Earthquakes Triggered by Fluid Diffusion and Boosted by Fault Reactivation in Weiyuan, China Due to Hydraulic Fracturing

Hydraulic fracturing has induced small-to-moderate-size earthquakes around the world. Identifying spatio-temporal evolution of microseismicity is important for understanding the physical processes that control hydraulic fracturing-induced seismicity. In this study, we build an enhanced earthquake catalog from continuous seismic data recorded by 1-year temporary deployment with an automatic procedure and relocate 18,663 earthquakes in the Weiyuan shale gas block in the southern Sichuan Basin, China. Our catalog, with a maximum M-L of 3.5, has a completeness magnitude (M-c) of 0.4, which is similar to 23 times more events than listed in the standard National Earthquake Data Center (NEDC) of China. Most earthquakes are clustered near hydraulic fracturing wells and delineate many pre-existing faults in the north-south direction. The space-time evolution of microearthquakes indicates fluid diffusion processes as the primary drivers for seismicity in this region. The fast earthquake migration patterns show that permeability within a fault zone could be affected by fault-valve behaviors and enhanced by earthquake rupture process. We find that the Gutenberg-Richter b-values increase systematically with depth, and b-values further away from hydraulic fracturing pads are generally low, especially for three strands with relatively high migration velocities. Our results confirm that earthquake clusters in Weiyuan are induced by hydraulic fracturing and the reactivated faults act as conduit networks for fluid flow, which promote triggering of earthquakes further away from the hydraulic fracturing pad. This study provides additional evidence for hydraulic fracturing-induced earthquake in the southern Sichuan Basin and advances our understanding of injection induced earthquakes.

Automated summary

This study enhances our understanding of induced earthquakes by hydraulic fracturing by providing evidence from seismic data in the Weiyuan shale gas block in China. It confirms that the earthquakes in this area are induced by hydraulic fracturing and identifies pre-existing faults near the clusters. The study also suggests that fluid diffusion processes are the primary drivers of seismicity in this region.