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Published October 2016 | Published + Supplemental Material
Journal Article Open

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


SKS 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.

Copyright and License

© The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society.


We acknowledge Brian Clements for providing the horizontal-component orientation measurements used in this study, and we thank Thorsten Becker for his help and sharing of codes. The OBS waveform data from the ALBACORE array are available from the IRIS Data Management Center. The IRIS Data Management System is funded through the National Science Foundation, specifically the GEO Directorate through the Instrumentation and Facilities Program under Cooperative Agreement EAR-1063471. The OBS deployment was made possible with instruments and logistical support of the IRIS Ocean Bottom Seismic Instrumentation Pool (OBSIP); in particular, thanks go to Jeff Babcock, Ernie Aaron, Phil Thai and Mark Gibaud at Scripps Institution of Oceanography. The ALBACORE OBS deployment and recovery cruises were made possible with the equipment and logistical support of the University-National Oceanographic Laboratory System (UNOLS) vessel fleet and staff support at Scripps with particular thanks to Jon Meyer, Brian Rowe and Meghan Donohue. MDK and DSW thank Captain Curl and the crew of R/V Melville for assistance during the 2010 OBS deployment cruise, and Captain Vullo and the crew of R/V New Horizon during the 2011 OBS recovery cruise. This work was supported by the National Science Foundation (grant # OCE-0825254).

Attached Files

Published - Geophys._J._Int.-2016-Ramsay-244-58.pdf

Supplemental Material - Ramsay_etal_Supplementary_GJI_05_18_2016.pdf


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August 22, 2023
March 6, 2024