A Comparative Seismological Study of the Banning and Mission Creek Strands of the Southern San Andreas Fault

We perform comparative analyses of seismic signals and fault-zone structures at the Banning Fault (BF) and Mission Creek Fault (MCF) strands of the Southern San Andreas Fault (SoSAF) in the Coachella Valley, California. The research utilizes data recorded by densely spaced linear and two-dimensional (2D) seismic arrays deployed across two faults. Continuous recordings are first segmented and classified into earthquakes, rail and road traffic events, and non-traffic noise using various detection algorithms. We then employ beamforming on traffic signals and noise to resolve apparent Rayleigh-wave velocities in the 5–10 Hz frequency band, imaging the upper $\sim$100 m of each fault damage zone. Exploiting the dual-2D array coverage, we further extend beamforming analyses to regional earthquakes to reconstruct body-wave propagation directions and quantify their azimuthal rotations induced by fault-zone structures at depths of $\sim$0.5–4 km. Our results reveal strongly asymmetric shallow damage zones on both strands, with velocity reductions of about 14.6% on the BF and 35.6% on the MCF toward the northeast. At greater depths, both faults are bimaterial interfaces that separate bounding rocks with higher seismic velocities on the northeast side, in agreement with previous large-scale P-wave tomography. Integrated observations document a depth-dependent reversal in velocity contrast across these bimaterial faults, consistent with rupture dynamics that promote northwestward propagation along the SoSAF and enhanced shallow damage accumulation on the stiffer block. The MCF, in particular, exhibits a pronounced asymmetric damage zone and increased wavefield scattering, affirming its role as the principal strand of the SoSAF in this region.