Overexploitation of ground water is no cause

By R. Prasad

CHENNAI, SEPT. 26. The earthquake which struck Chennai on Tuesday was only a moderate one measuring 5.5 on the Richter scale. Being a moderate one the possibility of aftershocks is very less, according to Mr. D.C. Mishra, Director of the National Institute of Geophysical Research, Hyderabad. The aftershocks would be so weak that they would be recorded only by seismograms. The possibility of aftershocks will arise only if the earthquake measures more than 6.5. Aftershocks as a rule are of an intensity lesser than the first earthquake.

Mr. Mishra ruled out overexploitation of groundwater as a likely cause for the earthquake. ``In that case, the earthquake would have had its epicentre somewhere in the landmass and not 50 km off the coast of Pondicherry,'' he asserted. ``Earthquakes due to overexploitation of ground water are of a very low intensity and would be recorded only by seismograms, and not like the one that happened on Tuesday.''

South India and the oceans which surround it are in a geologically stable area called cratons. Earthquakes very rarely happen in such a stable place as it is an intraplate region. Earthquakes generally occur at junctions where two plates come in contact with each other. ``Tuesday's earthquake had its epicentre at the junction of the continental slope and deep sea. And such regions are characterised by faults,'' he said. Faults are weaknesses on the earth's crust and are regions where earthquakes occur, be it interplate on intraplate.

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