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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. [ABSTRACT FROM AUTHOR]

The Koyna earthquake (M 6.3) of reservoir triggered seismicity (RTS) nature took place five decades back on December 10, 1967 in the Deccan Trap covered region of western India. This prompted a large number of both Indian and international geo-scientists to undertake various studies, particularly passive seismological studies with full vigour. In addition to these, active seismic studies, then known as deep seismic sounding (DSS), were carried out across and near the Koyna epicentral region to delineate the crustal velocity structure and geotectonic features of the region. Data acquisition along two DSS profiles was undertaken during 1975-78. The results of such rarely undertaken studies were widely utilised by the global earth scientists. Here we relook at the results published since 1981, and try to assess the global impact of DSS study on the research output during the last four decades. After giving a brief summary on the data acquisition, processing and results, we provide relevant information such as the Trap thickness, basement configuration, regional crustal and subcrustal velocity structures, and deep seated features like faults with a view to relate with earthquake activities. Some concluding observations have also been made discussing the accuracy and likely validity of the DSS results, obtained using the then acquired data and processing methods, and unlikely reprocessing of the data using upgraded tools. New active experiments in the Koyna region are being suggested, particularly to decide about the proper locations of the deep geophysical boreholes down to the hypocenters on the 'mapped' faults. [ABSTRACT FROM AUTHOR]

The Koyna region located in the west coast of India is a classic example of reservoir triggered seismicity (RTS) that started soon after the impoundment of the Koyna reservoir in 1962. Previous studies have shown that RTS can be explained in terms of stress and pore pressure changes due to poroelastic response of the rock matrix. The permeability of rock matrix is a key parameter for pore pressure diffusion which is mainly responsible for generation of stress perturbation related to seismicity. Based on the poroelastic theory, we employ 2-D finite element models to simulate the evolution of pore pressure up to 5 years after the reservoir impoundment in 1962, using a range in permeability, 10-10 m. Constraints on material properties of Deccan basalt and granitic rocks were taken from available studies. The results show the formation of pore pressure front and its propagation with depth and time since the reservoir impoundment as a function of permeability. While a permeability of 10 m does not produce any significant change in pore pressure, a ten-fold increase in permeability produces significant changes up to a depth of 2 km only beneath the reservoir after 5 years of impoundment. Permeability values between 10 m and 10 m are required to induce critical pore pressure changes in the range 0.1-1 MPa up to depth of 10 km, capable of triggering earthquakes in a critically stressed region. Studies on core samples of granitic basement rock down to a depth of 1522 m in the Koyna region provide evidences of fracture zones that may contribute to water channelization. Direct measurements of material properties through the ongoing deep drilling programme would help to develop more realistic models of RTS. [ABSTRACT FROM AUTHOR]