Precursory phenomena in the microearthquake sequence near the Osmansagar reservoir, Hyderabad, India

Abstract

Several precursors were noticed in a microearthquake sequence which occurred for 5 weeks during January and February 1982 near the Osmansagar reservoir, Hyderabad, India. These precursors are shown by velocity and radon anomalies and b-value change. Electrical resistivity changes in the telluric field were also noticed by Sarma et al. The sequence began with a 3.5 magnitude tremor on January 14, 1982 which was accompanied by a loud explosion sound. All the subsequent tremors also produced explosion-like sounds. With a local network of 5 seismic stations, 106 events were located. The events were confined in an area of 2 km × 4 km and had focal depth < 1.5 km. The t s t p ratios indicate a velocity anomaly one week prior to the strongest event of local code magnitude 2.5, which occurred during the period of detailed observations. During this period the b-value also showed a typical precursory decrease. Radon measurements on water samples from four shallow wells indicate a precursory increase 1 or 2 days prior to 5 events of local code magnitude ⩾ 2. A 10-day average measurement of radon concentration in soil gas from shallow holes indicates an increase in February, probably due to the high seismicity.