Spatiotemporal heterogeneity of hydraulic fracturing-induced earthquake stress drops modulated by geological setting and fluid operations: Evidence from multi-well observations at the Gonghe EGS, China

Summary

The spatiotemporal heterogeneity of stress drop in induced earthquakes is crucial for revealing fluid-rock coupling mechanisms, yet its controlling factors remain controversial. Specifically, the universality of the “low near-wellbore, high far-field” spatial pattern and the causal relationship between pore pressure diffusion fronts and high stress drops are debated. To address this, we analyze 1,728 induced earthquakes from three adjacent wells with contrasting geology—a homogeneous granite reservoir versus fault-developed reservoirs—at the Gonghe EGS, China. Our results reveal a dual-control model modulated by geological setting and fluid processes: (1) Temporal evolution is geology-dependent: stress drop increases with injection and peaks post-shut-in in the homogeneous reservoir, whereas strong fault heterogeneity masks this fluid-driven temporal signal in the faulted reservoirs. (2) The spatial “low near-wellbore, high far-field” pattern emerges exclusively post-shut-in in the homogeneous reservoir, absent during injection and in faulted areas. This indicates it is a dynamic product of post-shut-in re-equilibration rather than a universal attribute. (3) High stress drops do not accompany the pore pressure diffusion front during injection; instead, they occur near the front’s extension post-shut-in. This spatiotemporal decoupling suggests that high stress drops are associated with post-shut-in re-equilibration processes rather than the simple advancement of the diffusion front. These findings indicate that stress drop heterogeneity is modulated by the dynamic interplay between fluid diffusion and fault structure, providing a conceptual framework for interpreting diverse global observations and developing physics-informed seismic risk assessments.