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

Abstract

S U M M A R YSKS arrivals from ocean bottom seismometer (OBS) data from an offshore southern California deployment are analysed for shear wave splitting. The project involved 34 OBSs deployed for 12 months in a region extending up to 500 km west of the coastline into the oceanic Pacific plate. The measurement process consisted of removing the effects of anisotropy using a range of values for splitting fast directions and delay times to minimize energy along the transverse seismometer axis. Computed splitting parameters are unexpectedly similar to onland parameters, exhibiting WSW-ENE fast polarization directions and delays between 0.8 and 1.8 s, even for oceanic plate sites. This is the first SKS splitting study to extend across the entire boundary between the North America and Pacific plates, into the oceanic part of the Pacific plate. The splitting results show that the fast direction of anisotropy on the Pacific plate does not align with absolute plate motion (APM), and they extend the trend of anisotropy in southern California an additional 500 km west, well onto the oceanic Pacific plate. We model the finite strain and anisotropy within the asthenosphere associated with density-buoyancy driven mantle flow and the effects of APM. In the absence of plate motion effects, such buoyancy driven mantle flow would be NE-directed beneath the Pacific plate observations. The best-fit patterns of mantle flow are inferred from the tomography-based models that show primary influences from foundering higher-density zones associated with the history of subduction beneath North America. The new offshore SKS measurements, when combined with measurements onshore within the plate boundary zone, indicate that dramatic lateral variations in density-driven upper-mantle flow are required from offshore California into the plate boundary zone in California and western Basin and Range.

Coupled influence of tectonics, climate, and surface processes on landscape evolution in southwestern North America

“…This shallow eastward flow of mantle asthenosphere relative to an upper lithosphere that is undergoing BRP extension and transform margin-associated shear (Fig. 6 ) agrees with results inferred from shear-wave splitting observations 59 , 60 . Therefore, our 4-D simulations of the lithosphere evolution in SWNA includes this important basal velocity boundary condition (Fig.…”

Lithospheric Deformation and Asthenospheric Flow Associated With the Isabella Anomaly in Southern California

“…(a) Comparison of fast velocity directions (white bars) of P wave azimuthal anisotropy at 75‐km depth determined by this study with the distribution of delay times (colors) of shear wave splitting (SWS) measurements (Barak & Klemperer, ; Ramsay et al, ; Yang et al, ). (b) Comparison of the lithospheric thickness (Lekic et al, ) with P wave azimuthal anisotropy (white bars) at 200‐km depth determined by this study.…”

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

“…The signal is typically interpreted as being dominated by the asthenospheric mantle, with some contribution from the lithosphere and SAF shear (e.g., Savage et al., 2004) and small to negligible influence of the crust (Silver, 1996). While the stress field has nevertheless been invoked to explain SKS splitting in our study region (Polet & Kanamori, 2002), a more common explanation is large‐scale mantle flow and asthenospheric convection leading to lattice preferred orientation olivine fabrics (e.g., Becker, Schulte‐Pelkum, et al., 2006; Bonnin et al., 2010; Savage & Sheehan, 2000; Silver & Holt, 2002; Ramsay et al., 2016; Zhou et al., 2018). Alignment with shear as expected from absolute plate motions does not match the observed orientations well (Bonnin et al., 2010; Silver & Holt, 2002), but modeling of plate motion and density‐driven mantle flow, without any deep shear localization at the plate boundary, captures SKS patterns on scales of ≳200 km (Becker, Schulte‐Pelkum, et al., 2006).…”