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

Abstract:

We investigate the mechanism of local P-to-Rayleigh wave conversions recorded by a dense array deployed in Long Beach, Southern California, and derive high-resolution information on fault zone (FZ) structures along the Newport-Inglewood Fault Zone (NIFZ) by seismic waveform forward modelling. We observe a local Rayleigh wave having circular wavefronts at a velocity of 1 km/s arises from the Signal Hill structural pop-up in teleseismic P wave data from a large Fiji Islands earthquake. A group of high spatial frequency, low velocity 0.7 to 0.9 km/s Rayleigh waves having linear wavefronts also propagate from the NIFZ, indicating that P-to-Rayleigh wave conversions from fault damage zones are quite large in the Los Angeles Basin. We compute synthetic waveforms to constrain the fault zone parameters. A low velocity zone (LVZ) having a depth of 200-400m, an aperture of ~700m, with 40% reduction in Vp and ~30% reduction in Vs is situated under Signal Hill. The NIFZ is characterized by high velocity (HVZ) having a depth of 200-400m, a width of ~100-120m, with a ~20% increase in Vp and ~20% increase in Vs compared to the background model. The LVZ and HVZs dip toward the northeast at an angle of 60-70, deduced by the difference in scattering between two teleseismic earthquakes with different back azimuths. The clear signatures of damage zones in Long Beach improves our understanding of wave propagation complexities in the Los Angeles Basin and helps reveal geological structure of the NIFZ. 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.