The series of earthquakes in the vicinity of Koyna Dam in southwestern India, which included a very damaging magnitude 6.5 shock on Dec. 11, 1967 (Dec. 10 GMT), is of much interest. The events are exceptionally well documented, which is a tribute to Indian geologists, seismologists and engineers. To help in disseminating the information in the United States, this chapter presents a brief summary of the pertinent data and lists references.
Koynanagar( Koynatown),which began as the construction camp for the dam, is located about 140 miles southeast of Bombay and 70 miles south of Poona at 17.4°N and 73.7°E, about 40 miles inland from the Arabian Sea. Koyna River flows almost due south from its source a distance of about 38 miles and then makes an abrupt turn to the east and travels some 480 miles to empty into the Bay of Bengal. Koyna Dam is located about 3 miles north of this turn. The approximately north-south trending continental divide is a few miles west of Koyna and the terrain drops steeply about 1700 ft to where the hydroelectric generating station is located.
The reservoir behind Koyna Dam forms a long, narrow lake (Shivajisagar Lake) with mean water level (MWL) at 2160-ft elevation. The 345-sq-mile watershed receives an annual rainfall of about 200 in., most of which falls between mid-June and mid-October. The peak storage is 99 x 109 cu ft. A controlled flow passes under the continental divide in a tunnel and then down to the electric generating station about 5 miles WNW of the dam. The inflow not used for power generation is released to Koyna River. The Koyna project is a major source of electric power for Bombay and produces 540 MW at peak load.
The Indian Standards Institution has issued recommendations for earthquake-resistance design1 and this publication contains a seismic zoning map together with an epicenter map and geologic maps as substantiating information. The epicenter map, Fig. 1, shows that many large earthquakes have occurred in northern India, Tibet, and Pakistan, but practically no seismic activity is shown in peninsular India. The seismic zoning map, Fig. 2, divides India into seven seismic zones with,[ Fig. 1--Map of India showing epicenters for earthquakes haring magnitude greater than 5.0. From Indian Standard.1 (Available in full paper)],[Fig. 2-Seismic zones of India as specified by Indian Standard.1 Reproduced from Ref. 4 .See Table 1 for seismic design coefficient for the seismic zones. (Available in full paper)] Koyna in Zone 0 (reproduced from Ref. 2). It may be remarked that the seismicity of Zone 0 prior to 1967 was not inconsistent with that of Zone 0 on the U.S. seismic probability map. Since the Koyna earthquakes it has been pointed out that there are historical references to the occurrence of earthquakes in peninsular India which may not have been taken into account when the seismic zones were established. However, it is unlikely that consideration of these would have made significant changes in the zoning map prior to the Koyna events.
