Abstract

Earthquakes in Koyna–Warna region in western India are considered to be triggered by the reservoirs. However, our understanding of the tectonic processes responsible for causing these earthquakes is generally poor. With a view to understand these processes, in the beginning of year 2013 we installed five continuous Global Positioning System (GPS) sites close to the seismic zones of the region, which provide estimates of secular and seasonal deformation. Based on secular tectonic motion at these sites, we propose that the eastern block of Koyna–Warna fault zone probably moves faster (0.7  mm/yr) in the northeast direction which may be further confirmed from a better spatial and temporal coverage of GPS measurements and additional geophysical observations. Such anomalous secular motion loads the faults of the Koyna–Warna seismic zones and causes earthquakes with focal mechanisms consistent with this motion. Simulated deformation due to filling and emptying cycles of the reservoirs is consistent with the seasonal deformation obtained from the GPS observations and Interferometric Synthetic Aperture Radar (InSAR) analysis. Such seasonal deformation due to reservoir filling leads to reduction in strength on the faults of the Koyna–Warna seismic zones, thereby triggering of earthquakes on critically stressed faults and modulation of earthquake frequency.

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