Intraplate strain rate variations associated with reservoir-triggered seismicity: A case study from Koyna, Western India

The Koyna-Warna Seismic Zone (KWSZ) of western India is considered as a classical intraplate site to study reservoir-triggered seismicity (RTS). The area has been seismically active for more than six decades after the impoundment of the Koyna reservoir in 1962. Despite its location in a stable continental region, the ongoing seismic activity suggests active crustal deformation and strain accumulation in the area. Therefore, in the present study, we examine the ongoing deformation and strain accumulation in the KWSZ using GPS velocity data from 13 continuous and campaign-mode stations. In the realisation of the Indian Reference Frame (IRF), the residual velocities were estimated to range from 0.4 to 2.0 mm/year, with an uncertainty of ±0.1 mm/yr. The velocity variations, both in magnitudes and orientations, suggest localised deformation and heterogeneous strain distribution in the KWSZ. The estimated principal strain-rate field delineates alternating zones of compression and extension. The maximum compressional axis trends NE-SW, while the extensional axis trends NW-SE, reflecting a transpressional to transtensional stress regime. Elevated shear strain in the area between the Koyna and Warna reservoirs coincides with zones of dense seismicity (M_W < 4), suggesting active faulting and strain concentration. The positive dilatational (extensional) strain observed along the seismicity clusters may be attributed to reservoir-triggered pore-pressure diffusion and hydrological loading, which play a significant role in modulating the ongoing seismic activity in the region. The observed deformation pattern indicates that the KWSZ is governed by the combined influence of far-field India-Eurasia compressional stress and localised reservoir-induced stress perturbations. The coexistence of shear, compressional, and extensional strain components highlights a complex deformation regime even within the intraplate setting.