Poster #205, Seismology

Subsurface seismic properties across the southern San Andreas Fault in the Thousand Palms Canyon based on train-generated seismic waveforms

Hao Zhang, Haoran Meng, & Yehuda Ben-Zion
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Poster Presentation

2021 SCEC Annual Meeting, Poster #205, SCEC Contribution #11414 VIEW PDF
We use seismic waveforms generated by freight trains in the Coachella Valley and recorded by dense seismic array sensors to image the shallow structure of the Southern San Andreas Fault Zone (SoSAFZ). The array was deployed for one month with 322 nodes near the Thousand Palms Oasis Preserve, CA, and consisted of a 4-km-long linear array across the SoSAFZ and two 2D subarrays centered on the Banning Fault (BF) and Mission Creek Fault (MCF). Fifty train traffic events on a railway 5 km away in the Coachella Valley were manually detected and the associated waveforms analyzed for subsurface structural information. Particle motion analysis shows that the moving freight trains generate strong Rayl...eigh waves across the array. To model the wave propagation, we first determine the propagation direction of Rayleigh wave in a 60-sec-long moving window by obtaining the time delay of neighboring stations using cross-correlations of vertical waveforms band-passed filtered in the range 1-5 Hz. The location of each train event on the railway for the time windows of interest are determined by the propagation directions obtained at the two 2D subarrays and show continuous movement of the trains with about 42.2 km/hr. Rayleigh wave velocities are measured for each time window and each neighboring station pair based on the delay time and wave propagation direction. The results of the 50 train traffic events in different time windows indicate a consistent low-velocity zone around BF and a strong velocity reduction across MCF towards the northeast. The median amplitudes of the waveforms show strong variations across the array, including anomalies near the BF and MCF strands, which can be further utilized to estimate the Q-values across the fault zone.