HR: 1340h
AN: S43B-0998    [Abstracts]
TI: Array analysis of Taiwan short-period data and the transition zone morphology in the Fiji-Tonga region
AU: * Lin, P
EM: r90241303@ntu.edu.tw
AF: Institute of Oceanography, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Kuo, B
EM: byk@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, No.128, Academia Road Sec. 2, Nankang, Taipei, 115 Taiwan
AU: Chiao, L
EM: chiao@ntu.edu.tw
AF: Institute of Oceanography, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Shin,, T
AF: Central Weather Bureau, No.64, Gongyuan Road, Jhongjheng District, Taipei, 100 Taiwan
AU: Ho, M
AF: Central Weather Bureau, No.64, Gongyuan Road, Jhongjheng District, Taipei, 100 Taiwan
AU: Yin, Y
AF: Department of Earth Sciences,National Centeral University, No.300, Jhongda Road, Jhongli, 320 Taiwan
AU: Yang, S
AF: Department of Geological Sciences, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei, 106 Taiwan
AB: One hundred plus short-period, 3-component stations operated by the Central Weather Bureau of Taiwan form a perfect array for teleseismic events from Fiji-Tonga subduction zone to study the structure of the lower mantle and the transition zone. By beamforming the seismograms with correct slowness, secondary phases whose magnitudes may approximate the noise level of individual records can merge as coherent energy against the background of destructive interference of the noise. We analyzed only one and a half year of CWB data using array techniques. Vespagrams were constructed for events with depth $>$ 200 km in search for consistent secondary phases that bear significance of the dynamics and structures of the mantle. Up to date, we have identified coherent energy that may be attributed to conversion/reflection of P waves at 410 km and 660 km discontinuities, with delays corresponding to tens of km. The delays of these phases, i.e., S660P, s410P, and p410P, indicate the variations in the thickness of the transition zone deformed or penetrated by the subduction of the Tonga slab. With limited samples we observed large variations in the thickness along the strike of the slab, but verification of a systematics awaits further analyses. Some of these results were bolstered by the frequency-wavenumber analysis. The PcP phase is identifiable, whereas we cannot identify confidently any PcP precursors that may be generated by refraction from a positive discontinuity above the core-mantle boundary. It is likely that such a discontinuity is either fuzzy or absent in the vicinity mid-way along the path. Other energy that stands prominent on the vespagram remains unexplained with structures in the mantle.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8123 Dynamics, seismotectonics
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7207 Core and mantle
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting