Seismogenic structures and earthquake mechanisms in the Changning area, China: Insights from seismicity and tomography

The Changning area, located in southwestern Sichuan Basin, China, was previously known to be a seismically inactive region. However, in recent years, the region has experienced an increase in the rate of seismic activity. To gain a better understanding of the seismogenic structures and earthquake mechanisms in the region, we deployed 298 short-period seismic stations and collected 47 days of continuous waveform data following the 2019 Ms 6.0 Changning earthquake. Using a machine-learning-based earthquake location workflow (LOC-FLOW; Zhang et al., 2022), we detected and located over 60,000 seismic events. These events were then used for three-dimensional Vp and Vs inversion using a double-difference tomography method (tomoDD). The results show that the earthquake distribution has distinct zoning characteristics with different mechanisms. In the Changning-Shuanghe anticline, most earthquakes are distributed in areas with large velocity and Vp/Vs gradients, typi-cally occurring in regions with high velocity and low Vp/Vs, where tectonic stresses are prone to accumulate. Long-term water injections may promote fault activation in the southeast of Changning anticline. Earthquakes around the salt mining well were found to occur in sedimentary cover, indicating that the basement fault has not been activated but rather acts as a fluid-flowing path. In the Changning shale gas block, earthquakes showed different triggering mechanisms, accompanied by different velocity characteristics. The spatial variations in earthquake locations and velocity characteristics can be explained by the terrain at shallow depths, syncline structures, and the distribution of fluids and gas reservoirs. Our study provides new insights into the detailed seismogenic structures and earthquake mechanisms in the Changning area. These findings can be used to improve earthquake hazard assessment in the region.