Elevated in-situ Vp/Vs preceding M > 3 hydraulic-fracturing induced earthquakes

Hydraulic-fracturing (HF) induced seismicity has attracted growing global attention, with the recorded maximum magnitudes reaching up to M6.0 in the southern Sichuan basin, China. How to mitigate the induced seismic hazard is key for safe energy development. Three mechanisms are proposed to explain earthquake triggering during HF: fluid diffusion, poroelastic stress perturbations, and aseismic slip, which can act individually or in combination. Although fluid diffusion is widely regarded as the primary driver, tracking pore-pressure evolution in near real time and quantifying its role in the nucleation of moderate-to-strong earthquakes remains challenging. Here we apply a non-tomographic Vp/Vs method (Lin and Shearer, 2007) to the southern Sichuan Basin, China and analyze the spatiotemporal variations of near-source Vp/Vs during three moderate M3-M4 HF induced earthquake sequences. Benefiting from abundant clustered induced seismicity and dense seismic arrays, we resolve Vp/Vs changes at a high resolution of ~2 days and ~150 m. We observe a consistent increase in Vp/Vs from ~1.73 to ~1.80 prior to the moderate-sized earthquakes, suggesting progressive pore-pressure buildup that culminates in seismic slip. In addition, the elevated pore pressure precedes eventual seismic slip by ~5–10 days, highlighting a preparatory phase for earthquake nucleation, which could be a valuable time window for making injection parameter adjustments to mitigate seismic hazard. The ability to resolve observable changes that precede moderate seismic events on such time scales suggests that the in-situ Vp/Vs approach offers a promising near-real-time monitoring strategy for seismic hazard assessment in a HF setting.