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

Seismic hazard management in geo-energy development demands real-time assessment of fault instability. However, real-time monitoring of pore pressure change during fluid injection remains challenging. Here, we present a high-resolution ( ~150 m and ~2 days), non-tomographic strategy that tracks the seismic velocity ratio (Vp/Vs) as a proxy for pore pressure coevolution with induced seismicity clusters. Applying this strategy to the southern Sichuan Basin, China, we observe a distinct ~4% elevation in in-situ Vp/Vs that precedes M > 3 hydraulic-fracturing-induced earthquakes by several days of seismic quiescence. This quiescence reveals a prolonged diffusion phase, during which injected fluids gradually condition rupture zones by elevating pore pressure to critical levels. Such a preparation phase provides a valuable time window for operational intervention, offering more precise timing for hazard mitigation than traditional traffic-light protocols. Our results suggest a viable strategy for near-real-time seismic hazard assessment in diverse fluid-rich environments susceptible to induced or natural earthquakes.