Trans-Pacific upper mantle shear velocity structure

We use 50 Tonga‐Fiji events recorded at the broadband TriNet array, southern California, to develop a pure path upper mantle shear velocity model across the Pacific. At the epicentral distances of 70°–95°, multibounce S waves up to S5 are observed, including their triplicated branches, which become...

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Veröffentlicht in:Journal of Geophysical Research: Solid Earth 2007-08, Vol.112 (B8), p.n/a
Hauptverfasser: Tan, Ying, Helmberger, Don V.
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Sprache:eng
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Zusammenfassung:We use 50 Tonga‐Fiji events recorded at the broadband TriNet array, southern California, to develop a pure path upper mantle shear velocity model across the Pacific. At the epicentral distances of 70°–95°, multibounce S waves up to S5 are observed, including their triplicated branches, which become particularly clear after stacking. Since these S wave multiples turn at various depths, simultaneously modeling their differential traveltimes and waveforms provides strong constraints on the radial velocity structure. We parameterize the velocity model according to a priori information from the previous oceanic models, so that we can take a grid search approach, to fully investigate possible interdependencies among the model parameters. We construct synthetics with a reflectivity code and study both the SH and SV components. By modeling the whole recordings from events at different depths, with different mechanisms, we are able to separate shallow low‐velocity zone (LVZ) features from deeper structure. Our preferred model (PAC06) contains a fast lid (Vsh = 4.78 km s−1, Vsv = 4.58 km s−1) with a thickness of ∼60 km. The underlying LVZ is prominent with the lowest velocities Vsh = 4.34 km s−1, and Vsv = 4.22 km s−1 occurring at a depth of ∼160 km. These velocities are below the estimates of solid‐state LVZ, suggesting the presence of partial melt. The anisotropy (Vsv < Vsh) of PAC06 extends to ∼300 km depth, which is constrained by S5 turning at this depth. Besides the 406 km and 651 km discontinuities, PAC06 also has a small (∼1%) velocity jump at ∼516 km. We consider these main features of PAC06 to be well determined, since PAC06 explains a large data set from various events. Therefore it is ideally suited for comparing with mineralogical models.
ISSN:0148-0227
2156-2202
DOI:10.1029/2006JB004853