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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B08301, doi:10.1029/2006JB004853, 2007

Trans-Pacific upper mantle shear velocity structure

Ying Tan

Seismological Laboratory, California Institute of Technology, Pasadena, California, USA


Don V. Helmberger

Seismological Laboratory, California Institute of Technology, Pasadena, California, USA


Abstract

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 S 5 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 (V sh = 4.78 km s−1, V sv = 4.58 km s−1) with a thickness of ∼60 km. The underlying LVZ is prominent with the lowest velocities V sh = 4.34 km s−1, and V sv = 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 (V sv < V sh ) of PAC06 extends to ∼300 km depth, which is constrained by S 5 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.

Received 10 November 2006; accepted 25 April 2007; published 1 August 2007.

Keywords: upper mantle; lid; LVZ; transition zone; anisotropy; central Pacific.

Index Terms: 7208 Seismology: Mantle (1212, 1213, 8124); 7203 Seismology: Body waves; 7218 Seismology: Lithosphere (1236); 8120 Tectonophysics: Dynamics of lithosphere and mantle: general (1213).

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Citation: Tan, Y., and D. V. Helmberger (2007), Trans-Pacific upper mantle shear velocity structure, J. Geophys. Res., 112, B08301, doi:10.1029/2006JB004853.