Immediate Foreshocks Indicating Cascading Rupture Developments for 527 M 0.9 to 5.4 Ridgecrest Earthquakes

Abstract Understanding earthquake foreshocks is essential for deciphering earthquake rupture physics and can aid seismic hazard mitigation. With regional dense seismic arrays, we identify immediate foreshocks of 527 0.9 M 5.4 events of the 2019 Ridgecrest earthquake sequence, including 48 earthquakes with series of immediate foreshocks. These immediate foreshocks are adjacent to the mainshocks occurring within 100 s of the mainshocks, and their P waves share high resemblances with the mainshock P waves. However, attributes of the immediate‐foreshock P waves, including the amplitudes and preceding times, do not clearly scale with the mainshock magnitudes. Our observations suggest that earthquake rupture may initiate in a universal fashion but evolves stochastically. This indicates that earthquake rupture development is likely controlled by fine‐scale fault heterogeneities in the Ridgecrest fault system, and the final magnitude is the only difference between small and large earthquakes.