[S21-3] Seismic anisotropy tomography

[S21-3-02] Constraints on Anisotropic Velocity Structure of the Lithosphere-asthenosphere System in the Central Pacific from the NoMelt OBS Array

^○Pei-Ying Lin¹, James Gaherty², Joshua Russell², Ge Jin², Shu-Huei Hung³, John Collins⁴, Daniel Lizarralde⁴, Rob. Evans⁴, Greg Hirth⁵ (1.Taiwan Ocean Research Institute, National Applied Research Laboratories, Kaohsiung, Taiwan, 2.Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA, 3.Department of Geoscience, National Taiwan University, Taipei, Taiwan, 4.Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA, 5.Geological Sciences Department, Brown University, Providence, Rhode Island, USA)

Seismic anisotropy represents one of the primary mechanisms for probing deformation and melting processes in the earth. Seismic models of oceanic upper mantle are conflicting regarding the relative importance of these deformation processes: seafloor-spreading fabric is very strong just beneath the crust-mantle boundary at relatively local scales, but at global and/or ocean-basin scales, oceanic lithosphere is found to be weakly anisotropic when compared to the asthenosphere. There is little consensus on which of these factors are dominant, in part because observations of detailed lithosphere structure are limited. To address this discrepancy, we conducted the NoMelt experiment on ~70 Ma Pacific lithosphere with the aim of constraining upper-mantle circulation and the evolution of the lithosphere-asthenosphere system. The Rayleigh waves display strongest azimuthal anisotropy with within the high-velocity seismic lid, with fast direction coincident with seafloor spreading. A minimum in the magnitude of azimuthal anisotropy occurs within the middle of the seismic low-velocity zone. In no depth range does the fast direction correlate with the apparent plate motion. High-frequency Love waves suggest positive radial anisotropy (Vsh > Vsv) and azimuthal anisotropy with a strong 4-theta azimuthal signal in the upper 50 km of the mantle. To further evaluate the processes producing asthenospheric fabric, we measure teleseismic P- and S-wave multi-channel delay times to constrain lateral velocity variations beneath the array.