Earthquake focal mechanism studies in Koyna-Warna region in the last five decades — Current understanding on tectonics and seismogenesis

Abstract

On 10th December 1967, the world’s largest reservoir triggered seismic (RTS) event of magnitude 6.3 shook the Koyna region, the prime site of RTS globally. Ever since, several studies have attempted to infer the seismotectonics and to comprehend the actual causative mechanism of triggered seismicity in this region. Initial studies, including those of the 1967 Koyna main shock and its aftershocks, were based on the conventional P wave polarity or the first motion approach. These studies provided the first ever understanding of a predominantly strike-slip environment in the Koyna region, concurrent with the direction of ambient stress field due to the Indian plate motion. Subsequent studies pointed to a normal faulting environment in theWarna region further south, subsequent to impoundment later in 1985. A few studies did report solutions based on composite focal mechanisms, which however, only represent the average picture of the region. More recent studies based on modelling of seismic broadband waveform data provided more accurate focal mechanisms with unprecedented location accuracies including focal depths. A catalog of 50 focal mechanism solutions is now available for the earthquakes of magnitude ∼4 and larger that occurred during the last 50 years, which has paved way for a clear understanding of the stress field and the causative model of seismogenesis in this active intra-plate seismic RTS zone in western India. Based on stress inversion using this catalog, a new tectonic model depicting a periodically varying stress field and hence faulting mechanism has been inferred.