[S02-3] Studies of seismicity at Koyna, India

[S02-3-05] The seasonal variation regime of induced seismicity in the Koyna-Warna region, western India

^○Kusumita Arora¹, Rajender Chadha¹, Vladimir Smirnov^2,3, Srinagesh Davuluri¹, Alexander Ponomarev², I.M. Kartashov^2,3 (1.CSIR-National Geophysical Research Institute, Hyderabad, India, 2.Schmidt Institute of Physics of the Earth, RAS, Moscow, Russia, 3.Faculty of Physics, Lomonosov Moscow State University, Moscow, Russia)

The five decade earthquake catalogue of the Koyna-Warna reservoirs region in western India was analyzed to study seasonal variations in seismic activity related to the annual cycle of loading and unloading of these reservoirs. Earthquakes started occurring in the region during early 1960s coinciding with the construction of the Koyna dam and subsequent filling of the reservoir in stages. The seismic activity continues till today. Our study reveals regularities in the seismic structure in time where the activity peaks thrice every year, viz., in autumn (September), in winter (November-December) and in spring (February-April). The monsoon season starts during June and ends in September every year. Seasonal seismic activity is the lowest in May and June when the water level in the reservoirs is minimum. The first peak in autumn is explained as the instant reaction of reservoir filling during monsoon season which saturates the system and adds an additional load due to the weight of the water in the reservoirs. The following two peaks in winter and spring follow the draining phase of the reservoir with a delay caused by the diffusion of water to deeper levels.
It is shown that intensities of both instant and delayed reaction to seasonal variations of water level change in time, their correlation also changes in time. The possible factors that may influence the intensity changes of seasonal components of induced seismicity are discussed.