-l plane to the
retrieval of aspherical structure constraints. The method also yields
degenerate frequencies largely free from bias due to uneven data
coverage.
Black Forest Observatory
Germany
Email:
widmer@geophys.uni-stuttgart.de
poster/oral: poster
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The multiplet stripping technique introduced in the seminal paper of
Gilbert and Dziewonski in 1975 and applied to the WWSSN recordings of
the 1970 Columbian earthquake has yielded the bulk of the 1064
multiplet
degenerate frequencies used in the construction of the spherically
symmetric Earth models 1066A and 1066B (same paper) and subsequently
the ubiquitous PREM model (Anderson and Dziewonski, 1981)
For fundamental modes and for high-Q overtones the technique has
been
superseeded by techniques that explicitly account for aspherical Earth
structure. For high-l overtones, however, multiplet stripping
remains the method of choice.
I present a new multiplet degenerate frequency dataset including
1224 spheroidal modes up to 20 mHz. Backus-Gilbert style resolution
analysis
shows markedly improved resolution over previous datasets though
caution
because of remaining biases in the dataset (uneven data coverage) is
called for.
One subset of new mode observations is of particular interest: the
radial modes between 10 and 20 mHz. The frequencies of these modes
lie
consistently lower than predicted by PREM or more recent 1-D Earth
models.
One class of models that goes a long way to explaining the new radial
modes involves an additional first order discontinuity in the middle
of
the inner core.
Finally, with regionalized multiplet stripping (GJI, Vol 148,
201--213,
2002) we can unlock a new region in the -l plane to the
retrieval of aspherical structure constraints. The method also yields
degenerate frequencies largely free from bias due to uneven data
coverage.
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