[S-CG62] Structure, evolution, and deformation of island arcs associated with subduction processes

[SCG62-P04] Image of Philippine Sea plate beneath the Izu collision zone

*Makoto MATSUBARA¹, Hiroshi Sato² (1.National Research Institute for Earth Science and Disaster Resilience, 2.Earthquake Research Institute, the University of Tokyo)

Keywords:Izu collision zone, Philippine Sea plate, oceanic crust, oceanic mantle

The Philippine Sea (PHS) plate subducts beneath the Tokai region from south, however, the northern side of Izu Peninsula is the collision zone of the PHS and Eurasian (EUR) plates since the Izu Peninsula is on the PHS plate. The upper boundary of the PHS plate is estimated using the low-velocity (low-V) oceanic crust and high-velocity (high-V) oceanic mantle of the PHS plate and distribution of earthquakes with low-angle thrust focal mechanism (e.g. Hirose et al., 2007; Matsubara et al., 2021). Makajima and Hasegawa (2010) estimated continuous PHS plate from Kanto to southwestern Japan. The strike of the Nankai Trough is WSW-ENE, however, that within the Suruga Bay is almost NS direction and Izu peninsula on the PHS plate collides with EUR plate resulting in the overlapping of slab and separation of oceanic crust and accretion to EUR crust. In this study, we discuss the PHS plate beneath the northwestern part of Izu collision zone using the three-dimensional seismic velocity structures (Matsubara et al., 2021; Matsubara et al., 2022).
Map views clearly image the high-V Sambagawa belt and low-V Shimanto belt dipping to north. Low-V oceanic crust and high-V oceanic mantle of the PHS plate are also clearly imaged.
EW cross section clearly image the high-V oceanic mantle on the southern side of 35.7°N, however, subducting high-V zone is not clear on the northern sude of 35.8°N. NS cross section clearly image the subducting high-V PHS plate from south the Kanto region on the eastern side of 139.4°E, however, large low-V block exists at depths of 25-60 km from Izu peninsula to Izu collision zone on the western side of 139.5°E. High-V oceanic mantle of PHS plate at depths deeper than 40 km on the northern side of 35°N and western side of 138.2°E.
Comparison with the receiver function and reflection seismology found that the isovelocity line with 6.5 km/s of P-wave is consistent with the upper boundary of the PHS plate. This isovelocity line suddenly change at the Tanzawa Mountain range. This line lays at depths of 25 km on the eastern side, however, this line lays at depths of 15-20km on the western side and dipping to west.