Train Vibrations Reveal Internal Changes in a High-Speed Railway Viaduct System through Dispersion-Change Patterns of a Fast Guided Wave

High-speed trains generate highly repeatable seismic vibrations, whose changes are expected to reveal how railway infrastructure changes over time. From a dense array and 28 days of trains, we use seismic interferometry to extract a fast guided wave along the viaduct hour by hour. The wave travels at about 3000 m/s with a single S-shaped branch, and its dispersion has two change patterns: a diurnal cycle of up to 4% that reverses near 7 Hz, and a 0.6% multi-day change. Fitting attributes the diurnal cycle to the resonance frequency and the coupling parameter, both tracking air temperature.The multi-day change can come from the waveguide velocity and coincides with an irrigation-driven groundwater fall. Together, these observations indicate that the viaduct system is locally resonant, and the frequency pattern identifies which internal parameter of the system has changed. This offers a new seismological means for monitoring railway bridge health.