Triggered and tectonic driven earthquakes in the Koyna-Warna region, western India

Two strong M > 5.0 earthquakes within a span of six months occurred in a triggered seismicity environment in the Koyna-Warna region in western India in 2000. The region is experiencing continued seismicity since the last five decades indicating that this region is close to critical stresses and minor perturbations in the stresses due to reservoir loading and unloading can trigger earthquakes. In the present study we applied the technique developed for identification of prognostic anomalies for tectonic earthquakes to the Koyna-Warna catalogue prior to these two earthquakes with an aim to study the process of source preparation for triggered earthquakes. In case of tectonic earthquakes, unstable conditions in a source zone develop gradually leading to a metastable zone which shows variations in certain seismicity parameters known as prognostic anomalies. Our results indicate that the variations in seismicity parameters before the two strong earthquakes in the Koyna region have a pattern of prognostic anomalies typical of tectonic earthquakes. We conclude that initiation of failure in a metastable zone can be caused both, by external impacts, reservoir loading and unloading in our case, and internal processes of avalanche-like failure development.