Localized Double-Array Stacking Analysis of PcP: D" and ULVZ Structure beneath the Cocos Plate, Mexico, Central Pacific, an

Localized Double-Array Stacking Analysis of PcP: D" and ULVZ Structure beneath the Cocos Plate, Mexico, Central Pacific, and North Pacific Summary of P-wave velocity structures for the four study regions. The discrepancy in depth between the observed and synthetic PcP stack (when using the direct P-wave as the reference phase) was matched for each region. We do not model the absolute travel times of the P-waves, so the absolute velocities are not constrained. Perturbations from the reference 1D model are so small that relative depths of discontinuities are well constrained. This is not the case when modelling S-wave data. Our modelling has little sensitivity to density contrasts or the sign or magnitude of dVp/ dZ, however for some source-receiver geometries, there is some sensitivity to abrupt changes in dVp/dZ (Hutko et al. 2008). All discontinuities were modelled as sharp discontinuities. Distributing the velocity changes at the discontinuities across some depth can reduce the effective reflection coefficient by varying degrees, depending on the source-receiver geometry.
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A large, high quality P-wave data set comprising short-period and broadband signals sampling four separate regions in the lowermost mantle beneath the Cocos plate, Mexico, the central Pacific, and the north Pacific is analyzed using regional one-dimensional double-array stacking and modelling with reflectivity synthetics. A data-screening criterion retains only events with stable PcP energy in the final data stacks used for modelling and interpretation. This significantly improves the signal stacks relative to including unscreened observations, allows confident alignment on the PcP arrival and allows tight bounds to be placed on P-wave velocity structure above the core-mantle boundary (CMB). The PcP reflections under the Cocos plate are well-modelled without any ultra-low velocity zone from 5 - 20° N. At latitudes from 15 – 20° N, we find evidence for two P-wave velocity discontinuities in the D” region. The first is ~182 km above the CMB with a δlnVp of +1.5%, near the same depth as a weaker discontinuity (<+0.5%) observed from 5 - 15° N in prior work. The other reflector is ~454 km above the CMB, with a δlnVp of +0.4%; this appears to be a shallower continuation of the joint P- and S-wave discontinuity previously detected south of 15° N, which is presumed to be the perovskite to post-perovskite phase transition. The data stacks for paths bottoming below Mexico have PcP images that are well-matched with the simple IASP91 structure, contradicting previous inferences of ULVZ presence in this region. These particular data are not very sensitive to any D” discontinuities, and simply bound them to be <~2%, if present. Data sampling the lowermost mantle beneath the central Pacific confirm the presence of a ~15-km thick ultra-low velocity zone (ULVZ) just above the CMB, with δlnVp and δlnVs of around -3 to -4% and -4 to -8%, respectively. The ULVZ models predict previous S-wave data stacks well. The data for this region indicate laterally varying Vp discontinuities in D”, with one subregion having a δlnVp of 0.5% 140 km above the CMB. Beneath the north Pacific, the PcP arrivals are compatible with only weak ULVZ (δlnVp ~ 0 to -3%), and there is a weak D'' reflector with δlnVp = 0.5%, near 314 km above the CMB. These results indicate localized occurrence of detectable ULVZ structures rather than ubiquitous ULVZ structure and emphasize the distinctiveness between the large low shear velocity province under the central Pacific and circum-Pacific regions.
</p><p>References
</p><p>Hutko, A. R., T. Lay, and J. Revenaugh (2009). Localized double-array stacking analysis of PcP beneath the Cocos Plate, Mexico, central Pacific and north Pacific, Phys. Earth Planet. Int.r., 173, 6-74.
</p><p>Acknowledgements: This work was supported in part by the U.S. National Science Foundation under grants EAR-0453884, and EAR-0635570.</p>

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