Basin Structure Revealed in Receiver Functions from a Dense Nodal Seismic Array in the Northern Los Angeles Basins, Southern California

Abstract:

The northern basins in the Los Angeles area are well known for having a significant impact on ground motion and shaking from large earthquakes. Sedimentary rocks in the basins and the basins’ shape and depth are responsible for focusing and trapping seismic energy and amplify seismic waves which can enhance ground shaking. For an accurate earthquake hazard estimation, it is crucial to understand basin amplification, and a detailed basin structure is required for that. The goal of our study is to map the crustal structures underlying the San Gabriel (SG) and San Bernardino (SB) basins, the northern basins in this area. Four sets of deployment were conducted where 719 nodal seismometers were installed along 10 profiles in the two basins. Receiver functions were computed using recordings of six teleseismic earthquakes of magnitude >6.0 and distance 30º-90º for three seismic lines in the SB basin (SB2, SB3 and SB6) and two seismic lines in the SG basin (SG3 and SG4). Good receiver functions were resolved from 75-80% of the data in the SB basin and >95% data in the SG basin. Receiver function interpretations based on qualitatively consistent arrivals show the Moho Ps conversions at ~3 s in the north to ~5 s in the south for the N-S trending SB2 and SB3 lines. The E-W trending SB6 line shows Moho conversions at ~5 s with less variation. The SG basin lines show Moho conversions at ~3 s. A possible Moho offset is observed under the Indian Hill fault in SG3 and SG4. However, the fault cannot be traced to the sediment-basement interface for SG4 unlike SG3. 2 Hz receiver functions were computed to identify sediment-basement interfaces since they were not well resolved from 1 Hz receiver functions. A previous study using gravity data shows Peninsular Ranges batholithic rocks along SB2 and Pelona Schist basement along SB3. Our results show three different patterns of amplitude and frequency content in receiver functions along these lines which may indicate different crustal blocks. Another previously published SB4 line shows similar patterns in receiver function results which supports our interpretation of potential basement rock heterogeneity in the SB basin. CCP (Common Conversion Point) stacking technique will be applied to receiver function along all 10 profiles using an ambient noise shear wave velocity model to map basin depths along these profiles.