Varying degree of reservoir-induced seismicity as a seismoscope in the preliminary study of stress distribution in peninsular India

There exist stresses which maintain a disequilibrium in the varied topography of the earth. The stresses at any point within the Earth will primarily be determined by the weight of the overburden. The present-day stress field manifests itself in a variety of phenomena like rock bursts in mines and tunnels and tremors in seismic zones, when stresses are released. The additional stresses induced by the reservoir on the surface reactivate the region and thus enable it to release the accumulated stresses. Reservoir impounding is the most effective artificial source in triggering seismic events. The weight of the water in the reservoir as well as the orientation of the reservoir with respect to the Earth’s rotation which will introduce vertical and frictional stresses, respectively, will act as a stimulus to the already existing stress field beneath the crust, and the reflexive action of the earth locally is a response to the combined effect of these two types of stresses. From the study of peninsular India it has been found that the geographic location, the geologic set-up of the area, the geodynamic conditions of the region, the orientation of the reservoir with respect to the earth’s rotation and the topography of the area play an important role in deducing the nature of stresses in the area caused by reservoir impounding. The reservoirs located at the rivers flowing westwards show more seismicity than the reservoirs located at the rivers flowing eastwards. The reservoirs located deep in the shield show mild or no seismicity. The existing reservoirs themselves can be used as seismoscopes to study the reflexive action of the Earth. The seismic set-up of peninsular India may thus be broadly evaluated from the seismic behaviour of the reservoirs. This type of study will be supplementary to the studies already being made at the seismological stations.