How Induced Earthquakes Respond to Pre-Existing Fractures and Hydraulic Fracturing Operations? A Case Study in South China

Hydraulic fracturing in shale gas production can induce felt earthquakes, making it crucial to understand and mitigate induced earthquakes. The Cen'gong shale gas block in South China offers extensive data—3D seismic, geological structure, microseismic data, and detailed stimulation operations—allowing a comprehensive investigation into induced earthquakes by hydraulic fracturing. Using a dense temporary seismic array and deep-learning workflows, we build a high precision earthquake catalog and determine their focal mechanisms. Pre-existing fractures are identified through the Ant Tracking attribute derived from the 3D seismic data. We analyze the distribution, frequency, magnitude, and focal mechanisms of induced earthquakes, compare them spatially with the distribution of the pre-existing fractures, and track their temporal changes during and after hydraulic fracturing. Most induced earthquakes occurred along pre-existing fractures, exhibiting relatively larger magnitudes and persistent trailing seismicity. The number of trailing seismicity is proportional to the response time of stimulation earthquakes. The focal mechanism solutions suggest that the rupture mechanism of the trailing seismicity remained unchanged. By analyzing four clusters of earthquakes, we found that in two of these clusters, the induced earthquakes initiated from the far side of the fractures, then linearly migrated along the pre-existing fractures. This directional migration pattern is explained by stress rotation along the fractures. Our analysis suggests that both pre-existing fractures and stimulation operations significantly influence induced earthquake occurrences. Therefore, this work may enhance our understanding of pre-existing fractures, and optimizing stimulation operations can mitigate earthquake hazards in shale gas production.