Shift and Increase in Tidal Modulation of Seismicity in the Ridgecrest Fault Zone Before and After the 2019 Magnitude 7.1 Earthquake

We studied the tidal modulation of the rate of seismicity in the Ridgecrest, California area between 2008 and 2024, thus including the 2019 Mw 7.1 earthquake. We combined machine-learning-enhanced earthquake detection and location with a novel methodology to characterize tidal modulation. We identified significant modulation of seismicity in the Ridgecrest fault zone and in the neighboring regions, the Coso volcanic field and the Garlock fault, but tidal modulation in the fault zone exhibits a unique temporal evolution. Modulation in the fault zone shifts, around 2014–2015, from a stress-controlled regime where peak seismicity occurs at peak tidal stress to a stress-rate-controlled regime where peak seismicity occurs at peak tidal stress rate. In contrast, peak seismicity in Coso and along the Garlock fault consistently occurs at peak tidal stress throughout time. Moreover, the strength of tidal modulation in the fault zone increases before the 2019 Mw 7.1 earthquake, unlike in its surroundings. A possible interpretation based on experimental and theoretical work suggests that some characteristic frictional time scale (the nucleation time or the critical period of the fault) becomes shorter than the dominant tidal period after 2015. This change implies that the fault loading rate increases, which may be accompanied by a decrease in effective normal stress. Reduced effective stress, fault zone resonance, or increased mechanical compliance could cause the increase in modulation strength. We propose that these changes manifest a preparation phase before a large earthquake.