Crustal Magmatism and Deformation Fabrics in Northeast Japan Revealed by Ambient Noise Tomography

Mapping both isotropic and anisotropic velocity structures of the crust provides insight into the dominant mechanisms that produce the deformation. We performed ambient noise tomography for shear velocity V-s in the Tohoku region, Japan, where plate kinematics remains simple as the Pacific plate is subducting under the Okhotsk plate. Cross-correlation functions were retrieved from Hi-net recordings and phase velocities of 3-16 s were measured, followed by a one-step, wavelet-based multiscale inversion for both V-s and its azimuthal anisotropy. The isotropic model shows that high V-s anomalies underlie the Cretaceous aged Kitakami and Abukuma Mountains from shallow crust to lower crust. Low V-s is found to track the active volcanoes with the strongest anomalies concentrated at roughly 10 km depth, which we interpret as the shallow magma reservoirs of the Quaternary magmatic system. The resolved azimuthal anisotropy exhibits a switch of anisotropy in the upper crust from island-parallel to a random or suppressed state in 5-15 km depth interval and back to island-parallel. The island-parallel fast direction may map the structural fabrics of the crustal deformation, and the spatial correlation between velocity and anisotropy patterns suggests that continuous volcanic activities and magma ponding disrupt the otherwise consistent fabrics in the upper crust. Below 20 km depth, the anisotropy turns to convergence-parallel, which may result from shearing either imposed by the return flow in the mantle wedge or frozen-in from the last stage of extension of the continental margin that opened the Sea of Japan.