Stress drop variations of the 2019 ML 6.0 Changning earthquake and its aftershock sequence in the southern Sichuan Basin, China

The Lg-wave spectra are investigated for the 2019 Changning ML 6.0 earthquake and its 94 aftershocks with magnitudes above ML 2.8, which occurred in the southern Sichuan Basin, Southwest China. This earthquake sequence is potentially related to the water injection from nearby salt mining activities. Based on an established regional high-resolution broadband Lg-wave attenuation model, we correct the observed spectra for the atten-uation effect along the propagation path. By fitting the observed spectra from the Changning earthquake sequence with the theoretical source model, we estimate seismic moments and corner frequencies, from which the stress drops are calculated for the entire earthquake sequence. The results show that the stress drops are rather scattered, ranging from 0.16 MPa to 32 MPa with a median value of 0.78 MPa. Spatiotemporal variation of stress drops reflects strong fault heterogeneity and a complex stress release process, which is likely influenced by water injection and diffusion. Positive trends are observed between the stress drop and focal depth and between the stress drop and seismic magnitude. Relatively lower stress drops for most shallow events are possibly the indication of lowered normal stress by long-term fluid injection. No apparent relation is observed between the stress drop and the distance to the Shuanghe salt mine, where water was injected. Combining the actual water injection data and previous numerical simulation studies, we suggest that the salt mining-related water injection may lead to elevated pore pressure beyond the source region of the 2019 Changning earthquake. The increasingly cumulated underground water may have created pathways linking multiple fault systems, thus the possibility of future large induced-earthquakes cannot be ruled out.