A Geophysical Approach to the Problem of Seismicity in the Indian Shield

An earthquake is but a culmination of stress build-ups that go on over a period of time at various brittle-ductile transitions located mainly within the crust. Mapping such transitions must therefore form an integral part of any effort at understanding the seismicity of a region. However, this aspect has been grossly neglected, most of our concerns so far revolving round faults and allied structures that only provide a mechanism for stress dissipation as distinguished from stress accumlation.

As a matter of fact, any holistic approach to the problem must invariably include within its ambit an investigation of the thickness and composition of the crust on the one hand and surface heat flow on the other, since these are known to have a direct bearing on the seismic propensity of a region. For instance, a thicker than average crust or a higher than normal heat flow (with consequent thinning of the lighter upper crust) could signify a potential seismogenic area. The same thing could be said of areas where strong rheological stratification is suggested at depth by the presence of say, low velocity layers, seismic reflectors or electrical conductors.

There is a distinct possibility of many of these situations finding expression in gravity maps which could consequently provide a very reasonable basis for effctive selection of areas for detailed follow-up, by more complex methods. Adopting this principle, a broad geophysical strategy has been mooted for the active consideration of geoscientists in the country. While it may be debated and improved upon in regard to detail, its overall implementation as a national project is an imperative dictated by the ominous prospect of a more and more common recurrence of earthquakes in the peninsula in the years ahead.