Crustal 2-D Q_S tomography in Longtan reservoir，Guangxi region

In this study, we collected digital waveform records of 3 382 seismic events with M_L≥0 recorded by Longtan reservoir induced-earthquake monitoring network in Guangxi region from September of 2006 to December of 2016 and these data have been precisely located. By using 2-D attenuation tomography technique, we extracted the 2-D Q_S tomography of Longtan reservoir area. The result show that there is a significant lateral heterogeneity in Q_S distribution in Longtan reservoir area. Approximately, there are low Q_S value annular regions surrounding the reservoir, and most of the low Q_S value regions are located at the intersections of rivers and fault zones. From the point of view of lithology, the main distribution of the low Q_S value corresponds to the lithologic area with strong permeability. All the above phenomena may indicate that the low Q_S value regions are affected by reservoir water, fault zones, and the lithology. The preliminary conclusion is: the reservoir water infiltrates through rock fracture zones, joints and crack regions of faults, as well as the regions of rock stratum with strong permeability, filling with fluid in the rock pores, increasing the internal friction, and causing significant attenuation of seismic wave, resulting in the decline of Q_S value. The result also shows that most of the earthquakes occurred at the transitional edge regions of high and low Q_S. This is perhaps a result of accumulation of strain energy in the transitional regions of hard and weak medium zones. Due to the loading effect of water mass and the lubrication effect on the crack boundary of the water in the high and low Q_S transition region, the shear strength of the seismogenic faults is decreased, leading to the relative sliding easily and inducing earthquakes.