Earthquake behaviors for moderate to strong induced events are controlled by high-velocity bodies near shale reservoirs in southern Sichuan basin, China

Moderate to strong earthquakes have been induced worldwide by shale gas development. However, it is still unclear what factors control the occurrence and magnitude of moderate to strong earthquakes induced by hydraulic fracturing. By using a permanent local seismic network and a temporary dense seismic network, we reliably determined the source attributes of dozens of earthquakes with magnitudes M>3, and importantly, a high-resolution shear-wave velocity model is obtained for studying the detailed seismogenic structure of an unconventional oil/gas field using ambient noise tomography. These earthquakes are found to occur close to the target shale formations in depth, and along high seismic velocity boundaries. Especially, the 2018 Xingwen 5.7 and 2019 Gongxian 5.3 induced earthquakes nucleated around the edges of high velocity zones. These two M>5 earthquake magnitudes as well as co-seismic slip distributions are further determined jointly by seismic waveforms and InSAR data and are found correlated with the high velocity zones along the fault planes. Thus, the distribution of high velocity zones near the target shale formations, together with the stress state modulated by hydraulic fracturing controls induced earthquake behaviors and is critical for understanding the seismic potentials associated with hydraulic fracturing.