Seismic activity and fluid diffusion characteristics in the Weiyuan-Zizhong area, Sichuan Basin, China

The seismicity in the Weiyuan and Zizhong areas in Sichuan and their neighboring regions was analyzed using the unified catalog from the China Earthquake Networks Center. The maximum likelihood method was employed to investigate the temporal evolution of the b-value before and after the 2019 MS>5.0 earthquakes (MS5.4 in Weiyuan and MS5.2 in Zizhong), as well as its spatiotemporal correlation with shale gas extraction activities and hydraulic diffusion characteristics. The results indicate that the frequency of seismicity in the Weiyuan-Zizhong region surged in 2015, with events clustered in the shale gas extraction zone. The b-values exhibited varying degrees of decrease prior to the two MS>5.0 earthquakes. Using the relocated earthquake catalog, we further analyzed the clustering and migration characteristics of seismicity near the W204-H37, W204-H39, and W204-H42 well pads. Notably, the seismicity migration within a 10 km radius of well pad W204-H42 was consistent with the pore pressure diffusion process. When the hydraulic diffusion coefficient D ranges from 0.7 m2·s−1 to 0.8 m2·s−1, over 90% of the seismicity is located behind the diffusion front. The spatiotemporal distribution of seismicity in the Weiyuan-Zizhong area and its vicinity is highly synchronized with local hydraulic fracturing operations. Additionally, the distribution of seismicity near shale gas extraction well pads cannot be fully explained by fluid diffusion alone. After comparing different models and possible mechanisms, it is suggested that some seismic activities may be affected by other complex mechanisms.