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 depe ndence. The differential times of S-SKS exhibit relatively little scat ter and are used to investigate three-dimensional mantle models for th e central Pacific region. S-SKS times are corrected for Earth's ellipt icity, and residuals are computed with respect to the iasp91 reference model. Anomalously large S-SKS times, ranging up to 9 s larger than i asp91 predictions for DELTA > 100-degrees, suggest a slow lower mantle beneath the central Pacific. These results are compared to the predic tions of three tomographic models: MDLSH of Tanimoto (1990), model SH1 2-WM13 of Su et al. (1992), and model SH.10c.17 of Masters et al. (199 2). The three-dimensional whole mantle models have been parameterized into 11 spherical shells. Qualitative agreement between the tomographi c model predictions and observations is encouraging, varying from fair to good. However, inconsistencies are present and suggest anomalies i n the lower mantle of scale length smaller than the present 2000+ km s cale resolution of tomographic models. Laterally varying outer core st ructure is not necessary to explain the anomalies in this data set, al though such a scenario cannot be resolved with this data.