HR: 0800h
AN: T51B-0555    [Abstracts]
TI: Seismic velocity and attenuation in Izu-Bonin subduction zone inferred from BBOBS data
AU: * Shito, A
EM: azusas@eri.u-tokyo.ac.jp
AF: ERI, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Shiobara, H
EM: shio@eri.u-tokyo.ac.jp
AF: ERI, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Sugioka, H
EM: hikari@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan
AU: Ito, A
EM: iaki@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan
AU: Kawakatsu, H
EM: hitosi@eri.u-tokyo.ac.jp
AF: ERI, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Adam, C
EM: adam@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan
AU: Kanazawa, T
EM: kanazawa@eri.u-tokyo.ac.jp
AF: ERI, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AB: Seismic velocity and attenuation in the upper mantle and the subduction slab surrounding Izu-Bonin subduction zone is measured by using the waveform data recorded by Broad Band Ocean Bottom Seismometer (=BBOBS). As a part of Stagnant Slab Project, we deployed 12 BBOBS from October 2005 to October 2006 in and around the Philippine Sea. The sites were equipped with three components Guralp CMG-3T sensors recorded at 200 Hz. The BBOBS waveform data provide us much more information on physical property of subduction zone than previous on-land data. We analyze regional (< 15 degree) earthquakes that sample the Pacific slab, mantle above the Pacific slab (=mantle wedge), and the mantle beneath the Pacific slab. The waveform data from events in Izu-Bonin slab were recorded by the BBOBS array with adequate signal to noise ratio. We measured travel time residual dt and path integrated attenuation t*. The frequency dependent t* is measured using high frequency spectral decay and then path averaged attenuation Q is calculated. Time domain signals are windowed with the window length of 15 s; before 2.5 s and after 12.5 s onset of manually picked P wave. This window includes main pulse of the waveform and excludes later phases. Then amplitude spectra and the slope are calculated over the frequency range from 1.0 to 9.0 Hz. The results clearly show low velocity and high attenuation in the mantle wedge above the Pacific slab and high velocity and low attenuation in the Pacific slab. The estimated Q of P-wave at 1 Hz are 50-200 in the mantle wedge above the Pacific slab, 170-500 in the mantle beneath the Pacific slab, and 500-1000 in the Pacific slab. For all regions, data from shallower events have lower Q values. The intensity of P-wave low velocity anomaly in the mantle wedge above the Pacific slab is up to 10%. The high velocity anomaly in the Pacific slab is up to 8%. The mantle beneath the Pacific slab seems to have no velocity anomaly. The S-wave to P-wave velocity anomaly ratio is 1.1-1.3 in both mantle wedge and the Pacific slab. The observed low velocity anomaly in the mantle wedge above the Pacific slab is too large to be explained only by thermal anomaly. The temperature anomaly estimated from attenuation exceeds both wet and dry solidus of pyrolite. We may expect the existence of certain amount of partial melt in the mantle wedge above the Pacific slab.
DE: 5144 Wave attenuation
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting