Waveform inversion of broad-band data of local earthquakes in the Koyna-Warna region, western India

The Koyna-Warna region in western India is the best example of reservoir triggered seismicity. The world's largest triggered earthquake of magnitude 6.3 occurred in Koyna in 1967, followed by several moderate to small earthquakes ever since. A digital seismograph network deployed for seismic monitoring during 2005 August to 2008 December has indicated a shift in concentration of seismicity towards south in the Warna region including a new zone of seismic activity to the southwest. During the observation period 13 earthquakes of magnitude 4 and greater have occurred of which 11 occurred near the Warna region while only two occurred in Koyna. In this study we modelled broad-band waveform data of six of these earthquakes near Warna using waveform inversion approach. Initially a new velocity model was determined using a joint hypocentral determination approach that simultaneously solves for the velocity structure as well as the hypocentral parameters. A trap thickness of 1.2 km with a P-wave velocity of 4.40 km s(-1) and an upper crustal layer down to 10 km with a velocity of 5.96 km s(-1) are obtained. The new model not only provides the minimum residual error for the traveltimes, but consistently provides the least mismatch error in the waveform inversion of all the events, performing better than any of the previously determined velocity models. In general, focal mechanisms of normal type with NS to NNW-SSE oriented fault planes are obtained for all these events that are correlated with probable faults inferred from satellite images and aeromagnetic anomalies. Focal depths in the range of 5-6 km are obtained for earthquakes in the Warna region based on the sensitivity of whole waveform inversion at local distances. It is felt that joint inversion of waveform data of several earthquakes with a wider spatial distribution, along with the velocity structure would help in precisely characterizing the faulting mechanism and seismogenic depth range for the Koyna-Warna region in future.