high-quality magnetotelluric data at 100 stations, provide both regional information about the thickness of the Deccan Traps and the occurrence of localized density heterogeneities and anomalous conductive zones in the vicinity of the hypocentral zone. Acquisition of airborne LiDAR data to obtain a high-resolution topographic model of the region has been completed over an area of 1,064 km 2 centred on the Koyna seismic zone. Seismometers have been deployed in the granitic basement inside two boreholes and are planned in another set of six boreholes to obtain accurate hypocentral locations and constrain the disposition of fault zones.

Koyna, located in the Deccan Volcanic Province in western India, is the most significant site of reservoir triggered seismicity (RTS) globally. The largest RTS event of M 6.3 occurred here on December 10, 1967. RTS at Koyna has continued. This includes 22 M≥5.0 and thousands of smaller events over the past 50 years. The annual loading and unloading cycles of the Koyna Reservoir and the nearby Warna Reservoir influence RTS. Koyna provides an excellent natural laboratory to comprehend the mechanism of RTS because earthquakes here occur in a small area, mostly at depths of 2–7 km, which are accessible for monitoring. A deep borehole laboratory is therefore planned to study earthquakes in the near-field to understand their genesis, especially in an RTS environment. Initially, several geophysical investigations were carried out to characterize the seismic zone, including 5000 line kilometres of airborne gravity gradiometry and magnetic surveys, high-quality magnetotelluric data from 100 stations, airborne LiDAR surveys over 1064 km2, drilling of 8 boreholes of approximately 1500 m depth and geophysical logging. To improve the earthquake locations a unique network of borehole seismometers was installed in six of these boreholes. These results, along with a pilot borehole drilling plan, are presented here.

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.

Large man-made water-reservoirs promote fluid diffusion and cause critically stressed fault zones underneath to trigger earthquakes. Electrical resistivity is a crucial property to investigate such fluid-filled fault zones. We, therefore, carry out magnetotelluric (MT) investigation to explore an intra-plate earthquake zone, which is related to artificial reservoir triggered seismicity. However, due to surface access restrictions, our dataset has a gap in coverage in the middle part of the study area. This data gap region coincides with the earthquake hypocenter distribution in that intra-plate earthquake zone. Therefore, it is vital to fill the data gap to get the electrical signature of the active seismic zone. To compensate for the data gap, we develop a relation that connects resistivity with the ratio of seismic P- to S-wave velocity ( VP/ VS). Utilizing this relation, we estimate a priori resistivity distribution in the data gap region from known vp/vs values during inversion to compensate for the data gap. A comparison study of the root mean square (RMS) misfits of inversion outputs (with and without data gap filled) proves the effectiveness of the established relation. The inversion outputs obtained using the established relation brings out fault signatures in the data gap region. To examine the reliability and accuracy of these fault signatures, we occupy a portion of the data gap with new MT sites. We compare the inversion output from this new setup with the inversion output obtained from the established relation and observe that the electrical signatures in both outputs are spatially correlated. Further, a synthetic test on a similar earth model establishes the credibility and robustness of the derived relation.