Space-Time Variations in the Attenuation Field of Short Period Shear Waves in the Hindu Kush Area

摘要

This paper is concerned with space-time variations in the attenuation of shear waves in the Hindu Kush area. We examined the ratios of maximum amplitudes in S and P waves (the S/P parameter) obtained from records of deep-focus earthquakes made at the AAK station in 1993-2016 at epicentral distances of 700-800 km. The dependence of the amplitude on the radiation pattern for S and P waves was taken into account by averaging the S/P parameter over different time spans. Substantial space-time variations in S wave attenuation was found in different depth ranges in the zone of deep-focus seismicity. We show that the lowest attenuation in 2013-2015 prior to the great earthquake of October 26, 2015 (M-w = 7.5, h = 231 km) was observed for hypocenters above the rupture zone, at depths of 151-210 km, while the highest attenuation occurred at depths of 231-270 km. Following the earthquake, the attenuation rapidly decreased at depths of 231-270 km and increased in the depth range of 191-230 km. These effects are hypothesized to have been caused by dehydration of mantle rocks, as well as by migration of deep-seated fluids.