Abstract Laboratory experiments and theoretical models suggest that earthquakes are preceded by extended nucleation phases, perhaps by slow but accelerating slip. However, such nucleation phases are hard to observe before natural earthquakes. Here we identify clustered foreshock sequences that could be nucleation signatures. We develop a coherence‐based power metric to detect foreshock sequences along the San Jacinto fault zone (SJFZ) and then track the temporal evolution of foreshocks’ moment‐rate power. The results show that a small but significant fraction of M ≥ 2.5 earthquakes (19 out of 681) are preceded by 5 to 20‐s‐long clustered foreshock sequences, which may reflect extended nucleation phases. The sequences preferentially occur near the base of the seismogenic zone, which likely contains frictionally heterogeneous patches of varying sizes. We identify a build‐up of 2‐8‐Hz moment‐rate power during the sequences and consider some interpretations: that the growing power reflects accelerating aseismic slip or growing cascades of ruptures.

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