Structure of the Northern Los Angeles Basins Revealed in Teleseismic Receiver Functions from Short-term Nodal Seismic Arrays

The San Gabriel and San Bernardino basins are two wedge-shaped basins located adjacent to the San Andreas and San Jacinto fault zones in Southern California. Earthquake ground motions in the greater Los Angeles area are known to be affected by basin amplification and the channeling and focusing of seismic energy as it passes through the San Gabriel and San Bernardino basins in the northern part of this region. To help increase the accuracy of ground shaking models for the Los Angeles area, various studies have provided improvements to the shape of the northern basins, e.g., using gravity modeling, through finite difference simulations of ground motion and with active source seismic profiles across the San Bernardino basin. However, the current Southern California Earthquake Center velocity model, CVM-S4.26, still lacks sufficient detail and does not accurately represent the actual basin structure. Furthermore, simulated ground motions from a San Andreas fault earthquake are four times smaller than those measured with ambient noise cross-correlations, which is likely due to inaccuracies in the basin shape.

Our goal is to provide accurate estimates of the depths and shapes of the northern sedimentary basins in the greater Los Angeles area. Our data set is comprised of short period seismic data collected along three transects (202 nodes) in the greater Los Angeles area in 2017 and three additional transects (182 nodes) collected in 2018. In each year, the instruments recorded for 35 days. We use teleseismic receiver functions computed along three profiles to map the depth and shape of the sediment-basement interface and to identify possible deep fault offsets. The results show the Moho discontinuity, the bottom of the basement, and intermediary sedimentary layers. There are also indications of mid-crustal offsets along strike of the Red Hill and Raymond Faults. The results are compared to receiver functions from nearby permanent broadband stations and the 1993 LARSE profile. The images show that dense deployments of node-type sensors can be used to investigate structure at the basin scale in a noisy urban environment, and therefore have potential value for seismic hazard studies and hydrocarbon exploration.