In situ velocity-strain sensitivity near the San Jacinto Fault Zone analyzed through train tremors

We utilize train tremors as P-wave seismic sources to investigate velocity-strain sensitivity near the San Jacinto Fault Zone. A dense nodal array deployed at the Piñon Flat Observatory is used to detect and identify repeating train energy emitted from a railway in the Coachella valley. We construct P-wave correlation functions across the fault zone and estimate the spatially averaged dt/t versus strain sensitivity to be 6.25 × 10⁴. Through numerical simulations, we explore how the sensitivity decays exponentially with depth. The optimal solution reveals a subsurface sensitivity of 1.2 × 10⁵ and a depth decay rate of 0.05 km⁻¹. This sensitivity aligns with previous findings but is toward the higher end, likely due to the fractured fault-zone rocks. The depth decay rate, previously unreported, is notably smaller than assumed in empirical models. This raises the necessity of further investigations of this parameter, which is crucial to study stress and velocity variations at seismogenic depth.