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

Thursday, 18 December 2014
Jie Chen1, Doulgas A Wiens2, Erica Emry3, Songqiao Shawn Wei4, Chen Cai2, Yang Zha5, Spahr C Webb6, William H Menke7 and Yongshun John Chen8, (1)ITAG Institute of Theoretical and Applied Geophysics, Peking University, Beijing, China, (2)Washington University in St Louis, Earth and Planetary Sciences, St. Louis, MO, United States, (3)Pennsylvania State University Main Campus, University Park, PA, United States, (4)Washington University in St Louis, St. Louis, MO, United States, (5)Columbia University of New York, Palisades, NY, United States, (6)Lamont Doherty Earth Observ, Palisades, NY, United States, (7)Lamont-Doherty Earth Obs, Palisades, NY, United States, (8)Peking University, Beijing, China
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.