Seismic structure of the mantle beneath the Pacific Hemisphere

Abstract

Aside from xenoliths, the Earth's mantle is a region that is inaccessible directly, leaving us with limited tools to investigate its characteristics indirectly. Seismology is a tool well-suited for this purpose, and has provided valuable insight regarding many fundamental processes occurring within the mantle. It is fortuitous that the mantle is layered, and that these layers are often punctuated by distinct changes in density and/or velocity that are seismically detectable. By investigating the seismic structure of the mantle we are able to infer properties such as composition, temperature, anisotropy, and water content. Seismic tomography has informed our understanding of subduction and the fate of slabs, and we are beginning to realize that the lower mantle might also be rich with heterogeneity. Our picture of the Earth's mantle is becoming clearer, however, there is much that we do not understand. Receiver function studies of the oceans are fewer and suffer the common malady of looking beneath oceanic islands, not generic oceanic crust. Most of the detailed information regarding the seismic discontinuity structure of open ocean mantle comes from bottom-side reflections that are precursors to SS phases (a shear wave that has traveled from source to receiver with one bottom-side surface bounce in between). SS and PP (a compressional wave with a path analogous to SS) precursors offer extensive geographic coverage and good sensitivity to small velocity contrasts and reasonable localization. They do not perform well for shallow reflectors, or reflectors near the larger transition zone discontinuities. In our studies we use multiple ScS reverberations to gain better resolution of these features. The primary goals of this research are to study mantle discontinuities, and fill in some of the missing detail regarding mantle heterogeneity. We do this by examining the Pacific ocean, beginning with the open ocean mantle, then moving to the subduction zones in the west Pacific. This region, containing the Boso Triple Junction, is one of the most complex subduction zones on the planet. Finally we continue west beneath the Sea of Japan, the Sea of Okhotsk, and the northeast Chinese craton. The changes in mantle structure across the Pacific reveal many interesting differences between the open ocean mantle and the mantle in regions of subduction.