Configuration of Moho Discontinuity beneath Japanese Islands Estimated with Three-Dimensional Seismic Velocity Structure Obtained by Seismic Tomography Using Data from Dense Seismic Network

Abstract:

P-wave seismic velocity is well known to be up to 7.0 km/s and over 7.5 km/s in the lower crust and in the mantle, respectively. There is a large velocity gradient at the Moho discontinuity between the crust and mantle. In this paper, we investigates the configuration of Moho discontinuity defined as an iso-velocity plane with large velocity gradient derived from our fine-scale three-dimensional seismic velocity structure beneath Japanese Islands using data obtained by dense seismic network with the tomographic method (Matsubara and Obara, 2011).

We calculate the P-wave velocity gradients between the vertical grid nodes. The largest velocity gradient is 0.078 (km/s)/km at velocities of 7.2 and 7.3 km/s. In this study, we define the iso-velocity plane of 7.2 km/s as the Moho discontinuity. However, it is difficult to identify the Moho discontinuity of the Eurasian plate where the lower crust of the Eurasian plate contacts with the subducting oceanic crusts of the Pacific and Philippine Sea plates since there is no mantle high-velocity material. Beneath these contact zones, the Moho depth derived from the iso-velocity plane denotes the Moho within the subducting oceanic plates. We discuss the Moho discontinuity above the upper boundary of these subducting oceanic plates with consideration of configuration of plate boundaries of prior studies (Shiomi et al., 2006; Kita et al., 2012; Nakagawa et al, 2012).

The Moho discontinuity deepens over 35 km in the collision zone like as Kanto Mountains, the volcanic underplating zone as the Tohoku backbone range, and non-tension region like as Chugoku Mountains. These regions associated with deep Moho are characterized by the crustal seismicity within the depth range from 20 to 30 km.

The Moho discontinuity shallower than 30 km depth is distributed within the tension region like northern Kyushu and coastal line of the Pacific Ocean in the Tohoku region and the tension region at the Cretaceous as the northeastern Kanto district. These regions have low seismicity within the upper crust. Northeastern Kanto district has positive Bouguer anomaly and it indicates that the ductile material with large density in lower crust exist in the shallower portion and the crust is aseismic.