Shear Velocity Images of the Cascadia ETS Source Region

Shear Velocity Images of the Cascadia ETS Source Region Results and interpretation of shear velocity imaging of Western Washington from spectral ambient noise analysis. Shaded relief map in the background shows PASSCAL stations (black circles), locations of major cities and topographic features, and the depth to the subducting slab in km (black lines and numbers, from McCrory et al, 2004). Upper panel shows shear velocity profile along line B-B’, highlighting the low Vs zone in the North American plate directly above the portion of the subducting Juan de Fuca slab (thick black line, after Abers et al, 2009) expected to be under- going dehydration reactions. Lower right panel is a cartoon illustrating the main features of the Vs model near the center of the CAFE array and possible interpretations.
High-resolution 3D shear velocity (Vs) images of the western Washington lithosphere reveal structural segmentation above and below the plate interface correlating with transient deformation patterns. Using a spectral technique [Ekström et al, 2009], we extracted phase velocities from cross-correlated ambient noise recorded by the densely spaced “CAFE” Earthscope FlexArray. The spectral approach resolves shear velocities at station offsets less than 1 wavelength, significantly shorter than typically obtained by standard group-velocity approaches, increasing the number of useable paths and resolution. Tomographic images clearly illuminate the high Vs (>4.5 km/sec) subducting slab mantle. The most prominent anomaly is a zone of low Vs (3.0-3.3 km/sec) in the mid to lower continental crust, directly above the portion of the slab expected to be undergoing dehydration reactions. This low velocity zone (LVZ), which is most pronounced beneath the Olympic Peninsula, covers an area both spatially coincident with and updip of the region of most intense episodic tremor and slip (ETS). The low Vs and comparison with pub- lished P-wave velocity models indicate that Vp/Vs ratios in this region are greater than 1.9, suggesting a fluid rich lower crust. The LVZ disappears southward, near 47° N, coincident with sharp decreases in intraslab seismicity and ETS activity as well as structural changes in the slab. The spatial coincidence of these features suggests that long-term fluid-fluxing of the overriding plate via dewatering of a persistently hydrated patch of the Juan de Fuca slab may partially control slip on the plate interface and impact the rheology of the overriding continental crust.
</p><p>Abers G.A., L.S. Mackenzie, S.Rondenay, Z. Zhang, A.G. Wech, and K.C. Creager, 2009. Imaging the source region of Cascadia tremor and intermediated-depth earthquakes. Geology, 37, 1119-1122.
</p><p>Calkins, J.A., G.A. Abers, G. Ekstrom, K.C. Creager, and S. Rondenay. Shallow structure of the Cascadia subduction zone beneath western Washington from spectral ambient noise correlation. Submitted to J. Geophys. Res., 2010.
</p><p>Ekström, G., G. A. Abers, and S. C. Webb, 2009. Determination of surface-wave phase velocities across USArray from noise and Aki’s spectral formulation. Geophys. Res. Lett., 36, L18301.
</p><p>Acknowledgements: This work was funded by the National Science Foundation grant EAR-0544847.</p>


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