Evidence for upper mantle heterogeneity around 650 km depth beneath the Indian subcontinent

Abstract

Results of seismic ray direction anomalies as obtained from eighty earthquakes, occurring exclusively in the Hindukush focal region are reported. These earthquakes, which were recorded at Gauribidanur seismic array (GBA) in southern India, have identical epicentral distances and a narrow azimuthal range but varying focal depths from 10 to 240 km. Short-period P-wave recordings (up to 30 sec) of these events were processed using an adaptive cross-correlation filtering technique. Slowness anomalies of the order of 1.4 sec/ degr. and azimuthal anomalies of up to 6° have been found in this case. These deviations have been attributed to the upper mantle region. The results confirm the presence of lateral heterogeneities in the 400–750 km depth range beneath the Indian subcontinent where the seismic rays have bottommed. © 1980, All rights reserved.