HR: 17:30h
AN: S34A-07    [Abstracts]
TI: Imaging the Western Pacific Subduction Zones Using High-resolution Radon Transforms
AU: * Gu, Y J
EM: jgu@phys.ualberta.ca
AF: Department of Physics, University of Alberta, CEB Building, Edmonton, AB T6G2G7, Canada
AU: Schultz, R
EM: rschultz@phys.ualberta.ca
AF: Department of Physics, University of Alberta, CEB Building, Edmonton, AB T6G2G7, Canada
AU: An, Y
EM: yan@phys.ualberta.ca
AF: Department of Physics, University of Alberta, CEB Building, Edmonton, AB T6G2G7, Canada
AB: Underside reflections (SS, PP precursors) have been widely used in imaging seismic discontinuity structures since the early 1990s. While these waves provide a outstanding global coverage, some regions remain under- sampled, for example, much of the Western Pacific subduction zones and South America. Furthermore, the so called "cap centers" of common signal enhancement schemes do not always reflect the true centers of data, nor do they aim to reduce the effect of uneven coverage. We present results from analyzing the largest SS precursor dataset to date (from 1981-2007) along the Western Pacific subduction zones. This data set more than doubles the data used in the global study of Gu et al. (2003) in the same regions. We adopt variable cap sizes (hence variable resolutions) to equalize the data count, and apply a high-resolution Radon Transform method to simultaneously constrain the ray angle and timing of SS precursors. Our study shows strong depressions of the 660-km discontinuity along the subduction zones, and the lateral variations strongly correlate with regional seismic velocities. The lateral extent of the depressions is smaller than previously found, which we attribute to the high resolution of our new approach. The 410-km discontinuity is elevated beneath the northwestern Pacific, and the anti-correlation with the topography of the 660- km discontinuity reflect significant thermal variations. Overall, however, the correlation between the two transition zone discontinuities is poor in along the Western Pacific subduction zones. The presence of water, garnet phase transformation and the limited lateral dimension of subducted slabs could all affect the topography of these discontinuities, particularly the 410 km discontinuity. The thickness of the transition zone, however, appear to be a more robust measure of temperature: it is consistently narrower than the global average across these subduction zones, as would be expected from the olivine phase transitions. In addition to the transition zone discontinuities, our high-resolution Radon images also present clear evidence of mantle reflectors near 250 km and 850-950 km. The occurrence of the latter reflector is more common than previously proposed and could represent a potential global discontinuity. Phase transformation or chemical differentiation could contribute to this well-resolved interface.
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: 2007 Fall Meeting