Mantle layering from ScS reverberations 3. The upper mantle

This is the third paper in a four-part sequence that examines the nature of mantle layering using the multiple-ScS phases and internal reflections observed within the reverberative interval of SH-polarized seismograms. In this paper, migration techniques are applied to ScS reverberations to image discontinuities in shear impedance beneath a tectonically diverse study area that includes the western Pacific, Japan, the Philippine Sea, and Australasia. Between the M and 410-km discontinuities (Bullen's region B), the analysis reveals four reflectors. Overall, the results indicate an upper mantle where impedance layering varies substantially over a broad spectrum of scale lengths, but correlates with surface geology and plate structure down to at least 250 km (the L discontinuity), and perhaps to depths as great as 350 km (the X discontinuity). The lateral variability observed in region B reflectivity structure appears to be governed primarily by the thermal and chemical gradients related to continental deep structure and subduction. -from Authors