Monitoring P- and S-wave velocity changes in the San Jacinto fault zone (Southern California) using train tremors recorded by a long-term, dense nodal array

The San Jacinto fault is one of the most active faults in Southern California and a major cause of seismic hazard. Estimating its capability to generate large earthquakes requires a detailed understanding of its mechanical properties and of their temporal changes at seismogenic depths, which remain difficult to characterize with standard seismic data and methods. In this work, we use 2.5 years of seismic noise recorded by a dense array of 300 nodes deployed at the Piñon Flat Observatory (PFO) from April 2022 to October 2024 to monitor the San Jacinto fault zone with tremor-like signals generated by freight trains travelling in the nearby Coachella valley. Recent studies have shown that trains are powerful and repeatable sources of high-frequency (> 1 Hz) body waves that can be used for monitoring seismic velocity variations at seismogenic depths (5 - 10 km) by seismic interferometry, but they have only exploited P waves so far. Here we show that cross-correlating train tremors recorded on both sides of the San Jacinto fault zone enables us to retrieve both P and S waves, and therefore to measure relative changes of both V_P and V_S with time, from which we can derive relative changes of an effective V_P/V_S ratio in the sampled volume. While the 2.5-year-long observation period does not include significant earthquakes in the target area, our results show fluctuations of seismic velocities and of the derived V_P/V_S ratio, with long-term trends as well as more rapid changes. The quantitative interpretation of these variations remains to be specified, but they are likely related to changes in stress, porosity, or fluid pore pressure at depth. Combining these results with other observables (e.g., detailed catalogs of micro-seismicity) will provide valuable information on the dynamic mechanical behaviour of the San Jacinto fault, potentially yielding insights into the evolution of cracks and fluids in the fault zone.