The Isabella Anomaly Imaged by Earthquake and Ambient Noise Rayleigh Wave Dispersion Data: A Composite Anomaly of Sierra Nevada

The Isabella Anomaly Imaged by Earthquake and Ambient Noise Rayleigh Wave Dispersion Data: A Composite Anomaly of Sierra Nevada Batholith Root Foundering and Pacific Plate Slab-Flap Translation? Shear velocity image from inversio of ambient and earthquake Rayleigh wave dispersion measurements using SNEP and Transportable array data
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Sierra Nevada Earthscope Project (SNEP) and Earthscope Transportable Array (TA) Rayleigh wave dispersion data are inverted to construct a shear velocity model of the Sierra Nevada and San Joaquin valley. The Rayleigh wave dispersion dataset was measured using the two-plane wave method with earthquake records and the parametric Bessel-zeros method [Ekstrom et al., 2009] with correlated ambient noise records. Two starting velocity models have been tested: a uniform (4.4 km/s) starting model and a starting model with the Moho mapped by Pn station time terms [Buehler and Shearer, in review].
</p><p>With respect to previous body and surface wave tomograms, the Isabella anomaly is imaged as more geometrically rich. This observation leads us to consider a composite explanation of the anomaly as a Pacific plate slab-flap (Monterey microplate) and the foundering roots of the southern Sierra Nevada batholith. The slab flap is identified as a 4.4- 4.6 km/s NW-SE striking 140 km wide planar anomaly imaged at 60-110 km depth beneath the San Joaquin valley. The foundering southern Sierra batholithic root is identified as an N-S trending 4.4-4.5 km/s high velocity region beneath the southern Sierran foothills. This anomaly is bowed down beneath the high standing southern Sierra block to form a wedge filled with 4.2 km/s mantle which is interpreted as in-flowed asthenosphere. The pros and cons of this composite interpretation will be discussed.
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Comparison of our velocity model with the only other joint ambient/earthquake surface wave model [Moschetti et al., in review] finds that the two models are well correlated. Comparison of our velocity model with teleseismic body wave tomograms reveals substantial differences in the geometry and depth extent of the Isabella anomaly related to differences in the resolution of surface and body waves.
</p><p>References
</p><p>Ekström, G., G. A. Abers,and S. C. Webb, Determination of surface-wave phase velocities across USArray from noise and Aki's spectra formulation, Geophys. Res. Lett., 36, L18301, 2009.</p>

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