HR: 10:35h
AN: V52A-02    [Abstracts]
TI: Seismological constraints on mantle upwelling in Japan: Implications for the genesis of arc magmas
AU: * Nakajima, J
EM: nakajima@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 9808578, Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 9808578, Japan
AB: The Japan subduction zone is one of the most-studied areas in the world by seismological approaches. In particular, the mantle-wedge structure beneath northeastern (NE) Japan has been investigated and an inclined low-velocity zone is clearly imaged in the mantle wedge, sub-parallel to the down-dip direction of the slab. The zone is interpreted to be an upwelling flow (return flow) mechanically induced by the subduction of the Pacific slab [Zhao et al., 1992; Nakajima et al., 2001; Hasegawa and Nakajima, 2004]. A quantitative analysis of the inclined low-velocity zone was done in terms of thermal anomaly and fluid content [Nakajima et al., 2005]. The results show a systematic change in melt-filled pore shapes with depth, suggesting the existence of 3-6 vol% melts as grain boundary tubules at a depth of 90 km, 0.04-0.05 vol% melts as thin cracks or dikes with aspect ratio of ~0.001 at a depth of 65 km, and 1-2 vol% melts as cracks or dikes with aspect ratio of 0.02-0.04 at a depth of 40 km. Based on these observations Hasegawa and Nakajima (2004) proposed a conceptual model for fluid- transportation path and magmatism in NE Japan. The objectives of this study are (1) to estimate detailed 3D seismic velocity structures beneath Tohoku and construct a more accurate model of arc magmatism and (2) to confirm the existence of mantle upwelling beneath Hokkaido and Kyushu where a clear volcanic front is well developed. In this study, we apply the tomography method by Zhao et al. [1992] to high-quality arrival-time data obtained by a nation-wide seismograph network in Japan. In Tohoku, we collected 626,463 P and 382,796 S-wave arrivals from 10,623 earthquakes that occurred from October 1997 to August 2006. A number of seismograph stations used in this study is 879. The number of arrival- times is thee-times larger than those used in our previous work (Nakajima et al., 2001). Grid nets with horizontal and vertical spacing of 0.15-0.25 degrees and 10-15 km, respectively, were set up in the model space. The obtained velocity structure shows a narrower low-velocity zone in the mantle wedge compared to that by Nakajima et al. (2001). The thickness of the zone is ~ 20 km. This observation suggests that a mantle upwelling is localized in the core of the mantle wedge. In case of Kyushu, arrival-time data of 238,340 for P wave and 182,495 for S waves were collected from 5635 earthquakes that occurred around the Kyushu district. Grid spacing is 0.2 degrees in the horizontal and 10-20 km in the vertical directions. The preliminary results show that an inclined low-velocity zone exits in the mantle wedge even in Kyushu.
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8178 Tectonics and magmatism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting