Foreshock-induced slip transients set mainshock nucleation timing

Foreshocks are sometimes observed before earthquakes^1-13, yet their role in controlling rupture nucleation remains unclear^1,11,14. Classical models often assume that nucleation arises from slow, quasi-static slip governed primarily by fault weakening^15-21, typically neglecting impulsive precursory events. Here we show, using laboratory experiments and a rate-and-state-based Griffith-like rupture framework²², that foreshocks, when they occur at the onset of or during nucleation, can fundamentally regulate earthquake initiation. We find that the slip burst induced by foreshocks imparts a transient sliding velocity, V_min, whose magnitude is set by foreshock size and which robustly predicts both nucleation duration and spatial length. Larger foreshocks generate higher V_min and trigger a more rapid transition to dynamic rupture, whereas smaller foreshocks produce long-duration quasi-static growth and very small impulses lead to ruptures entirely arresting. Extending our theoretical framework to tectonic faults, we show that foreshock and associated slow-slip sequences preceding natural earthquakes seem to follow the same scaling. These observations allow us to constrain realistic characteristic nucleation slip distances of 0.3-3.0 mm, orders of magnitude smaller than those inferred for dynamic rupture²³. Our results demonstrate that foreshock-induced transients set the timing and potential detectability of earthquake nucleation²⁴.