Effects of Shallow-Velocity Reductions on 3D Propagation of Seismic Waves

Abstract

We perform 3D simulations of seismic wavefields to clarify effects of strong reductions of shallow velocities on long-period seismic waves. The simulations use a reference Community Velocity Model of southern California and a modified version with strong velocity reductions in the top 500 m of the Los Angeles basin. Differences between wavefields generated by 10 earthquakes in the reference and perturbed models are analyzed. Velocity changes are estimated by measuring relative time shifts between reference and perturbed seismograms using wavelet cross-correlation spectra. The results indicate that strong localized temporal velocity drops near the surface, such as those observed during strong ground motions, may generate regional perturbations of wavefields at periods up to 20 s. These perturbations may be misinterpreted as generated by temporal changes at seismogenic depths. The results also have important implications for waveform tomography studies.

Spatial Dependence of Dynamic Nonlinear Rock Weakening at the Alpine Fault, New Zealand

“…The low characteristic pressures we observe are consistent with previous laboratory (Johnson & Jia, 2005;Zinszner et al, 1997), field (Rubinstein & Beroza, 2005;Sens-Schönfelder & Eulenfeld, 2019;Wu et al, 2009), and numerical (Juarez & Ben-Zion, 2020) studies that show nonlinear shear modulus decreases from large dynamic strains are confined to the top several hundred meters of the crust. While these shallow changes are important for explaining phenomena such coseismic velocity decreases, our results do not preclude the possibility of nonlinear weakening in these rocks influencing fault processes at greater (nucleation) depths.…”

Monitoring Seasonal Shear Wave Velocity Changes in the Top 6 m at Garner Valley in Southern California With Borehole Data

“…Our analysis based on data from densely deployed borehole instruments indicates V S variations up to 25%, concentrated only in the top 6 m. Modeling results (Juarez & Ben-Zion, 2020;Yang et al, 2019) show that temporal changes in shallow structures may alter the wavefield over larger scales, and that changes in shallow materials may be improperly interpreted as variations in deep structure. In situ observations indicate that material variations tend to concentrate at shallow surface layers, while the bedrock below remains largely unchanged (e.g., Qin et al, 2020;Rubinstein & Beroza, 2005).…”

“…Near‐surface materials have extremely low shear wave velocities ( V S ) of 100–400 m/s (e.g., Boore et al., 2011; Tanimoto & Wang, 2021; Zigone et al., 2019), and sustain variations caused by earthquakes (e.g., Nakata & Snieder, 2011; Wu et al., 2010), temperature (e.g., Oakley et al., 2021), and water level changes (e.g., Mao et al., 2022). The properties and high susceptibility to failure of shallow materials affect near‐surface processes, influence the stability of vast infrastructure, alter the seismic recordings at the surface, and can mask information from deep structures (Juarez & Ben‐Zion, 2020; Yang et al., 2019). Imaging properties of near‐surface materials and monitoring how they respond to loadings can provide important constraints on the rheology of shallow materials with implications for seismic hazard estimates, structural engineering design, and various near‐surface studies.…”

Effect of Merging Multiscale Models on Seismic Wavefield Predictions Near the Southern San Andreas Fault

“…However, with the Ely geotechnical layer, the actual near-surface velocities are lower than 200 m/s. Even for long periods, the shallow low velocity layer is known to influence the seismic wavefield and, as such, represents a source of error (Juarez & Ben-Zion, 2020). Also, the Salton Sea, which should be an acoustic domain, is not incorporated in the simulations.…”