Physics
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999pepi..114..197o&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 114, Issue 3-4, p. 197-210.
Physics
15
Scientific paper
We analyzed SKS/SKKS waveforms recorded by permanent, broadband stations in Australia and found a lack of birefringence: Signals in general are absent on the transverse (T) component of high-quality data over a wide range of back-azimuths. At periods longer than 6 s, including the dominant periods of 8-10 s for SKS/SKKS, there is no exception out of 89 high-quality observations. At periods between 1.3 and 3 s, eight observations show appreciable signal on the T component. The anomalous signals on the T component, however, are unlikely to be a result of azimuthal anisotropy in the upper mantle. (1) For a particular earthquake-station pair, anomalous signal is present only in either SKKS or SKS, but not in both. Since the Fresnel zones of these two phases overlap in the upper mantle, this observation alone rules out simple, homogeneous layers of azimuthally anisotropic upper mantle as the main cause of anomalous signals. (2) The small number of anomalous observations are restricted to high-frequencies over limited back-azimuths and distances. (3) Signals on the T component do not resemble the time-derivative of waveforms on the radial component. These observations and an analysis of Fresnel zones suggest that the anomalous waveforms are probably caused by scattering associated with small-scale heterogeneities in a corridor trending roughly east-west in the lower mantle. If the structure lies near a depth of approximately 1000 km, the source of anomalous propagation correlates with the estimated position of Paleo-Pacific lithosphere that was subducted more than 30-60 Ma ago, suggesting remnant slab in the lower mantle as a plausible cause of anomalous SKS/SKKS arrivals. The lack of azimuthal anisotropy beneath Australia may be related to its high speed of current plate motion in the hotspot frame of reference, a feature unique to the Indo-Australian plate. Sparse SKS/SKKS data from the Indian shield also support this view.
Chen Wang-Ping
Özalaybey Serdar
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