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

Effective management of seismic hazard in geo-energy development demands real-time estimates of subsurface fault instability. However, real-time monitoring of pore pressure change during subsurface fluid injection remains challenging. Here, we present a novel high-resolution, non-tomographic monitoring strategy that tracks the ratio of seismic wave speeds (Vp/Vs) as a proxy for pore pressure coevolution with induced seismicity clusters. Applying this strategy to the southern Sichuan Basin, China, we observe distinct elevated in-situ Vp/Vs ratios that precede the occurrence of M>3 hydraulic-fracturing (HF) induced earthquakes by an intervening period of several days of seismic quiescence. This quiescent period reveals a prolonged nucleation phase, during which injected fluids gradually condition the M3–M4 rupture zones by elevating pore pressure to critical levels. Such a prolonged nucleation phase could provide a valuable time window for operational intervention, offering more precise timing for hazard mitigation than traditional magnitude-based traffic-light protocols. Our results suggest a viable strategy for near-real-time seismic hazard assessment in diverse fluid-rich environments susceptible to either induced or natural earthquakes.