Seismicity near Bhatsa reservoir, Maharashtra, India

Abstract

Bhatsa dam has been under construction since 1977 on the west coast of Maharashtra, India and is about 90 km northeast of Bombay. The dam is about 200 km north of Koyna with a similar geological setting. The maximum height of the Bhatsa dam, when completed, will be 88.5 m and have a capacity of 957 × 10 6 m 3. The seismic activity at Bhatsa began in May 1983. Subsequent bursts of seismicity in August and September 1983 closely followed a rapid 18 m rise in water level to a height of 57.5 m with a storage of 356 × 10 6 m 3. The three earthquakes of magnitude M L 4.4, 4.9 and 3.9 on August 17, September 15, 1983 and January 7, 1984, respectively, caused damage in nearby villages. The number of tremors recorded and felt have decreased considerably after the spurt of activity in August/September 1983, and now is at a relatively low level with occasionally felt earthquakes. One microseismic station near the Dam began in July 1983 and a network of eight seismic stations was established by October 1983. More than 400 well recorded earthquakes of magnitude of about − 1.2 and greater have been located with HYPO 71 computer program. More than 13 000 microearthquakes have been recorded of which 342 were felt. Epicentres are mainly confined in a zone of 5 × 7 km area in between Khardi and Bhatsa and cover most of the portions of the reservoir. The epicentres are related to the major NW-SE running faults as well as smaller faults/fractures in the area. Composite fault plane solution indicate normal faulting along north-northwest nodal plane with the western block going down relative to the eastern block in conformity with the local tectonics. The velocity anomalies are noted for felt earthquakes in January, February, May, June and July 1984. Radon measurements also showed some increase during March–April 1984, when the seismicity was comparatively high. The background level of radon in the Bhatsa area is established to be about 40 tracks week −1 mm −2. The b-value is found to be lower prior to a magnitude 3.9 earthquake on January 7, 1984. Initiation of seismicity, correlation of seismicity with water levels, intense seismicity following 50 m reservoir depth, confinement of epicentres in close vicinity of the reservoir and shallow depths of hypocentres indicate that the seismicity is reservoir induced. Seismic characteristics like foreshock-aftershock pattern, b-value, slow rate of decay of seismicity and growth of epicentral area at a rate of 6 × 10 4 cm 2 s −1 further support the view that the seismicity is reservoir induced.