Fissures and Fractures in the Koyna Seismogenic Zone, Western India

Abstract

ABSTRACT The 1967 M6.3 earthquake near the Koyna dam produced a surface rupture zone following a NNE-SSW structural trend. The rupture zone is documented in a report by the officers of the Geological Survey of India (GSI, 1968). The surface expressions of the rupture comprising fissures, en-echelon fractures, oblique or diagonal tensional cracks and mole tracks, termed here as the Koyna Fissure Zone (KFZ), were reported over a limited area, about 1000m in length and 200 m in width, located to the southeast of the Koyna dam between Donichawadi and Kadoli. The Koyna region was revisited recently for investigating the fissures, to help in locating potential sites for scientific deep drilling. Most of the reported surface deformation features are obliterated due to intense weathering and laterization of the exposed basalt surface during the past 48 years. However, careful field investigations in the Donichawadi-Kadoli area as well as in the surrounding region show that fissures and fractures are still preserved at several locations. The fissures trend between N30°W to N50°E, similar to that reported previously. The mean orientation of these fissures is 25°/89°-E. Transposition of one set of fissures by another set provide strong evidence for repeated brittle deformation in the area. Physical and microstructural studies on core samples of granite-gneiss basement rock underlying Deccan basalt in the area, obtained from a 1522 m deep borehole at Rasati, confirm the extension of the surface fissures to depth. Occurrences of secondary mineralization along these fissures indicate the percolation of water through fractures within basement granitoids. The evidences for water channelization may have direct implications for the occurrence of reservoir triggered earthquakes in the Koyna region for the past five decades.