Stress drop variations of triggered earthquakes at Koyna-Warna, western India: A case study

We have implemented the spectral ratio technique (SRT) for the estimation of source parameters of local earthquakes (M-L 0.5-4.0) of the Koyna-Warna region, western India, a well-known site of artificial water reservoir triggered seismicity. The SRT uses the concept of empirical Green's function and it takes care of the path and site effects; thus, it contributes to the optimal estimates of the source parameters. Here, we have calculated the corner frequencies and stress drops of a new earthquake sequence that occurred during 1 May-25 June 2017 and compared these estimates with the stress drop behaviour of the existing seismicity cluster (ES) in the region. We also tested the dependency of stress drop with increasing seismic moment of these earthquakes. A total of 689 P-wave spectra of earthquakes recorded by a short-period borehole and a broad-band surface seismic network are utilized for this purpose. We found that the corner frequency varies from 1 to 25 Hz and stress drops vary from 0.01 to 14 MPa for earthquakes of both the clusters. Interestingly, the depth dependence of median stress drop of the new earthquake cluster is slightly different from the ES. The combined trend of median stress drop from both the clusters follows a constant stress drop scaling as widely accepted for natural tectonic earthquakes.