Physics
Scientific paper
Jun 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993georl..20.1135s&link_type=abstract
Geophysical Research Letters (ISSN 0094-8276), vol. 20, no. 12, p. 1135-1138.
Physics
13
Earth Core, Earth Mantle, P Waves, S Waves, Seismographs, Signal To Noise Ratios, Data Bases, Fourier Transformation, Ground Stations, North America
Scientific paper
We have examined the amplitude ratio SKKS/SKS from two deep focus events recorded by the APT89 portable seismic experiment. For one of the events (beneath the Solomon Islands), we obtain a profile of 18 stations in the distance range 99-108 deg. We observe large systematic variations in this ratio as a function of distance. From 99 to 106 deg the ratio is fairly constant and in the range 0.1 to 0.5, but it then rapidly increases to the range 0.5-3.5 between 106 and 108 deg. Such variations are in fact predicted by standard earth models, such as PREM, although this feature of the wavefield has not previously been observed. It is due to the momentary drop in SKS amplitude at the ray parameter where the reflected phase ScP is critically reflected. Because ray theory is not valid in this distance range, we use full wave theory to model the amplitudes. We have been able to match the distance pattern reasonably well if we use a model with the P-velocity at the base of the mantle reduced by 1.5 percent compared to PREM. Although fewer stations are available the second event shows the same basic distance pattern, except that the abrupt increase in the ratio occurs at a closer distance. This suggests an even lower P-velocity for this region (about 3.5 percent slower than PREM) and is consistent with lateral heterogeneity at the base of the mantle of a few percent.
Bina Craig
Silver Paul G.
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