Ground Electrical and Electromagnetic Studies in Koyna-Warna Region, India

Abstract

ABSTRACT In this paper, we present results of the Ground Electrical and Electromagnetic (GEE) studies (MT, CSAMT, DRS) carried out in the Koyna-Warna zone since 1976 and discuss their relevance in understanding the seismicity of the Koyna-Warna seismic zone. Though there were not many GEE studies carried out in this region, the existing ones provided a vital information, in particular, on the subsurface crustal structure, the Deccan Trap thickness and its variation and also the nature of the basement in this region. The GEE studies rule out the presence of any subtrappean sediments in this region. An interesting feature that has been identified from MT studies is the presence of a well-defined crustal block structure, characterized by high resistive blocks interspersed with moderately conductive features. All these structural features clearly point out to the intensity of tectonic disturbance that the region was subjected to in the geological past. The conductive features are correlatable with some of the known seismogenic structural features, for e.g., the Konya Fault Zone, the west coast fault, the Donachiwada fault. The moderately resistive zone sitting over the high resistive blocks as seen both in MT& DRS models is inferred to be related to the generally fractured nature of the shallow crustal column. The conductive linear features bordering the resistive blocks represent fluid filled fracture/fault zones. It is inferred that because of the NE to NS oriented compressive stress regime in the Indian shield, due to the Himalayan collision tectonics, some of these structural features may become the locales of stress accumulation which may get released due to fluid filling of these zones under the influence of nearby reservoirs, resulting in triggering of seismicity. Results from different GEE studies conducted in the study area are found to be highly consistent with each other as well as with the Rasati bore hole data, thus bringing significant validity to the subsurface model derived. Further, the 3D modelling of the MT data acquired in the Koyna-Warna region together with airborne gravity gradient and magnetic studies carried out under the Deep drilling program would open up new gate ways to accomplish multi-parametric three dimensional modeling, that will provide still more detailed and relevant subsurface image of this important RTS zone.