Offshore Southern California lithospheric velocity structure from noise cross‐correlation functions

Abstract

A new shear wave velocity model offshore Southern California is presented that images plate boundary deformation including both thickening and thinning of the crustal and mantle lithosphere at the westernmost edge of the North American continent. The Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment (ALBACORE) ocean bottom seismometer array, together with 65 stations of the onshore Southern California Seismic Network, is used to measure ambient noise correlation functions and Rayleigh wave dispersion curves which are inverted for 3‐D shear wave velocities. The resulting velocity model defines the transition from continental lithosphere to oceanic, illuminating the complex history and deformation in the region. A transition to the present‐day strike‐slip regime between the Pacific and North American Plates resulted in broad deformation and capture of the now >200 km wide continental shelf. Our velocity model suggests the persistence of the uppermost mantle volcanic processes associated with East Pacific Rise spreading adjacent to the Patton Escarpment, which marks the former subduction of Farallon Plate underneath North America. The most prominent of these seismic structures is a low‐velocity anomaly underlying the San Juan Seamount, suggesting ponding of magma at the base of the crust, resulting in thickening and ongoing adjustment of the lithosphere due to the localized loading. The velocity model also provides a robust framework for future earthquake location determinations and ground‐shaking simulations for risk estimates.

Matched Field Processing of Three‐Component Seismic Array Data Applied to Rayleigh and Love Microseisms

“…In a second step the velocity in offshore regions is calculated as where v c is the 3.05 and 3.2 km/s for Rayleigh and Love waves, respectively, and d ( x , y ) is the spatially dependent bathymetry in meters with x and y being the spatial coordinates. The right‐hand side term reduces the velocity linearly with depth and at 4‐km depth the velocity is reduced by 1 km/s, which yields the observed Rayleigh wave phase velocity (Bowden et al, ). Love waves are assumed to be roughly 9% faster (Lin et al, ), which is in agreement with the above equation.…”

“…For secondary microseisms (8 s), where a large difference between the land and offshore phase velocity was observed (Bowden et al, ), the transition from oceanic to continental velocity can be significant and induce raypath bending. For this purpose a bathymetry‐dependent phase velocity is generated (in accordance with Bowden et al, ) and an example of the Love wave phase velocity map is shown in Figure a. Rayleigh and Love wave maps at 8 s are constructed in a two‐stage approach.…”

“…For secondary microseisms (8 s), where a large difference between the land and offshore phase velocity was observed (Bowden et al, 2016), the transition from oceanic to continental velocity can be significant and induce raypath bending. For this purpose a bathymetry-dependent phase velocity is generated (in accordance Journal of Geophysical Research: Solid Earth 10.1029/2018JB015526…”

“…with Bowden et al, 2016) and an example of the Love wave phase velocity map is shown in Figure 3a. Rayleigh and Love wave maps at 8 s are constructed in a two-stage approach.…”

Anisotropy from SKS splitting across the Pacific-North America plate boundary offshore southern California

“…The data from the OBSs are used to distinguish among contrasting upper-mantle geodynamic scenarios that predict large-scale mantle flow patterns beneath western North America. In addition to anisotropy, seismic velocities are being obtained through inversion of dispersion curve data calculated from surface waves and cross correlation of ambient noise functions (Bowden et al 2016), as well as receiver function analysis (Reeves et al 2015).…”