Scientific Deep Drilling in Koyna, India

Startdate: 14 October, 2017
Enddate: 16 October, 2017

Status: done

The Koyna region, located in the ~65 Ma old Deccan Traps of western India is a classical site of artificial water Reservoir Triggered Seismicity (RTS). Triggered earthquakes started occurring soon after the impoundment of the Shivaji Sagar lake formed by the Koyna Dam in 1962 and have continued till now. The seismicity includes the largest triggered earthquake of M 6.3 on December 10, 1967, 22 M≥5 earthquakes, more than 200 M~4 earthquakes and  several hundred smaller earthquakes. A strong association of the earthquake activity is observed with the annual  loading and unloading cycles of the Koyna and nearby Warna reservoirs (impounded in 1985). The entire seismic activity is restricted to ~20 km x 30 km area and there is no other earthquake source within ~50 km of the Koyna  Dam. The depth of these earthquakes is limited to about 10 km with ~80% occurring between 2 and 9 km depths.  Earthquakes of M~4 are found to be preceded by well-defined nucleation and real time identification of nucleation  has led to short term forecasts. During the past few decades, a considerable shift in the seismic activity has been  observed in the region to the south. However, it may be noted that even before the impoundment of the Warna  reservoir in 1985, triggered earthquakes had occurred in the vicinity of Warna river as a consequence of loading of  the Koyna Dam. Moment tensor inversion studies show that focal mechanisms are dominated by strike-slip faulting  in the vicinity of Koyna Dam and immediately south of it and normal faulting in the Warna region.

A major programme on scientific deep drilling in the Koyna region was conceived by the CSIR-National Geophysical  Research Institute (NGRI) and Ministry of Earth Sciences (MoES), Government of India in consultations with  International Continental Scientific Drilling Program (ICDP), to investigate the causative mechanisms of the RTS. The  first international workshop, held at Hyderabad and Koyna during March 21-25, 2011 provided the necessary impetus  and inputs for development of the project on scientific drilling to study reservoir triggered earthquakes occurring in  the Koyna region for the past five decades.A preparatory phase of the project was recommended. Under the  programme, exploratory drilling and geophysical investigations were carried out during 2012-14. The second  workshop held at Koyna during May 16-18, 2014 reviewed the work carried out during 2011- 2014 based on the  recommendations of the 2011 workshop and suggested the future course of action, including drilling of a 3km deep pilot hole in the Koyna seismic cluster and detailed planning for a full deep drilling proposal. In late 2014, Ministry of  Earth Sciences initiated the setting up of Borehole Geophysics Research Laboratory (BGRL) at Karad. BGRL was  mandated with the task of building necessary R&D infrastructure and expertise to carry out scientific drilling  investigations, including planning and execution of the Koyna deep drilling programme. The drilling of a 3000 m –  deep pilot borehole followed by downhole measurements have been completed recently. After the successful  completion of the pilot borehole drilling phase, an ICDP International Workshop was held during  October 14-16, 2017  to discuss the results obtained from the pilot borehole and chalk out the future course of  activities including setting  up of a deep borehole observatory and down hole as well as laboratory measurements and modelling.