TeleseismicP-to-Rayleigh Conversions from Near-Surface Geological Structure along the Newport–Inglewood Fault Zone in Long Beach, California

Abstract

ABSTRACTWe investigate near-surface 3D structure giving rise to P-to-Rayleigh wave conversions from teleseismic P waves recorded by the Long Beach array, southern California. Our previous study demonstrated that a local Rayleigh wave having circular wavefronts with phase velocities of ∼1 km/s arise from Signal Hill in array P wave data from a large Fiji Islands earthquake. A group of high-spatial frequency, low velocity 0.7–0.9 km/s Rayleigh waves having linear wavefronts also propagate away from strands of the Newport–Inglewood fault zone (NIFZ), suggesting that P-to-Rayleigh wave conversions from fault damage zones can also be observed. We compute synthetic waveforms using 3D finite difference to show that topography of Signal Hill accounts for much of the circular P-to-Rayleigh wave conversions. The NIFZ is best modeled by low-velocity, vertical tabular features above a depth of 500 m, with a width of 100–120 m, and ∼15% reduction in VP and VS compared with the background model. We observe that structure from the northwestern part of the inferred southwest boundary fault of the Signal Hill anticline dominates the scattering from fault damage zones. It is remarkable that the combination of low, near-surface velocity with relatively small-scale heterogeneity can significantly affect the signature of long horizontal wavelength teleseismic P waves, suggesting additional complexities in interpreting receiver functions for stations on deep sedimentary basins or in areas of significant topography.