earthquake will happen after that. It changes the equation somewhat.
Yeah. Yes. One of the things that's fairly consistent about earthquake population dynamics is that they often occur in clusters. And we can understand the physics of that in terms of stress redistribution caused by small earthquakes or bigger ones, and the fact that that then makes earthquakes more likely to occur. So how much more likely is an earthquake to occur if there's been an earthquake swarm, as they're called, beforehand? So it can be significantly above the background rate. So instead of once in 100 years, it might be once in a few months or something like that. So the probability could be on the order of up to, say, 1% that a small earthquake or series of small earthquakes might trigger a bigger one that would cause significant damage.
So the probability of an earthquake happening after a series of these events could go up by more than 100-fold, but it still goes up to a very small amount, 1% or so.
Yes, it's a significant probability gain...
What happened in the L'Aquila earthquake case and why were scientists sentenced?
but it's still low absolute probability. There's on the order of or less than a 1 in 100 chance of a swarm ending in a larger earthquake or a smaller earthquake triggering a bigger one. It's very rare, but it does occur.
So 99 times out of 100, a series of tremors won't mean there's about to be a larger earthquake. But this time, there was. Now, in the past in L'Aquila, after numerous tremors, some residents would leave their houses and sleep in their cars. However, on this occasion, comments by the authorities prompted some of those people to change their usual response and stay in their homes. This case isn't about the scientists' ability to predict earthquakes. It's about their statements communicating the risk of an earthquake. The Italian seismologists thought an earthquake was unlikely, but in a press conference the message seemed to be that that meant there was nothing to worry about at all. This is the falsely reassuring statement which formed part of the case against them.
The scientists do have the right to appeal, but their jail sentences have been viewed with surprise and in some cases outrage by statisticians and scientists worldwide, like David Spiegelhalter, Professor of the Public Understanding of Risk at Cambridge University in the UK.
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