Seismic Discontinuities in the Mantle Beneath the Western Pacific: Evidence from ScS Reverberations

Earthquakes generate seismic waves that travel through the Earth and can be reflected by changes in density and/or seismic velocity that may relate to changes in the phase or chemical composition of the mantle. To study these discontinuities we use the ScS reverberative interval of seismic records. This is accomplished by inverting zeroth order waves for whole mantle properties that are used to create a profile of reflectivity vs. depth through the mantle. This profile includes the expected transition zone discontinuities. We then create a second profile through a process of iterative forward modeling to locate the depth of additional mantle discontinuities. We have selected three source-receiver paths in the western Pacific as our research area because of its high seismic activity, and the widespread coverage of seismometers. We found the expected transition zone discontinuities at 410 km and 660 km in all three paths. We found discontinuities at 83 and 288 km in path A, which samples the oceanic crust between the Marianas trench and Hawaii. In path B, which samples an area between the Izu-Bonin trench and Hawaii, we found discontinuities at 77, 321, and 2645 km. Path C, which samples the Sea of Japan and includes the subducted Pacific plate contains discontinuities at 337, 558, 850, and 2782 km. Many of these discontinuities have been detected in previous studies using different methods [e.g., Zang et al., 2003; Revenaugh and Jordan, 1991]. We believe the discontinuities near 300 km in paths A and B signal the presence of stishovite in eclogitic mid-upper mantle. The negative reflectivity of the feature at 337 km in path C is interpreted as evidence of a partially molten zone above the 410-km discontinuity, as has been indicated in previous studies [e.g., Revenaugh and Sipkin, 1994]