Depth Variation of Upper Mantle Seismic Discontinuities in the Region of the Tonga Subduction Zone

In order to study the mantle transition zone structure near the Tonga subduction zone in the southwestern Pacific, we analyzed receiver functions from teleseismic P waves recorded by 17 seismic stations on islands and 50 ocean bottom seismographs deployed as part of the RIDGE2000 Lau Basin Imaging Project. The orientations of the OBSs were derived from a combination of the results of ambient noise correlation and Rayleigh-wave polarization orientation methods. These orientations were used to rotate the seismograms into a ray-centered (LQT) coordinate system, and receiver functions were calculated with an iterative deconvolution in the time domain [Ammon, 1999]. Then a 3-D stacking approach [T. J. Owens, 2000] was adopted to stack those receiver functions for all station-event pairs. We binned the study area in 0.25° by 0.25° regions and stacked the traces within 1.25° radius from each bin at depths with a 5-km increment. The Tonga slab subducts with the fastest known convergence velocity, and a flat slab with scattered seismicity is found to the west of the main Tonga slab in tomographic models. A shallowing of the 410-km and a depression of the 660-km discontinuities are expected at and near the slab since the olivine transitions are perturbed by the cold slab at these depths, although metastabliity of the phase transitions may produce additional complications. We will present preliminary results for the variation of the 410-km and 660-km discontinuities in the region.