TRAVEL-TIMES OF S AND SKS - IMPLICATIONS FOR 3-DIMENSIONAL LOWER MANTLE STRUCTURE BENEATH THE CENTRAL PACIFIC

The travel times of S and SKS phases from deep-focus Fiji-Tonga events recorded in North America by Canadian Station Network and World Wide Seismographic Station Network stations exhibit a strong azimuthal dependence. The differential times of S-SKS exhibit relatively little scatter and are used to investigate three-dimensional mantle models for the central Pacific region. S-SKS times are corrected for Earth's ellipticity, and residuals are computed with respect to the iasp91 reference model. Anomalously large S-SKS times, ranging up to 9 s larger than iasp91 predictions for DELTA > 100-degrees, suggest a slow lower mantle beneath the central Pacific. These results are compared to the predictions of three tomographic models: MDLSH of Tanimoto (1990), model SH12-WM13 of Su et al. (1992), and model SH.10c.17 of Masters et al. (1992). The three-dimensional whole mantle models have been parameterized into 11 spherical shells. Qualitative agreement between the tomographic model predictions and observations is encouraging, varying from fair to good. However, inconsistencies are present and suggest anomalies in the lower mantle of scale length smaller than the present 2000+ km scale resolution of tomographic models. Laterally varying outer core structure is not necessary to explain the anomalies in this data set, although such a scenario cannot be resolved with this data.