Lateral Variations Across the Southern San Andreas Fault Zone Revealed From Analysis of Traffic Signals at a Dense Seismic Array

We image the shallow seismic structure across the Southern San Andreas Fault (SSAF) using signals from freight trains and trucks recorded by a dense nodal array, with a linear component perpendicular to SSAF and two 2D subarrays centered on the Banning Fault and Mission Creek Fault (MCF). Particle motion analysis in the frequency band 2-5 Hz shows that the examined traffic sources can be approximated as moving single- or multi-point sources that primarily induce Rayleigh waves. Using several techniques, we resolve strong lateral variations of Rayleigh wave velocities and Q-values across the SSAF, including 35% velocity reduction across MCF toward the northeast and strong attenuation around the two fault strands. We further resolve 10% mass density reduction and 45% shear modulus decrease across the MCF. These findings suggest that the MCF is currently the main strand of the SSAF in the area with important implications for seismic hazard assessments.

Automated summary

We used signals from freight trains and trucks to image the shallow seismic structure of the Southern San Andreas Fault (SSAF). Our findings reveal strong lateral variations of Rayleigh wave velocities and Q-values across the SSAF, including a 35% velocity reduction across the Mission Creek Fault (MCF) towards the northeast and strong attenuation around the two fault strands. We also observed a 10% mass density reduction and 45% shear modulus decrease across the MCF.