by Harsh Gupta, Georg Dresen, Sukanta Roy, Hans-Joachim Kümpel, Serge A. Shapiro, William L. Ellsworth, Marco Bohnhoff

The Koyna region located in the ca. 65 Ma old Deccan Traps of western India is a classical site of artificial water reservoir triggered seismicity (RTS) because of (i) persistent seismicity in the vicinity of the reservoir for the past five decades since the impoundment of the Shivajisagar (Koyna) lake in 1962, with 22 M 5 earthquakes, more than 200 M ca. 4 earthquakes and several thousand smallerearthquakes, (ii) the largest RTS earthquake so far - M6.3 on 10 Dec 1967, (iii) strong association of the earthquake activity with the annual loading and unloading cycles of the Koyna and nearby Warna reservoirs, and (iv) the isolated nature of the seismic zone, restricted to ca. 20 x 30 km2 area and up to 10 km in depth, with no other source of activity within ca. 50 km of the Koyna Dam. Although the Koyna region has been subjected to geological and geophysical probing during the past few decades, a model to explain the triggered earthquakes remains elusive. Our knowledge, about the physical properties of rocks and pressure of fluids in the fault zones and their role in triggering earthquakes for extended period, is limited by the lack of near-field data. Two international workshops, organized by the Ministry of Earth Sciences (MoES), India and ICDP during March 2011 and May 2014, backed up by a comprehensive exploratory phase of geophysical investigations and drilling during 2012-14 provide a strong justification for setting up of a deep borehole observatory to study pre-seismic, co-seismic and post-seismic changes in critical physical and chemical parameters in the hypocentral region. Exploratory drilling of ten boreholes, up to 1522 m depth, covering the seismic zone have brought forth critical new information on subsurface geology including the variations in thickness of the Deccan Trap cover in the region, absence of infra-Trappean sediments, and the nature and properties of the flood basalt and the underlying basement rocks. Airborne gravity gradiometry and magnetics, broadband magnetotelluric studies and information from drilling have provided a regional 3D subsurface structure of the region. New information on the geothermal regime and physical properties of the rock formations are available from the geophysical well logs. Seismometers have been installed in the granitic basement underlying the Deccan Traps in five boreholes and three other boreholes are to be instrumented by March 2015 for a better assessment of the focal parameters. Based on the recommendations of the 2014 ICDP International Workshop, pilot boreholes are proposed to be drilled to a depth of 3 km at two locations - one in the Koyna seismic cluster and another in the Warna seismic cluster, prior to undertaking drilling of the main boreholes. The pilot boreholes will provide critical parameters such as in-situ stress regime, pore fluid pressure, fluid/gas properties and hydrological parameters of basement rocks, and geothermal regime. Seismometer arrays installed in the two boreholes.

• The scientific objectives of the present pilot borehole drilling and related studies proposal are linked with the overarching goal of the scientific deep drilling project in the Koyna area, to comprehend the genesis of reservoir triggered earthquakes in an intraplate geotectonic setting. This is a fore-runner to the main phase involving scientific drilling to ~5 km depth to set up a deep fault zone observatory. The specific objectives of the present proposal are to:
• improve the locations of hypocentres and delineate the active fault zone(s) in the Koyna-Warna seismic zone that will guide the trajectory of the main holes to sample active fault zone and establishing a fault zone observatory investigate in-situ physical properties of rocks, hydrological parameters, temperature and other parameters from interpretation of downhole geophysical logs determine the state of in-situ stress in the Koyna-Warna seismic zone through hydrofrac tests and image logs study the state of and temporal variations in pore fluid pressure delineate subsurface structures in the vicinity of seismic zone model the thermal regime in the upper crust using heat flow, thermal properties and radiogenic heat production measurements on rocks quantify the frictional properties of basement rocks The proposed drilling project addresses a key ICDP scientific theme “Active faulting and earthquakes”. The project will generate new information and knowledge about artificial water reservoir triggered seismicity within an intraplate setting but also complement the other ICDP active fault zone drilling projects including the SAFOD (USA), Nojima fault (Japan), NanTroSEIZE (Japan), Chelengpu Fault (Taiwan), Gulf of Corinth (Greece), Alpine Fault (New Zealand) and the GONAF (Turkey) projects.

Asia, Deep Borehole Observatory, Drilling, India, KOYNA, Reservoir-Triggered Seismicity

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