Structural Controls on Earthquake Clustering in Hydraulic Fracturing: Insights from Velocity Model and Seismic Reflection Data

The spatial distribution of hydraulic fracturing-induced seismicity is controlled by regional tectonics and local geological structures. In this study, we integrated a high-resolution shear velocity model from ambient noise imaging with 3D seismic reflection data to investigate structural influences on induced seismicity near an active hydraulic fracturing (HF) platform in the Sichuan Basin, China. We conducted continuous seismic monitoring throughout the fracturing period and located over 1,000 earthquakes within the 7 weeks of active stimulation. Tomographic model reveals a distinct first-order, EW-striking velocity boundary near the HF well. This lateral velocity discontinuity aligns closely with the 3D curvature attribute identified in seismic reflection data, with high-curvature areas corresponding to disrupted geological features like small-scale faults or stratigraphic discontinuities. Further quantitative analysis reveals that in addition to the spatial clustering near high-curvature areas, 70% of earthquakes are distributed on the high-velocity side and concentrated within a range of 500 meters from the HF well. Based on these observations, we infer that 1) the high-velocity anomalies are mechanically stronger and more susceptible to the release of cumulative elastic energy, and 2) pronounced attribute variations delineate the principal seismogenic structures responsible for hosting induced earthquakes. Consequently, regions with brittle rock properties and significant structural variations are more seismically sensitive under external fluid injection. Future work will involve applying the ETAS model to better understand the triggering mechanisms of induced seismicity, aiming to provide insights into the interaction between external fluid injection and localized stress perturbations.

These observations highlight the interplay between velocity heterogeneity, structural attributes, and localized stress perturbations in driving induced seismicity. Similar correlations between local velocity structure and earthquake nucleation are observed at a nearby platform, where the majority of over 6000 detected earthquakes are preferentially located near a NE-SW oriented high-velocity structure east of the injection well. Interestingly, both platforms are characterized by sharp topographic relief, with their maximum gradient well aligning with the velocity boundaries. These integrated structural features may prove crucial in identifying local geological structures that are prone to instability and assist strategy development toward risk mitigation of HF-induced seismicity.