Seismic Anisotropy beneath Cascadia and the Mendocino Triple Junction: Interaction of the Subducting Slab with Mantle Flow
Seismic Anisotropy beneath Cascadia and the Mendocino Triple Junction: Interaction of the Subducting Slab with Mantle Flow Figure. Regional splitting pattern overlain on the vertically averaged upper mantle velocity anomaly. Our splitting results are shown in black and those of previous studies are in grey (Long et al., 2009; Wang et al., 2008; West et al., 2009; Zandt and Humphreys, 2008 and references therein). The splitting pattern is shown to be uniform trench-normal throughout Cascadia. The velocity anomaly shown is a vertical average of the velocity anomaly from the DNA09 P-wave model over the 100-400km depth range (Obrebski et al., 2010). The slab is imaged as the north-south high velocity feature. The splitting measurements rotate around the southern end of the slab. Curved black lines on the G-JdF plate represent the direction of its absolute plate motion.
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Mantle flow associated with the Cascadia subduction zone and the Mendocino Triple Junction is poorly characterized due to a lack of shear wave splitting studies compared to other subduction zones. To fill this gap data was obtained from the Mendocino and FACES seismic networks that cover the region with dense station spacing. Over a period of 11-18 months, 50 suitable events were identified from which shear wave splitting parameters were calculated. Here we present stacked splitting results at 63 of the stations. The splitting pattern is uniform trench normal (N67°E) throughout Cascadia with an average delay time of 1.25 seconds. This is consistent with subduction and our preferred interpretation is entrained mantle flow beneath the slab. The observed pattern and interpretation have implications for mantle dynamics that are unique to Cascadia compared to other subduction zones worldwide. The uniform splitting pattern seen throughout Cascadia ends at the triple junction where the fast directions rotate almost 90°. Immediately south of the triple junction the fast direction rotates from NW-SE near the coast to NE-SW in northeastern California. This rotation beneath northern California is consistent with flow around the southern edge of the subducting Gorda slab.
</p><p>References
</p><p>Eakin, C.M., M. Obrebski, R.M. Allen, D.C. Boyarko, M.R. Brudzinski, R. Porritt, Seismic Anisotropy beneath Cascadia and the Mendocino Triple Junction: Interaction of the subducting slab with mantle flow, in review.
</p><p>Acknowledgements: This work was funded by NSF EAR-0745934 and EAR-0643077. The work was facilitated by the IRIS-PASSCAL program through the loan of seismic equipment, USArray for providing data and the IRIS-DMS for delivering it.</p>