Thursday, August 25, 2016

"Ask a statistician: Why can we not predict earthquakes?"

To explain the first half of the headline here's Statistics Views last October:
The latest issue of Significance magazine introduces a new feature “Ask a Statistician”, in which readers have their burning statistical questions answered by an expert.,,,
I know, I was all atingle too.

From Significance Magazine (a member benefit for joining the American Statistical Association or the Royal Statistical Society):
Hundreds of people have been killed since the start of the year as a result of earthquakes - including those who died this week following a 6.2-magnitude quake in Italy. With all the data we have on these natural disasters, why can we not reliably predict their occurrence?

An earthquake-damaged monument in Kathmandu, Nepal. Photo credit: US Geological Survey
Robert Matthews of Aston University writes: It’s certainly not because of lack of data or effort: attempts to predict earthquakes go back centuries. The problem lies in extracting from this data the tell-tale signs – “precursors” – that reliably foretell when, where and how strong an earthquake will be.

Earthquakes are triggered when two slabs of rock meeting at a fault-line can no longer resist the forces acting on them, and slip.

Much effort has been devoted to identifying symptoms of the conditions likely to trigger earthquakes. Over the centuries everything from changes in groundwater to seepages of naturally occurring radioactive gas have been put forward as potential precursors. To date, only one has been found to be reliable: the quake itself. The rupturing rock emits so-called primary waves (P-waves), which travel faster than the more destructive secondary waves (S-waves). The emergence of P-waves thus foretells of impending disaster – albeit only by seconds. Even so, this has long been used to protect Japan’s bullet train, by cutting power and reducing the risk of derailment.

No reliable longer-term precursor has ever been found. That does not rule out such a possibility. However, the theory underpinning any putative prediction method does not give grounds for optimism.

In essence, prediction of an earthquake is like a medical diagnosis: it should give insights reliable enough to take appropriate action. Like a medical test, it may occasionally “cry wolf” or miss genuine events, so minimising false positive and false negative rates is clearly important if people are to trust the prediction enough to, say, order an evacuation. But the very nature of earthquakes makes it unlikely that any prediction system will be reliable enough to overcome the so-called base rate problem....MORE
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