Rayleigh Wave Ellipticity and Off-Great-Circle Propagation Across the Los Angeles Basin via Dense Seismic Surveying

We deployed a temporary nodal array in and around the Los Angeles basin, comprising 273 nodal geophone seismometers, structured by two dense lines (interstation spacing of ∼0.6 km) and a 2D distributed or shotgun array (spacing of ∼2 km), which significantly densified the station coverage in the area. We compute multicomponent ambient-noise cross correlations between all stations available in Southern California, including broadbands and accelerometers from the regional networks and broadband and nodal stations from temporary experiments. We observe clear fundamental-mode Rayleigh waves in the period band of 5–9 s and measure the Rayleigh wave ellipticity, or horizontal-to-vertical (H/V) amplitude ratios. Furthermore, we correct the measurements to account for off-great-circle propagation due to lateral velocity variations. The measured Rayleigh wave ellipticity depicts the basin's lateral boundaries, where higher and lower H/V are generally observed within and outside of the basin, respectively. We compare our findings with the predicted H/V ratios from the Statewide California Earthquake Center Community Velocity Model (CVM). Clear discrepancies are observed near the Central and the West Coast basins, where the depth velocity gradients of the CVM are likely too low and too high, respectively. Because an accurate basin model is important to earthquake ground-motion prediction, our newly acquired results demonstrate the utility of dense nodal arrays for shallow imaging in a densely populated urban setting for seismic hazard assessment.