Abstract Seasonally frozen soil, a major component of the cryosphere in high-altitude regions, exerts a strong control on hydrological, ecological, and geotechnical processes, with important implications for slope stability, infrastructure performance, and subsurface carbon cycling. Monitoring its freeze–thaw dynamics is therefore critical for understanding surface–subsurface coupling and assessing the stability of near-surface materials under climate variability. Here, we combine passive seismic observations with thermal diffusion modeling to investigate the temporal evolution of the active layer in the central Tibetan Plateau. Using two years of continuous seismic data from three broadband stations, we apply ambient noise autocorrelation functions and horizontal-to-vertical spectral ratios (HVSRs) to quantify seasonal variations in seismic velocity (dv/v) and impedance contrast. The dv/v results exhibit pronounced seasonal cycles, with increases of up to 5% during winter freezing and decreases of up to 2% during summer thaw, closely correlated with near-surface temperature and modeled frozen soil depth. A 1D heat conduction model incorporating latent heat and phase-change effects reproduces the observed thermal evolution, indicating an active-layer thickness of approximately 2.5 m. Seasonal HVSR amplitude variations further confirm impedance changes associated with freeze–thaw transitions. These findings demonstrate that passive seismic monitoring provides a robust, noninvasive approach for tracking active-layer dynamics and offers a scalable framework for evaluating subsurface freeze–thaw processes across cold regions worldwide.
Zhao et al. (Mon,) studied this question.