[S-IT02_29PO1] Aqueous fluids and melts in subduction zones: Experiment, modeling, and geophysical observations

[SIT02-P01] Later phase observations and seismic velocity structure in the subducting crust of the Pacific slab beneath Hokkaido

*Takahiro SHIINA¹, Junichi NAKAJIMA¹, Genti TOYOKUNI¹, Toru MATSUZAWA¹, Saeko KITA² (1.RCPEV, Grad. Sch. of Sci., Tohoku Univ., 2.NIED)

Keywords:subducting crust, later phase, guided wave, the Pacific slab

The subducting crust at the uppermost part of the oceanic lithosphere is considered to play important roles for generation of intraslab earthquakes (e.g., Kirby et al., 1996) and arc-magmatism in the mantle wedge (e.g., Nakajima et al., 2013), because the crust involves a large amount of water in form of hydrous minerals and these hydrous minerals affect seismic velocities in the crust (e.g., Hacker et al., 2003). Therefore, to understand water circulation in the subduction zones and genesis of intermediate-depth earthquakes, it is important to reveal where dehydration reaction of hydrous minerals occurs in the crust. However, it is generally difficult to obtain the detailed velocity variation in the crust because the thickness of the crust is ~7 km.Later phases, such as mode-converted wave and guided wave, are sensitive to heterogeneous structure in the crust because of their longer propagation paths in the crust, and hence they are very useful to resolve small-scale seismic velocity structure in the crust (e.g., Matsuzawa et al., 1986; Abers, 2005).At the Hidaka mountain range, middle of Hokkaido, northern Japan, some later phases are reported from earthquakes that occurred in the Pacific slab (e.g., Shimizu and Maeda, 1980). A later phase (Xp phase) recorded in this region has some characteristics: 1) amplitudes of Xp phase are similar to or larger than those of the P wave, 2) Xp-P time lies in a range of 2-10 s and increases with epicentral distances. Shiina et al. (2013, SSJ) discussed the origin of the Xp phase with numerical modeling and interpreted the Xp phase as guided P-wave that propagated in the crust. Moreover, we identified a later phase (Xs phase) that arrives several second after the theoretical initial S waves, and such a phase usually appears in seismograms with guided P-wave. We interpreted the Xs phase as guided S-wave by comparison characteristics of guided P-wave and results of numerical modeling.In this study, based on these interpretations for later phases that observed in the western part of Hidaka mountain range, we estimated P- and S-wave velocity distributions in the subducting crust beneath the eastern part of Hokkaido. The number of arrival times of guided P- and S-waves picked in this study is 117 records and 56 records, respectively. Then, we obtained Vp of 6.8-7.7 km/s and Vs of 3.5-4.0 km/s at depths of 50-100 km in the crust. The obtained Vp in the crust is similar to that observed beneath Tohoku (Shiina et al., 2013) and lower than that expected for fully-hydrated MORB materials (e.g., Hacker et a., 2003). This result suggests that aqueous fluids may co-exist with hydrous minerals in the crust beneath the eastern part of Hokkaido.