Pre-existing Fault Regulates the Distribution and Behavior of Hydraulic Fracturing-Induced Seismicity in southern Sichuan, China

A thorough understanding of geological and hydraulic fracturing aspects can offer significant insights into the physical mechanisms governing earthquakes. In this study, we conducted seismic monitoring near a hydraulic fracturing well-pad in the southern Sichuan Basin. The monitoring adopts a dense seismic array that consists of 60 three-component stations, and last for a duration of 53 days. Accordingly, we resolved and located over 1,000 events (-1.43<M<2, Mc=-0.72). Most events (~ 70%) distributed near the southwest direction of the injection wells, delineating a series of NE-SW trending structures. Our high-resolution hypocenter locations and statistical analysis reveal two distinctive clusters: (i) one linearly-distributed cluster characterized by larger magnitudes, deeper focal depths and a b value (1.09) comparable to tectonic earthquakes; (ii) one relatively scattered cluster with smaller magnitudes, shallower depths and a higher b value (1.29). We speculate that deeper events are more consistent with seismicity occurring on pre-existing fault(s), whereas shallower events occur within a fracture network.

The detailed structures are further evaluated with resolved focal mechanisms and 3D seismic reflection imaging. The deeper events unanimously support a right-lateral, steep strike-slip fault, consistent with the fault geometry depicted by high-resolution hypocenter locations. In comparison, focal mechanisms of the shallower earthquakes are more complex and diverse, showing a mixture of normal fault and strike-slip events. In the vicinity of the two clusters, seismic reflection data indicates a ~3 km-length fault that strikes in approximately north-south (NS) orientation. Therefore, we suggest that the damage zone along the NS fault enhanced the connectivity and provided additional hydraulic channel for fluid migration during shale gas extraction. Overall, the distinct characteristics of the two earthquake clusters could be well-explained by their spatial proximity to the fault zone: shallower earthquakes occur on dense fractures near the main fault, whereas the deeper cluster occur on a distant small-scale fault. This study sheds light on the complex relationship between hydraulic fracturing, geological factors, and earthquake occurrence, and may assist strategy development toward risk mitigation of HF-induced seismicity.