Seismic Activity Reveals the Coexistence of Multiple Mechanisms of Fault Reactivation Induced by Hydraulic Fracturing in the Gonghe EGS Project in Qinghai, China

Abstract The application of hydraulic fracturing technology in deep geothermal development can activate pre-existing faults and induce significant earthquakes, impacting disaster management and project commercialization. The mechanisms of fault reactivation are complex and include pore pressure diffusion, poroelastic stress effects, aseismic slip, and shear stress transfer. Thorough theoretical research and case study accumulation are critical. This article investigates the fault reactivation mechanisms associated with fluid injection in the first enhanced geothermal system project in Gonghe, Qinghai, China. Using high-precision microseismic location data and hydraulic fracturing construction data, we successfully identified seven seismic clusters. These clusters exhibit typical characteristics of fault reactivation, such as spatial arrangement along specific structures, temporal clustering, occurrence of larger magnitude events, consistency between the earthquake rupture surface and the fitted fault plane, and a b-value less than 1.0 in the magnitude–frequency distribution. Further using repeated earthquake identification technology, we confirmed that these clusters correspond to five independent faults. The fault activation mechanism inferred through spatiotemporal seismic migration analysis indicates that in the early stage of hydraulic fracturing, two faults are activated, which may be related to the poroelastic stress effect. Such a phenomenon is rarely reported in previous cases. Another two faults showed typical hydraulic diffusion characteristics, indicating activation by pore pressure diffusion. One fault’s reactivation may result from the combined effects of pore pressure diffusion and aseismic slip. We hypothesize that the differing mechanisms and sequences of fault reactivation reflect significant variations in the critical stress states of faults at the same site. The study also revealed two faults undergoing repeated reactivation, emphasizing that fault properties mainly control reactivation modes and behaviors. Our findings provide a scientific basis for designing risk mitigation measures for induced seismicity.