Continued seismic activity at the Koyna reservoir site, India

Following the impounding of the Shivaji Sager Lake in 1962, tremors became prevalent in the Koyna region, considered previously to be aseismic. During ensuing years the tremor frequency appears to have been dependent on the rate of increase of water level, maximum water level reached, and the period for which high levels were retained. This culminated in a burst of seismic activity from September 1967 to January 1968 following the record water levels in the reservoir and included the earthquake of September 13, 1967 with magnitude 5.5 and the damaging December 10, 1967 earthquake of magnitude 6.0. During the next five years water levels were kept low and no significant earthquakes occurred subsequent to the October 29, 1968 earthquake of magnitude 5.

The reservoir was filled to maximum capacity during September 1973 and this was followed by a conspicuous increase in seismic activity which included an earthquake of magnitude 5.1 on October 17, 1973. However, seismic activity during 1973 was much less severe than that of 1967. This relative decrease in seismicity may indicate that (a) the “threshhold level” for relatively large magnitude earthquakes had increased; (b) a major portion of the accumulated strains had been released; and/or (c) the importance of the longer period of high loading in 1967. Similar observations have been made at other seismically active reservoir sites.