Seismogenic properties of the crust inferred from recent studies of reservoir-induced seismicity – Application to Koyna

Recent studies of reservoir-induced seismicity have led to a better understanding of the seismogenic crust. These studies reveal that many parts of the seismogenic crust are critically stressed and small strength changes induced by reservoir impoundment can lead to seismicity. In the vicinity of the reservoir impoundment causes an immediate undrained response to the loading, and a delayed response due to the diffusion of pore pressure. Analytical calculations of strength changes at Monticello Reservoir, South Carolina show that diffusion is the predominant mechanism in triggering seismicity. Seismicity associated with pore pressure diffusion occurs in a crust with seismogenic permeability, ks, ~ 2 to 200 mD. Analyses of seismicity data at Koyna reveal that the rocks may have stressmemory (they remember the largest load that they were subjected to), and that seismicity occurs within a block enclosed by NNE-SSW trending Koyna River fault zone on the west, NE-SW trending Patan fault and a subsidiary fault on the east and a series of NWSE trending faults to the north and south. Continued monitoring of water levels in observation wells and their correlations with lake levels will likely produce the next breakthrough in understanding the protracted seismicity in the Koyna–Warna area.