Earthquakes continue to occur in the vicinity of Shivaji Sagar Lake since its creation by the Koyna Dam in 1962. The seismicity peaked in 1967 with a M 6.3 earthquake which claimed over 200 human lives and destroyed the Koyna township. Earthquakes of M`4 occur every year following an increase of water level in the reservoir. During 1973During , 1980During and 1993 earthquakes exceeding magnitude 5 occurred. Most earthquakes of M`4 are associated with pronounced foreshocks and aftershocks. Starting Sepember 1993, seismic monitoring was vastly improved with the deployment of additional close-by stations (analog and digital). The focal parameters now available have enabled delineation of the active faults and deciphering of the earthquake nucleation process. During 1995 -96, 13 boreholes were drilled to depths of 130 to 250 m and measurement of water levels in these wells was initiated. A preliminary analysis of one year's data from a borehole 1 km south of Koyna reveals tidal signatures, indicating connection of the well to a confined aquifer which is favorable for detection of pore pressure anomalies induced by crustal strain. We hope to improve our understanding of the genesis of reservoir-induced earthquakes at Koyna with these new measurements.
The Dharwar craton, Cuddapah basin and the Godavari graben characterise three diverse geological and tectonic settings in the peninsular shield of India. Owing to their contiguous proximity, they offer a unique opportunity to document differences, if any, in their seismic wave attenuation characteristics that might have a bearing on the seismogenic nature of the crust in a craton, basin and a rift-like graben structure. An attempt is made here to bring out these differences using constraints from coda-Q. We considered local earthquakes with epicentral distances ranging from 14 to 150 km recorded at the digital broadband stations at Dharwar (DHD), Cuddapah (CUD) and Kothagudem (KGD) regions to derive the frequency-dependent coda-Q relations. Using the single scattering method, we obtained the frequency-dependent Q C relationship (Q C = Q 0 f n )for each of the three geological units separately: DHD: Q C = (730.62 ± 0.09)f (0.54 ± 0.01) ; CUD: Q C = (535.06 ± 0.13)f (0.59 ± 0.01) and KGD: Q C = (150.56 ± 0.08)f (0.91 ± 0.01) . The Q C values obtained for all the three sub-regions show moderate to strong frequency dependence and essentially reflect the level of crustal heterogeneities to varying degrees.
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