Search

The Koyna region, located in the Deccan Flood Basalt Province of western peninsular India has been experiencing reservoir triggered seismicity since the impoundment of the Shivajisagar water reservoir in 1962. Scientific drilling carried out to 1522 m depth in the vicinity of the seismogenic zone exposed the granitic basement that lay below the Deccan Traps and provided a unique opportunity to study the rock types, petrological characteristics and microstructures. Cores obtained from drilling at four sites considered to be representative of the Koyna region, were studied. The boreholes include KBH-1 (Rasati) in the northern part, KBH-5 (Phansavle) in the western part, KBH-6 (Ukhalu) and KBH-7 (Panchgani) in the eastern part of the region. Each borehole penetrates the entire pile of Deccan basalt and pass through a few hundred metres of the granitic basement. The salient results are as follows: (i) The basement granitoids are dominantly composed of granite-gneiss, granite and migmatitic gneiss, typical of cratonic gneiss exposed in peninsular India. (ii) Petrology and microstructure study confirm the occurrence of strained quartz and unstrained plagioclase feldspars in the basement granitoids. (iii) Localized fault zones within the basement section, with prominent evidences of fault breccia, fault gouge, slicken lines with slickensides and pseudotachylite veins are observed in the individual boreholes. (iv) Anastomosing fracture network within these fault zones are good pathways for water channelization, which is supported by the higher abundances of ferruginous and siliceous secondary precipitations following the fractures.

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.