Tomography of the S wave attenuation in the upper crust of Three Gorges Reservoir region

A 2-D S-wave Q_S values of the upper crust of Zigui-Badong segment beneath the Three Gorges Reservoir region are estimated by means of S wave attenuation tomography based on more than 6000 S wave-rays of 1300 small seismic events with M_L>1.5 recorded by digital seismic network of Three Gorges from January 2009 to February 2015. The results show that Q_S exhibits a significantly lateral heterogeneity distribution in Three Gorges Reservoir region, with high Q_S value mainly in the nearby riversides and low Q_S value in the surrounding area in the northeast, northwest, southwest and southeast directions. Particularly, the low Q_S value is closely related to seepage of reservoir water due to the conversion and fragment of rock stratum. The variation of Q_S value in this paper is in accord with Q value of rock stratum from rock-basement to the bottom of the upper crust from the Wushan-Zigui deep seismic sounding (DSS) profile, which reflects that the deep media of upper crust have influence on the seismic wave attenuation. The M_S5.1 Badong earthquake swarm in 2013 and the M_S4.5, M_S4.7 Zigui earthquake swarms in 2014 were basically converged to transitional edge regions of high and low Q_S values. That is because that, the physical property of the crustal medium is located on the weak-soft transitional region, strain energy is prone to accumulate, and Q_S value also decreses due to the permeation of the water, furthermore, the shear strength of the seismogenic fault is reduced so as to induce earthquakes.