[S02-3] Studies of seismicity at Koyna, India

[S02-3-02] Scientific deep drilling investigations to probe reservoir triggered seismicity in the Koyna seismogenic zone, western India

^○Sukanta Roy¹, Brijesh Bansal², Vyasulu Akkiraju¹, Surajit Misra¹, Deepjyoti Goswami¹, Nagaraju Podugu¹, Satrughna Mishra¹, Pinki Hazarika¹, Amrita Yadav¹, Sanjay Tiwari¹, Harsh Gupta³, Shailesh Nayak² (1.Borehole Geophysics Research Laboratory, Ministry of Earth Sciences, Karad 415 114, India, 2.Ministry of Earth Sciences, New Delhi 110 003, India, 3.CSIR-National Geophysical Research Institute, Hyderabad 500 007, India)

invited

The Koyna region of western India is an outstanding natural laboratory to test hypotheses and model reservoir triggered seismicity through scientific deep drilling and monitoring in an intraplate tectonic setting. Persistent seismic activity during the past five decades including the M~6.3 Koyna earthquake, 22 M ≥ 5 earthquakes, 200 M ≥ 4 earthquakes and a few thousands smaller earthquakes define a seismogenic zone covering an area of 20km x 30km. Most earthquakes are located in the depth range 2-7 km. The frequency of earthquakes in the region is modulated by the annual loading and unloading cycles of the Koyna and Warna reservoirs. A preparatory phase of investigations including drilling at 9 sites up to 1522 m depth has uncovered the Precambrian basement granitoids underlying the Deccan basalt in the region and shed new light on the subsurface geology and structure. Geological studies of basement granitoids are bringing out new lines of evidence to constrain the deformation mechanisms associated with recent and ongoing seismicity.

The Ministry of Earth Sciences, Government of India, has undertaken a two-phase scientific deep drilling and investigations plan. During the pilot phase, two boreholes up to 3000m depth are planned next to each other; one for making repeat measurements of geophysical, mechanical and hydrological parameters as well as multi-disciplinary laboratory studies on borehole samples, while the other would house a permanent array of seismometers and piezometers for long-term monitoring. Drilling of the first pilot borehole at a carefully chosen site is underway. The datasets acquired from this phase will be analysed together with the data from the existing network of 6 borehole seismometers (up to 1522 m depth) and 20 surface seismometers in the region, which will facilitate the design and set-up of a fault zone observatory at depth of ~5000m during the next phase.