SEISMOTECTONICS OF THE KOYNA-WARNA AREA, INDIA

Reservoir-induced seismicity has been observed near Koyna Dam, India s ince the early 1960s. In order to understand the seismotectonics of th e region we analyzed available seismicity data from 1963 to 1995. Over 300 earthquakes with M greater than or equal to 3.0 were relocated us ing revised location parameters (station locations, velocity model, st ation delays and V-p/V-s ratio) The spatial pattern of earthquakes was integrated with available geological, geophysical, geomorphological d ata 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 Wa rna Rivers can be divided into several seismogenic crustal blocks, und erlain by a fluid-filled fracture zone. This zone lies between similar to 6 and 13 km and is the location of the larger events (M greater th an or equal to 3.0). The seismicity is bounded to the west by the Koyn a River fault zone (KRFZ) which dips steeply to the west. KRFZ lies al ong the N-S portion of the Koyna River and extends S10 degrees W for a t least 40 km. It was the location of the 1967 Koyna earthquake. The s eismicity 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 similar to 45 degrees to the NW and was the lo cation of the M 5.4 earthquake in February 1994. The bounding KRFZ and Patan fault are intersected by several NW-SE fractures which extend f rom near surface to hypocentral depths. They form steep boundaries of the crustal blocks and provide conduits for fluid pressure flow to hyp ocentral 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.