HR: 0800h
AN: T11E-1323    [Abstracts]
TI: Evidence for a Sharp Lateral Boundary at the Southern Border of the Pacific Superplume
AU: * To, A
EM: toh@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, 215 McCone Hall, UC Berkeley, Berkeley, CA 94720 United States
AU: Romanowicz, B
EM: barbara@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, 215 McCone Hall, UC Berkeley, Berkeley, CA 94720 United States
AU: Capdeville, Y
EM: capdevil@ipgp.jussieu.fr
AF: Institut de Physique du globe de Paris, 4, place jussieu, Paris, 75005 France
AU: Takeuchi, N
EM: takeuchi@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Recently, sharp lateral transitions in the velocity structure have been reported at the borders of the lowermost mantle low velocity "superplumes". Most of the findings are associated with the African superplume (Ni et al, 2002), although there is also some evidence in the east-central Pacific (Br\'{e}ger and Romanowicz, 1998). We here report that a sharp lateral boundary exists at the southern edge of the Pacific superplume. The set of SHdiff waveforms, which graze the South Pacific, have similar features to those observed previously at the southeastern edge of the African superplume. Both waveform sets show a rapid shift of the arrival time with respect to the azimuth as previously reported in the case of the African superplume. In addition, we report that both waveform sets show a secondary pulse that follows the direct Sdiff phase. The coupled mode/spectral element method (Capdeville et al, 2002), which can handle strong lateral variations of the velocity in D", is used to construct synthetic waveforms. The computationally heavy spectral elements are only used in the target region, which is the bottom 370km of the mantle in this study (Capdeville et al, 2003). This allows one to extend computations to much higher frequencies than if spectral elements are used for the whole earth. The postcursors can be explained by a simple structure in the D'' region with a sharp quasi vertical boundary aligned almost parallel to the ray path. The existence of these pulses indicates that modelling of heterogeneity outside of the great circle path can help constrain the 3D structure at the base of the mantle. The similarity of the two observed SHdiff waveform sets at relatively high frequencies indicates that the low velocity regions in the lower mantle under Pacific and Africa, observed as the strong degree-2 pattern in shear velocity tomographic models, have a similar nature also at finer scales.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting