Computer Science
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999e%26psl.170..497n&link_type=abstract
Earth and Planetary Science Letters, Volume 170, Issue 4, p. 497-507.
Computer Science
13
Scientific paper
Seismic waveforms observed in South Africa containing the first arrival crossover of S to SKS (70° to 110°) are analyzed. The data consist of analog records from the World Wide Seismographic Station Network (WWSSN) of deep events beneath South America. The S-waves arrive 2 to 3 s early relative to PREM at ranges from 70° to 95° and then become increasingly delayed, becoming 5 to 6 s late at 110°. The SKS phase is late by 3 to 5 s over the entire range. This pushes crossover between S and SKS, normally observed at about 81°, out about 2° to 3°, which is the most anomalous shift ever reported. To model such features, we modified Grand's tomography model [Grand et al., GSA Today 7 (1997) 1-7], and generated 2D synthetics to match the data. The overall shape and position of the lower mantle low-velocity anomaly proposed by Grand predicts good results if lower mantle anomalies are enhanced to a level of about 4%. This results in a complex tabular structure extending upward from the core-mantle boundary about 1500 km into the mantle. These features appear to be consistent with a large young plume which is erupting off the CMB.
Ding Xiaohua
Gurnis Michael
Helmberger Donald V.
Ni Shuang
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