Depth-Dependent Stress Sensitivity and Its Influence on Temporal Variations in Low-Frequency Rayleigh-Wave Velocities

Ambient noise interferometry is widely used to detect subsurface changes. These measurements primarily rely on Rayleigh waves or coda waves, with the latter typically interpreted as Rayleigh waves through the application of the diffusion approximation based on the selected lapse window. Converting or inverting the detected changes in Rayleigh- or coda-wave velocity into stress changes requires an understanding of how body-wave velocity responds to stress changes—a relationship defined by stress sensitivity. In this study, we extract stress sensitivity from laboratory experiments and demonstrate that it decreases dramatically with confining pressure and, consequently, with depth. We further collect field measurements to support this relationship. On this basis, we conduct synthetic tests to show that temporal changes in low-frequency (0.05–1 Hz) Rayleigh-wave velocity related to static stress changes may predominantly reflect conditions within the uppermost 1 km of the crust, as shallow S-wave velocities are much more sensitive to stress perturbations than deeper ones. We review previous seismological observations and identify inconsistencies in some interpretations when the depth dependence of stress sensitivity is overlooked, suggesting that the depth corresponding to the detected velocity change may be much shallower than claimed in some previous studies. Our results provide a theoretical foundation by establishing the quantitative relationship between body-wave velocity changes and stress changes, which can improve the interpretation of temporal variations in wavespeeds and emphasize the need for careful analysis given the complexities of real-Earth observations.