Seismotectonics of the Koyna-Warna Area, India

Reservoir-induced seismicity has been observed near Koyna Dam, India since the early 1960s. In order to understand the seismotectonics of the region we analyzed available seismicity data from 1963 to 1995. Over 300 earthquakes with M]3.0 were relocated using revised location parameters (station locations, velocity model, station delays and Vp /Vs ratio). The spatial pattern of earthquakes was integrated with available geological, geophysical, geomorphological data and observations following the M 6.3 earthquake in December 1967, to delineate and identify the geometry of seismogenic structures. From this integration we conclude that the area lying between Koyna and Warna Rivers can be divided into several seismogenic crustal blocks, underlain by a fluid-filled fracture zone. This zone lies between 6 and 13 km and is the location of the larger events (M]3.0). The seismicity is bounded to the west by the Koyna River fault zone (KRFZ) which dips steeply to the west. KRFZ lies along the N–S portion of the Koyna River and extends S10°W for at least 40 km. It was the location of the 1967 Koyna earthquake. The seismicity is bounded to the east by NE–SW trending Patan fault, which extends from Patan on the Koyna River, SW to near Ambole on the Warna River. Patan fault dips 45° to the NW and was the location of the M 5.4 earthquake in February 1994. The bounding KRFZ and Patan fault are intersected by several NW–SE fractures which extend from near surface to hypocentral depths. They form steep boundaries of the crustal blocks and provide conduits for fluid pressure flow to hypocentral depths. Sharp bends in the Koyna and Warna rivers (6 km south of Koyna Dam and near Sonarli, respectively) are locations of stress build-up and the observed seismicity.