HR: 0800h
AN: U41B-0408    [Abstracts]
TI: Probing two low velocity regions with PKP b-caustic amplitudes and scattering
AU: * Thomas, C
EM: tine@liv.ac.uk
AF: University of Liverpool, Department of Earth and Ocean Sciences, 4, Brownlow Street, Liverpool, L37 3HD, United Kingdom
AU: Kendall, M
EM: gljmk@bristol.ac.uk
AF: University of Bristol, Dept. of Earth Sciences Wills Memorial Building Queen's Road, Bristol, BS8 1RJ, United Kingdom
AU: Helffrich, G
EM: george@geology.bristol.ac.uk
AF: University of Bristol, Dept. of Earth Sciences Wills Memorial Building Queen's Road, Bristol, BS8 1RJ, United Kingdom
AB: Knowledge of the lowermost mantle region (D") is important in understanding the dynamics of the mantle and the core-mantle interactions. In several regions reflections from the D" discontinuity are observed, however, the imaging in regions that show slow velocities in tomographic models is still hampered by the source-receiver combinations currently available. Some of the regions with slow seismic velocities have been found to exhibit strong scattering. The scatterers are thought to be close to the core-mantle boundary, but scattering throughout the mantle has also been reported. We are probing low velocity regions of the lower mantle beneath the southwest Pacific using PKP waves recorded at the temporary EAGLE array (Ethiopia) and the GRSN (Germany) with epicentral distances between 130 and 143 degrees. The earthquake locations are in the South Pacific for both arrays. The PKP waves to Ethiopia show strong scattering of PKPab waves which arrive as precursors to PKPdf and in addition show large PKP-b caustic diffracted amplitudes. In contrast, the events recorded at the GRSN show scattering of PKPab as precursors to PKPdf but smaller PKP- b diffracted amplitudes. To understand the observed amplitudes we test a large set of models with velocity and density anomalies in both mantle and core. Explanations for the observed waveforms and amplitudes are low velocity regions just above the core-mantle boundary, on the order of 50 km thick for the South Pacific to Ethiopia path with velocity reductions of around 5% for P and larger reductions for the S-wave velocity. The best fitting models require a change in attenuation in the outer 30 km of the inner core (Qp around 100). The low velocity regions act as waveguides for the PKP-b caustic diffracted waves. They cannot be smooth layers but must exhibit a certain roughness to produce the high-frequency scattering observed in the data. The path to Germany also requires small scatterers but the low velocity region is less pronounced compared with that for the South Pacific - Ethiopia path. Cumulatively, these results suggest that low-velocity-zones at the base of the mantle are regions of small-scale heterogeneity populated with low-velocity inclusions.
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 9355 Pacific Ocean
SC: Union [U]
MN: 2007 Fall Meeting