Why disasters like this cannot be foretold
Tuesday 11 October 2005
Earthquake prediction is still in its infancy and few scientists expect that it will be possible in the foreseeable future to give the sort of warnings that would have saved lives in the disaster affecting Pakistan and India.
Scientists estimate earthquake probabilities by studying the history of large earthquakes in a specific area and the rate at which strain accumulates in the rock. But such probabilities are next to useless in giving the sort of meaningful warnings to residents about precisely when and where an earthquake will occur.
They are also notoriously bad at assessing the probability of earthquakes in a region with hidden underground faults or the many regions that are not adequately monitored by seismic instruments. The best that scientists can do at present is to assign a probability based on past events.
So if four earthquakes greater than magnitude seven occurred in a region during 200 years of recorded history, and if these shocks occurred randomly, then the probability of another occurring in the next 50 years is put at 50 per cent. Even this vague idea of probability is based on the assumption of random shocks in fact when seismic strain is released in one part of a geological fault to cause an earthquake, it can non-randomly increase the strain elsewhere in the zone.
Chinese seismologists claimed to have predicted an earthquake on 4 February 1975 in Haichen when an evacuation warning was given the day before. The scientists said that in the preceding months they monitored changes to land elevation and ground water, as well as seeing peculiar animal behaviour. Other seismologists are sceptical of the claims, which were made during the Cultural Revolution, and whatever measures were taken by the Chinese they were not enough to warn people of the 1976 earthquake in Tangshan which killed 250,000 people.
Brian Baptie, head of UK seismic monitoring at the British Geological Survey in Edinburgh, said: "We are a long way from predicting exactly where and when an earthquake will strike. But we do know where they occur and which faults are active The best way of avoiding deaths is to spend money on building earthquake-proof buildings."
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