Construction and Location of the Dam
The Koyna Dam and its reservoir, Shivaji Sagar Lake, are located in the watershed of the Krishna River in Maharashtra, India. Maharashtra is a state in the Koyna region located near Bombay, India. The Koyna Dam, and the reservoir formed by it, went under construction in 1962 and were completed in 1963. The project is comprised of an 800 meter long, 110 meter high concrete dam, an intake structure, an underground powerhouse, a transformer hall, a control valve house, and a tail race tunnel. The reservoir is 103 meters high and has a water capacity of 2.8 billion cubic meters. Seasonal fluctuations of the reservoir level are typically 30 to 35 meters and are dominated by monsoon rainfalls (Guha 1971; Gupta 1992; McCully 1997; Sharma Home page).
Geological Data
The Koyna Dam and its reservoir are part of the Deccan Plateau of western India, which is located in the Peninsular Shield region. The Peninsular Shield is one of three regions that make up the Indian plate (the Himalayan Front and the Indo-Gangetic Plains are the other two regions). The Deccan Plateau is overlain by basalt, or a basic volcanic rock, extruded in the late Cretaceous period (a period beginning about 135 million years ago and lasting about 70 million years). The thickness of the basalt is anywhere from 1800 meters to 3000 meters, and is thickest on the west coast, decreasing towards the east. The Deccan Plateau is part of the Precambrian block, which is the geologic time period prior to the Cambrian period (a period about 570 million years ago), and also represents the largest areas of exposed rocks in this region. The Koyna Dam and its reservoir are located along the same region as the Holocene Fault and the Koyna River Valley Fault, which is also known as the Donichiwada Fault (Allen 1982; Amateur Seismic Centre; Clark and Stiegeler 2000; Guha 1971; Gupta 1992; Reddy 2000; The Facts on File Dictionary of Earth Science 2000).
Earthquake Event
Prior to the year 1962, the Koyna Dam had been considered free from any significant seismic (earthquake) activity, but a seismic zoning map of India, which was prepared by the Indian Standards Institution in 1962 and later revised in 1966, showed this region to be very seismic. Soon after the impounding of the Koyna reservoir in 1962, reports of earth tremors near the dam site became prevalent. The frequency of these tremors increased considerably from the middle of 1963 onwards. These tremors were invariably accompanied by sounds similar to those of blasting. The strongest of these tremors would rattle windows, disturb utensils, etc. To monitor these earthquakes, a close network of four seismological observatories was established and it was found that the hypocenters were clustered near the reservoir, at a very shallow depth (Gupta 1992).
On December 10, 1967 at approximately 22:51 UTC (4:21 a.m. local time on December 11, 1967) the Koyna region experienced an earthquake at a magnitude of 6.6 to 7.5 on the Richter scale (the earthquake’s recorded magnitude differed between different agencies). It was found by Gupta (1992) that the heavy earthquake damage was confined in a very small area, while the area where the shock was felt was comparatively much larger; therefore, a focal depth of 8 to 10 kilometers was assigned (various agencies have assigned this earthquake different focal depths anywhere from 9 to 32 kilometers). The large area that felt the earthquake did so because of the efficient transmission of seismic energy in the Peninsular Shield. The earthquake was felt over distances as far as 700 kilometers from the epicenter. Investigations after the earthquake revealed that only a small, much more localized area actually suffered damage from the event (Gupta 1992).
The earthquake was associated with left-lateral faulting on the Koyna River Valley Fault in Koyna Nagar near the site of the Koyna Dam. Between 10-18 kilometers of the northern end of this fault ruptured in the main shock and the earthquake was followed by approximately 100 aftershocks. The earthquake shook the western half of Peninsular India, and it was distinctly felt as far as Surat and Ujjain in the north, Nagpur and Hyderabad in the east, and Calicut and Bangalore in the south and southeast. Studies indicate that the epicenter of the earthquake was located near the vicinity of the Koyna Dam, which gives rise to the speculation that the reservoir may have been the sole cause of the earthquake (Bapat 2000; Gupta 1992; Times of India 1967(E)).
