An investigation of earthquakes in Koyna region, Maharashtra, for the period October 1973 through December 1976

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Abstract
Twelve earthquakes of Ms ≧ 4.0, their foreshocks and aftershocks, which occurred during the period October 1973 through December 1976 in the vicinity of the Koyna Dam, Maharashtra have been investigated using the seismograms from the Koyna seismic network, WWSSN seismic station at Poona (POO), and the NGRI seismic station (HYB) at Hyderabad. In all 71 hypocenters are located. Due to paucity/poor quality of data, the locations are mainly fair to poor in quality. Inferred focal depths are less than 15 km. These hypocenter locations indicate the possibility of the existence of a N-S trending fault at 73°45′E longitude. An empirical relation between signal duration (τ) and surface-wave magnitude (Ms), Ms = −2.44 + 2.61 log τ, is obtained for the region. This relation yields more reliable estimates of magnitudes. Composite focal mechanism solutions could be obtained for eight earthquakes with Ms ≧ 4. These solutions are mostly consistent with a N-S trending fault. Energy release patterns have been investigated for four sequences. A major portion of energy is released through the main shock.

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