Environmental Controls on the Near-Surface Seismic Velocity Changes in the South China Coastal Area

Abstract Monitoring the seismic velocity changes (Δv/v) is crucial for deciphering tectonic and environmental activities. Yet, the contribution of various factors to the measured Δv/v is often difficult to disentangle, as different processes could operate simultaneously. In this study, we take advantage of a dense nodal array deployed in the South China coastal area—a tectonically inactive region with limited seismicity and no volcanic activity—and apply the single-station ambient noise monitoring approach to obtain the Δv/v of the near-surface. We employ the sequencing clustering analysis on the obtained Δv/v series associated with each station over the entire region and identify patterns of velocity changes that correlate primarily with rainfall. The two intense rainfalls during the observation period disrupt the otherwise periodic Δv/v series, which align with thermal and tidal variations. Focusing on the semidiurnal component of the Δv/v that is most sensitive to tidal strain, we obtain a sensitivity of −5.8×103 across our study region. Our results imply that the elastic properties of the near-surface are primarily affected by precipitation in regions without tectonic activity. The obtained Δv/v could also provide valuable baseline references when tectonic effects on the near-surface are of interest.