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 Pi & ntilde;on 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 x 104. Through numerical simulations, we explore how the sensitivity decays exponentially with depth. The optimal solution reveals a subsurface sensitivity of 1.2 x 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. The speed at which seismic waves travel can be affected by Earth's tidal strains. Understanding this relationship is beneficial for studying tectonic strain accumulation and earthquake nucleation. When freight trains run, they produce powerful seismic energy that can be detected tens of kilometers away. We use these signals to measure how solid Earth tides affect seismic wave speed. Our study focusing on the San Jacinto Fault Zone in southern California reveals that the velocity-strain sensitivity is consistent, albeit at the higher end of previously reported values measured in other regions. Additionally, our numerical simulations examine how this sensitivity varies with depth. We find that the rate at which sensitivity decreases with depth is smaller than what is typically assumed. Stable P-wave correlation functions are constructed from selected train tremors The covariance between tidal strain and P-wave travel-time is used to estimate the velocity-strain sensitivity Full-waveform simulations of correlation functions are performed to constrain the depth dependence of the velocity-strain sensitivity