Seismic Velocity Contrasts at the Top Surface of the Subducting Pacific Slab Down to the Deep Upper Mantle ; Implications for Water Flow Into the Mantle Transition Zone

HR: 09:24h
AN: DI51B-08 [Abstracts]
TI: Seismic Velocity Contrasts at the Top Surface of the Subducting Pacific Slab Down to the Deep Upper Mantle ; Implications for Water Flow Into the Mantle Transition Zone
AU: * Tonegawa, T
EM: tonegawa@eri.u-tokyo.ac.jp
AF: University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Hirahara, K
EM: hirahara@kugi.kyoto-u.ac.jp
AF: Kyoto University, Oibun-cho, Kitashirakawa, Sakyo-ku, Kyoto, 606-8502, Japan
AU: Shibutani, T
EM: shibutan@rcep.dpri.kyoto-u.ac.jp
AF: Kyoto University, Gokasyo, Uji, 611-0011, Japan
AU: Shiomi, K
EM: shiomi@bosai.go.jp
AF: NIED, 3-1, Tennodai, Tsukuba, 305-0006, Japan
AU: Kanamori, H
EM: hiroo@gps.caltech.edu
AF: California institute ofTechnology, MC 170-25, 1200 E. California Blvd, Pasadena, CA 91125, United States
AB: It has recently been proposed that the mantle transition zone is a water reservoir in the Earth’s interior. Several experimental researches have shown that transition zone minerals, such as wasleyite and ringwoodite, can contain significant amounts (~several wt%) of water in the crystal structure, whereas the solubility of water in olivine is less than 0.2 wt%. Hydrous minerals above and inside the subducting slab may play an important role in carrying water into the mantle transition zone. To image seismic velocity contrasts at the upper mantle discontinuities including the subducting Pacific slab, we applied a receiver function (RF) analysis to the Hi-net tiltmeter recordings. Usually, radial RF is stacked to image seismic discontinuities. In this study, we also used transverse RF, which is effective for detecting P-to-S phase conversion at (or P-to-S converted phases from) a dipping layer, by limiting back azimuth of the teleseismic events between 120° and 180°. We combine radial RF with transverse RF in seismic imaging. We applied a band-pass (0.02-0.16 Hz) filter and CCP (common conversion point) stacking for imaging. We present a receiver function image of the subducting Pacific slab with undulations of both the 410 km and 660 km discontinuities beneath the Japanese Islands. Especially, the top surface of the Pacific slab indicated by positive RF amplitudes can be seen down to the mantle transition zone, implying that the subducting slab is faster than the mantle wedge in seismic velocity. In synthetic forward modeling, the seismic velocity contrast at the top slab surface is approximately 8-10 % at depths of 200-300 km. This could be attributed to the existence of hydrous minerals on the slab; these minerals possibly carry water into the mantle transition zone. Furthermore, in addition to the top slab surface, we could also image the uplift of the 410 km discontinuity. This means that we could successfully identify both the top slab surface and the 410 km discontinuity at depths of 300- 400 km.
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting