Toward Recognizing the Waveform of Foreshocks

The identification of seismic precursors remains a fundamental challenge. Foreshocks are often indistinguishable from regular seismic sequences, making it difficult to determine whether they precede a larger rupture. We show that the ground velocity envelope recorded after several Mw6+ foreshocks exhibits an anomalous sawtooth pattern, distinct from typical post-mainshock signals. This pattern suggests the presence of rate-weakening fault patches approaching instability, promoting stress transfer and aftershock migration into neighboring critically stressed regions. A similar signature was observed in multiple events, including the 2011 Mw9.1 Tohoku earthquake and the 2014 Mw8.1 Iquique sequence. To assess the systematic occurrence of this anomaly, we introduce an index Q based on the first 45 min of waveform data. Analyzing 68 M6+ earthquakes in selected regions since 2011, we find that 10 of 11 foreshocks preceding a larger event exhibit anomalous Q values, while only 4 of 57 other events show similar behavior. These findings suggest that foreshock waveform characteristics may provide insight into seismic rupture processes.