Low deformation rate in the Koyna–Warna region, a reservoir triggered earthquake site in west-central stable India

Abstract

We analyse nine years of GPS measurements of crustal deformation from the Koyna–Warna region within the stable India plate. The Koyna–Warna region experienced a strong earthquake on 10 December 1967 (M 6.3) that is considered to have been induced by the impoundment of the Koyna reservoir and the continuing earthquake activity in the region is considered to be associated with the Koyna and Warna reservoirs. The earthquakes occur in a very small region of 30 × 10 km² in two well defined seismic zones, the NNE–SSW trending Koyna Seismic zone, and the NNW–SSE trending Warna Seismic Zone. These zones are characterised by predominantly left-lateral strike slip motion and normal motion, respectively. In 2003, we initiated campaign-mode GPS measurements in the region. Analysis of the GPS data collected over nine years indicate low to moderate deformation rate (<2 ± 0.5 mm/year) at a few sites within and close to the fault zones and no resolvable deformation elsewhere. This has been seen in many intra-plate seismic regions of the world with varying causative mechanism for the deformation. In the Koyna Warna region, the observed surface displacement rates of up to 2 mm/year near the fault zones are consistent with a fault slip rate of about 7 mm/year, and with the inferred sense of motion on the faults. The inferred fault slip rate is consistent with the total moment release during earthquakes of past six years in the Koyna Warna region which may imply that the ongoing earthquake activity causes the deformation in the region.