Heterogeneities of the Short-Period S-Wave Attenuation Field in the Lithosphere of the Himalayas, Indian Plate, and Southern Tibet and Their Relation to Seismicity

The characteristics of the attenuation field in the lithosphere of South Asia are considered. Records of local earthquakes, obtained at the NIL station, and ratios of maximum amplitudes of S_n and P_n waves within the distance range of ~300–1900 km are analyzed. About 200 earthquake seismograms were processed in aggregate. It is established that generally lower attenuation is observed in the uppermost mantle beneath the Indian Plate (for the meridional profile directed toward the source zone of large Bhuj earthquake of January 26, 2001, with M_W = 7.7). Considerably higher attenuation corresponds to the regions of Himalayas and, especially, Southern Tibet. It is shown that increased attenuation is observed in the source zone of the recent large Nepal earthquake of April 25, 2015 (M_W = 7.8). Additionally, lower and intermediate attenuation is reported within the source zones of large and great interplate events (M_W = 7.0–8.1), occurred in the Himalayan region in 1897–1930. Conversely, substantially decreased attenuation corresponds to the source zone of the Bhuj intraplate earthquake. These new results are consistent with earlier data, which indicate concentration of mantle fluids below source zones prior to large interplate earthquakes, as well as ascent of fluids into the crust after these events. High attenuation zones are distinguished in the regions of West Himalayas and central Pakistan, where large earthquakes have not occurred for a long time. It is suggested that processes related to the preparation of large seismic events can occur there.