Investigating body waves radiated by urbanized sedimentary basins for seismic monitoring

Seismic velocity monitoring using body waves has recently emerged as a promising approach to investigate temporal variations in crustal structures. In particular, body waves generated by specific, high-frequency (> 1 Hz) anthropic sources, such as train traffic, have been shown to provide stable and repeatable illumination of the crust at seismogenic depths (5 – 10 km). However, the applicability of this approach is geographically limited to targets located in the vicinity of these strong and continuous train-induced seismic sources. This limitation motivates the search for other opportune sources of continuous, high-frequency body waves for seismic monitoring.

To this end, we revisit the dense array data of the FaultScan experiment, originally deployed at the Piñon Flat Observatory (Southern California) to study the San Jacinto fault zone via seismicity analysis and velocity monitoring using train-induced signals. In this work, we look at quiet periods, in the absence of strong seismic events (no earthquakes, no train signals), in order to characterize the remaining ambient noise at high frequency (1 – 10 Hz), using beamforming and time-frequency analysis. During these quiet periods, we consistently observe body waves (both P and S waves) coming from the direction of the Los Angeles and Riverside urbanized sedimentary basins. The amplitude of these body waves exhibits a clear weekly pattern, suggesting that anthropogenic activity is their primary source mechanism. We hypothesize that anthropic sources within the basin may excite basin resonance and lead to the scattering of body waves at the basin-bedrock interface. We further demonstrate that these basin-radiated body waves can be successfully extracted by cross-correlation in the frequency band 2 – 6 Hz.

This wave field provides a good opportunity to investigate the characteristics of the basin resonance through seismic observations from outside the basins. Besides, by characterizing the spatiotemporal properties of body-wave energy from sedimentary basins, this study could extend the applicability of passive monitoring techniques using body waves to a wider range of tectonic and urban environments, including regions lacking strong specific anthropogenic sources such as trains.