Eikonal Tomography of the Southern California Plate Boundary Region

Abstract

We use Eikonal tomography to derive phase and group velocities of surface waves for the plate boundary region in Southern California. Seismic noise data in the period range 2 and 20 s recorded in year 2014 by 346 stations with ~1‐ to 30‐km station spacing are analyzed. Rayleigh and Love wave phase travel times are measured using vertical‐vertical and transverse‐transverse noise cross correlations, and group travel times are derived from the phase measurements. Using the Eikonal equation for each location and period, isotropic phase and group velocities and 2‐psi azimuthal anisotropy are determined statistically with measurements from different virtual sources. Starting with the SCEC Community Velocity Model, the observed 2.5‐ to 16‐s isotropic phase and group dispersion curves are jointly inverted on a 0.05° × 0.05° grid to obtain local 1‐D piecewise shear wave velocity (Vs) models. Compared to the starting model, the final results have generally lower Vs in the shallow crust (top 3–10 km), particularly in areas such as basins and fault zones. The results also show clear velocity contrasts across the San Andreas, San Jacinto, Elsinore, and Garlock Faults and suggest that the San Andreas Fault southeast of San Gorgonio Pass is dipping to the northeast. Investigation of the nonuniqueness of the 1‐D Vs inversion suggests that imaging the top 3‐km Vs structure requires either shorter period (≤2 s) surface wave dispersion measurements or other types of data set such as Rayleigh wave ellipticity.

Tectonic Inheritance During Plate Boundary Evolution in Southern California Constrained From Seismic Anisotropy

“…While regional‐scale surface wave inversions for crustal azimuthal anisotropy are not yet available (Qiu et al., 2019), crustal radial anisotropy shows anomalous negative values (vertically polarized shear wave speeds higher than horizontally polarized shear wave speeds) in the mid‐to lower crust, mostly south and west of the SAF (K. Wang et al., 2020). There are several possible explanations for this observation.…”

Shallow Crustal Shear Velocity and Vp/Vs Across Southern California: Joint Inversion of Short‐Period Rayleigh Wave Ellipticity, Phase Velocity, and Teleseismic Receiver Functions

“…(2018). In addition to Rayleigh‐wave H/V ratio measurements from 6 to 10 s periods, we use 3–10 s periods Rayleigh‐wave phase velocities from previous ambient‐noise‐based eikonal tomography (Qiu et al., 2019) extracted at the inversion grid point nearest to each station.…”

“…The isotropic H/V ratio and uncertainty are determined from the mean and standard deviation of the mean, respectively, for each station with at least 20 measurements remaining after removing outliers; more details can be found in Berg et al (2018). In addition to Rayleigh-wave H/V ratio measurements from 6 to 10 s periods, we use 3-10 s periods Rayleigh-wave phase velocities from previous ambient-noise-based eikonal tomography (Qiu et al, 2019) extracted at the inversion grid point nearest to each station.…”

Tectonic Inheritance With Dipping Faults and Deformation Fabric in the Brittle and Ductile Southern California Crust

“…The main additional active strike‐slip faults south of the Big Bend in the SAF include the San Jacinto Fault (SJF) and Elsinore Fault (EF) lying between the SAF and the coast to the southwest. The subsurface geometry of strike‐slip faults including the SAF is generally treated as vertical outside of fault bends (e.g., Langenheim et al., 2005; Plesch et al., 2007), but recent work proposed dipping geometry at depth along most of the SAF including straight sections in Southern California (Barak et al., 2015; Fuis et al., 2012, 2017; Qiu et al., 2019; Share et al., 2019) and for parts of the SJF (Ross et al., 2017), with all dips to the northeast except for a SW dipping segment just south of the central SAF (Fuis et al., 2012).…”