Triggering of earthquakes at Koyna, India and hammer drilling for the pilot borehole

Koyna located near the West Coast of India is an outstanding case of artificial water reservoir-triggered seismicity. It is observed that annually earthquakes get triggered in the area associated with loading during the monsoon and later unloading of the reservoir. The rate of unloading increased in the third week of March 2015 at Koyna from 0.053 to 0.170 m/day and at Warna from 0.065 to 0.170 m/day. The increase in the weekly number of earthquakes of ML ≥ 0.8 from 10 to 25 compared to the second week of March 2015 is found well corroborated with similar observations of unloading during the years 2016, 2017 and 2018. Drilling of 3-km-deep pilot borehole using hammer drilling technique that reached to the sub-basaltic granitic basement on 1 February 2017 showed increase in the number of earthquakes of ML ≥0.8 to 323 during March 2017–June 2017 compared to 138, 130 and 186 earthquakes in the respective period of 2015, 2016 and 2018. In addition, 116 earthquakes (ML ≥ 0.8) were located within 10 km of the pilot borehole during March 2017–June 2017 in comparison to 68 in 2015, 75 in 2016 and 71 in 2018. Spectral analysis of ambient noise data recorded by the Rasati seismic station at a distance of 9.71 km away from the pilot borehole also supports the fact that Koyna-Warna region is critically stressed.