Deep Crustal Structure at Koyna, Maharashtra, Indicated by Deep Seismic Soundings

Deep Seismic Soundings (DSS) were carried out by the National Geophysical Research Institute, Hyderabad, during December 1975-April 1976, along a profile across Koyna. The studies of the data reveal a number of reflection horizons below the Deccan Traps up to the Moho discontinuity. Below the Deccan Traps, the crustal section along this profile is cut into two blocks by a deep fault west of Koyna. The eastern block is further cut by another deep fault which affects only the deeper horizons including Moho. Recent movements along the first of these deep faults, west of Koyna, appear to be responsible for the major Koyna earthquake of 1967 and subsequent seismicity in this region. The Moho depth in the western block is around 40 km in the vicinity of the deep fault and is of about 30 km at the west coast of India. In the eastern block the Moho boundary lies at an average depth of 36 to 38 km.

The thickness of the Deccan Traps along Koyna profile varies from o.4 km in the east to about 1.5 km near the west coast. The velocity in Deccan Traps is found to vary between 4.7 to 4.9 km/sec, and in the Pre-Trap basement it varies between 5.9 to 6.1 km/sec.