High-precision seismic relocation and three-dimensional shallow crustal structure in the southern Sichuan basin

Since 2014, rates of earthquakes have increased rapidly in the Weiyuan and Changning regions of the southern Sichuan basin, where the shale gas and salt mine have been operated. We perform double-difference seismic tomography using P- and S-wave arrival times from 44,759 events recorded by the permanent seismic networks and two local networks. Data augmentation is offered by a temporary broadband array deployed recently in the Weiyuan region. Relocated seismicity is concentrated at depths of 2–8 km, and mainly occurs in and around hydraulic fracturing well pads or salt wells. Some moderate earthquakes occur relatively deeper. The distribution of M 3.0+ earthquakes clearly reveal several pre-existing faults in the Weiyuan region. Our retrieved three-dimensional shallow crustal models show that although the Vp and Vs values at earthquake locations vary between the Weiyuan and Changning shale gas demonstration zones, the Vp/Vs ratios are coherently low. Such low Vp/Vs values of the source regions suggest the presence of gas-saturated cracks with relatively large aspect ratios under high pore pressure, rather than a great number of liquid-filled cracks. •Events are mainly confined in and around hydraulic fracturing wells or salt wells.•Some moderate events occur relatively deeper, indicating large pre-existing faults in basement.•Earthquakes in the Weiyuan region depict several pre-existing faults.•The Vp/Vs ratios of source regions are coherent low beneath two shale gas fields.•Low Vp/Vs ratios cannot be explained by presence of liquid-filled cracks alone.