Whole-Mantle Isotropic and Anisotropic Tomography Beneath Japan and Adjacent Regions

Abstract

The Japan Islands and surrounding regions have complex structures and tectonics due to strong interactions of four lithospheric plates, including the subducting Pacific and Philippine Sea slabs beneath the Okhotsk and Amurian plates, which have caused catastrophic interplate and intraplate earthquakes, as well as arc and intraplate volcanoes. The subducted Pacific slab becomes flat within the mantle transition zone, forming a big mantle wedge above the flat slab, which stimulates intraplate volcanic activities in East Asia. Furthermore, subslab hot mantle upwelling (SHMU) beneath the subducting Pacific slab may contribute to various seismic and volcanic activities. However, SHMU was sometimes claimed to be an artificial ghost resulting from isotropic tomography. Seismic anisotropy tomography is crucial for addressing such issues, because it is a powerful tool to visualize flow field in the mantle. In this study, we first use an existing isotropic tomographic method called multiscale global tomography to reveal whole-mantle isotropic P-wave velocity structure beneath Japan and its surrounding regions. Then we develop a novel program that incorporates seismic anisotropy into multiscale global tomography, revealing whole-mantle P-wave azimuthal anisotropy beneath the study region. As a result, we find for the first time that, even when anisotropy is considered, SHMU beneath the subducting Pacific slab is clearly imaged, indicating that SHMU is not a ghost of isotropic tomography. Furthermore, beneath the Izu-Bonin region, our anisotropic tomography clearly shows that the subducted slab is split into two blocks vertically in the lower mantle, indicating intermittent slab subductions.