IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Poster presentation 2176
Lower mantle structure from scattered and diffracted PKP waves
Dr. Christine Thomas
Earth and Ocean Sciences University of Liverpool IASPEI
J.-Michael Kendall
The lowermost mantle region (D") is important to understand the dynamics of the mantle and the coremantle
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 sourcereceiver
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. In this study we are testing new regions
of the lower mantle using PKP waves recorded at the temporary EAGLE array in and the GRSN () with
epicentral distances between130 and 143 degrees. The earthquake locations are in the South Pacific for
both arrays. The PKP waves to show strong scattering of PKPab waves which arrive as precursors to
PKPdf and in addition large PKP-b diffracted amplitudes. In contrast, the events recorded at the GRSN
show scattering of PKPab as precursors to PKPdf but much smaller PKP-b diffracted amplitudes.
Explanations for the observed waveforms and amplitudes are strong low-velocity regions just above the
core-mantle boundary, only a few 10s of km thick for the South Pacific to path. These strong low
velocity regions act as waveguides for the PKP-b diffracted waves. They cannot be smooth layers but
must show a certain roughness to produce the high-frequency scattering observed in the data. The path
to needs small scatterers but there is no specific requirement for strong low velocity zones.
Keywords: lower mantle, diffracted waves