P wave azimuthal and radial anisotropy of the Hokkaido subduction zone

Abstract

We present the first three‐dimensional P wave radial anisotropy tomography of the Hokkaido subduction zone, as well as P wave azimuthal anisotropy and S wave tomography, which are determined by inverting 298,430 P wave and 233,934 S wave arrival times from 14,245 local earthquakes recorded by 344 seismic stations. Our results reveal significant velocity heterogeneity, seismic anisotropy, and upwelling flows beneath the study region. In the mantle wedge, prominent low‐velocity (low‐V) anomalies exhibit trench‐normal fast‐velocity directions (FVDs) and a negative radial anisotropy (i.e., vertical velocity > horizontal velocity), which may reflect upwelling mantle flows. Fan‐shaped FVDs are found at depths of 65–90 km, and a detailed 3‐D mantle flow pattern is revealed, which may be caused by a combination of oblique subduction of the Pacific plate and collision of the Kuril arc with the Honshu arc beneath southern Hokkaido. The radial anisotropy changes at ~100 km depth, which may reflect variations in temperature and fluid conditions there. The subducting Pacific slab exhibits a positive radial anisotropy (i.e., horizontal velocity > vertical velocity), which may reflect the original fossil anisotropy when the Pacific plate formed at the mid‐ocean ridge.

Implications of Seismic Velocity Structure at the Junction of Kuril‐Northeastern Japan Arcs on Active Shallow Seismicity and Deep Low‐Frequency Earthquakes

“…In particular, the LVZs beneath active volcanoes have high Vp/Vs (Figure b). LVZs with a high Vp/Vs beneath volcanoes are also shown at a depth of 35 km (Figures c, c, and c), similar to the other active volcanoes in northeastern and northern Japan (e.g., Nakajima et al, ; Niu et al, ). The LVZs extend from beneath the active volcanoes to the nonvolcanic LFE regions at depths of 22.5 and 35 km.…”

P wave anisotropic tomography of the Alps

“…Our results agree well with the traditional paradigm in geodynamic interpretation of the upper mantle anisotropy with V sh > V sv for horizontal flow and V sh < V sv for vertical flow, due to A‐type olivine LPO after large‐strain deformation [ Wang and Zhao , ]. This feature is different from the results of Vp radial anisotropy in the Japan subduction zone [ Wang and Zhao , ; Niu et al , ] which show a positive radial anisotropy within the subducting Pacific and Philippine Sea slabs. This difference in the slab radial anisotropy may be caused by the discrepancy in the dipping angle of the subducting slabs beneath the Alps and Japan.…”

“…The subduction of the European slab and the Adriatic slab is nearly vertical, and so the movement of the slabs is mainly confined in the vertical direction, leading to the negative radial anisotropy (i.e., V pv > V ph ). In contrast, the Pacific slab and the Philippine Sea slab are subducting beneath Japan with a lower angle, hence resulting in a positive radial anisotropy (i.e., V pv < V ph ) [ Wang and Zhao , ; Niu et al , ; D. Zhao et al , ].…”

Mantle Dynamics of Western Pacific and East Asia: New Insights from P Wave Anisotropic Tomography

“…However, this SWS technique lacks depth resolution and so it is very difficult for us to distinguish the fine anisotropic structures in the crust, mantle wedge, and subducting Pacific slab beneath the Japan arc. The anisotropic tomography methods (e.g., Wang & Zhao, , ; Liu & Zhao, ; Liu & Zhao, ) can overcome the shortcoming of the SWS method, and many researchers have used the tomographic methods to investigate the 3‐D Vp azimuthal anisotropic structure of the Japan subduction zone in the past decade (e.g., Ishise & Oda, ; Wang & Zhao, , ; Wei et al, ; Huang, Zhao, & Liu, ; Liu & Zhao, ; Liu & Zhao, ; Niu et al, ). Most of these results show that the subducting Pacific slab exhibits mainly trench‐parallel FVDs beneath the Japan arc, whereas both the mantle wedge and subslab mantle exhibit trench‐normal FVDs.…”