Space-time variations of the shear wave attenuation field in the upper mantle of seismic and low seismicity areas

Abstract

This paper deals with characteristics of the short period S -wave attenuation field in the rupture zones of 37 large and great earthquakes with M s = 7.0-8.6, as well as in low seismicity areas. We estimate the effective quality factor from Sn and Lg coda envelopes in two time intervals ( Q 1 and Q 2 ). The quantity Q 1 is a measure of shear wave attenuation in the uppermost mantle, at depths of down to approximately 200-250 km, while Q 2 is relevant to deeper horizons of the upper mantle. We studied variations in the attenuation field in the rupture zone of the 1950 Assam earthquake. We examined the parameters Q 1 , Q 2 , and Q 1 / Q 2 as functions of the time ∆ T elapsed after a large earthquake. It is shown that the parameter Q 2 in rupture zones is practically inde- pendent of ∆ T , while the quantities Q 1 and Q 1 / Q 2 increase until ∆ T ~ 20-25 years, especially rapidly for normal, normal-oblique, and strike-slip earthquake mechanisms. This analysis provides evidence that, as ∆ T increases, so does the quality factor in the upper mantle for shear waves. It is supposed that this is related to the rise of mantle fluids to the crust. Geodynamic mechanisms are discussed that can support a comparatively rapid "dry- ing" of the upper mantle beneath earthquake rupture zones.