Monday, April 27, 2015

About prediction of earthquakes.

Several years ago soon after the 18Sep2011 (6.9R) Sikkim earthquake, I received a rather unusual request from an organization in Siliguri - they wanted me to come there from Kalimpong to assure a section of the public on TV that another big quake would not take place in the near future. Because many people, undoubtedly suffering from some form of PTSD were unable to sleep especially in high rise apartments. Needless to say I declined the offer.
A similar situation is again persisting today, in much of North Bengal and many parts of Bihar. In Darjeeling district schools are closed for the next 2 days in anticipation or more correctly fanned by rumors that major aftershocks are expected within the next 48hrs. I have also been dogged by local media asking whether this is possible.
Having been involved as a lay person in Disaster Management, all I can say is, unlike many other disaster forms (landslides, droughts, floods, cyclones, and so on), earthquakes are notoriously unpredictable and this has been substantiated by two senior scientist friends of mine who have spent almost their entire lives studying geo-hazards.
Placed below is an extract from 'Fundamentals of Geology' by Vladimir Obruchev about the predictability of earthquakes :-

An earthquake prediction implies that an earthquake in a specific magnitude range will occur in a specific region and time window. Predictions are considered as such to the extent that they are reliable for practical, as well as scientific, purposes. Although there is evidence that at least some earthquakes in some tectonic regimes are predictable with useful accuracy of time and space, the reliability and reproducibility of prediction techniques have not been established beyond the level of conjecture. We are still far away from predicting an earthquake with any amount of accuracy. Besides, any attempt to predict an earthquake precisely means deciding well in advance when and where it will occur. It will also be necessary to suggest about the magnitude of the shock and what could be the likely damage. An element of conjecture is always associated with any prediction.
It may be easier to predict where a major earthquake is likely to hit rather than the time when it will occur. Most seismologists do not believe that a system to provide timely warnings for individual earthquakes has yet been developed, and many believe that such a system would be unlikely to give significant warning of impending seismic events. More general forecasts, however, are routinely used to establish seismic hazard. Such forecasts estimate the probability of an earthquake of a particular size affecting a particular location within a particular time span.
In an effort to predict earthquakes, people have tried to associate an impending earthquake with such varied phenomenon as seismicity patterns, electromagnetic fields, weather conditions and unusual clouds, radon or hydrogen gas content of soil or ground water, water level in wells, animal behaviour. Thus far, earthquake prediction is controversial because data are sparse and there is little evidence or verified physical theory to link observable phenomena to subsequent seismicity. The frequent practice of polishing predictions after the fact further complicates the matter. Most assessments rely on chance models for earthquake occurrence, models that are difficult to test or validate, because large earthquakes are so rare, and because earthquake activity is naturally clustered in space and time. The best advice that has been provided by many seismologists is to consider the earthquakes as natural phenomena, and the people living in zones of high seismicity should learn to live with these unfriendly events, as we do in case of a violent storms and cyclones.

My thanks to
Prof Ananda Chakrabarti (retd),
Associate Professor,
Presidency Univ.

for his valuable inputs.

Praful Rao,
Dist Darjeeling.

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