HR: 0800h
AN: DI41A-1253    [Abstracts]
TI: Determination of Mantle Discontinuity Depths beneath the South Pacific Superswell As Inferred Using Data From Broadband OBS Array
AU: * SUETSUGU, D
EM: dai@jamstec.go.jp
AF: Institute for Frontier Reseach on Earth Evolution of JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: SHIOBARA, H
EM: shio@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute of University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: SUGIOKA, H
EM: hikari@jamstec.go.jp
AF: Institute for Frontier Reseach on Earth Evolution of JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: KANAZAWA, T
EM: kanazawa@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute of University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Fukao, Y
EM: fukao@jamstec.go.jp
AF: Institute for Frontier Reseach on Earth Evolution of JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AB: We determined depths of the mantle discontinuities (the 410-km and 660-km discontinuities) beneath the South Pacific Superswell using waveform data from broadband ocean bottom seismograph (BBOBS) array to image presumed mantle plumes and their temperature anomalies. Seismic structure beneath this region had not previously been well explored in spite of its significance for mantle dynamics. The region is characterized by a topographic high of more than 680 m (Adam and Bonneville, 2005), a concentration of hotspot chains (e.g., Society, Cook-Austral, Marquesas, and Pitcairn) whose volcanic rocks have isotopic characteristics suggesting deep mantle origin, and a broad low velocity anomaly in the lower mantle revealed by seismic tomography. These observations suggest the presence of a whole-mantle scale upwelling beneath the region, which is called a 'superplume' (McNutt, 1998). However, the seismic structure has been only poorly resolved so far and the maximum depth of anomalous material beneath the hotspots has not yet been determined, mainly due to the sparseness of seismic stations in the region. To improve the seismic coverage, we deployed an array of 10 BBOBS over the French Polynesia area from 2003 to 2005. The BBOBS has been developed by Earthquake Research Institute of University of Tokyo and are equipped with the broadband CMG-3T/EBB sensor. The observation was conducted as a Japan-France cooperative project (Suetsugu et al., 2005, submitted to EOS). We computed receiver functions from the BBOBS data to detect Ps waves from the mantle discontinuities. The Velocity Spectrum Stacking method (Gurrola et al., 1994) were employed to enhance the Ps waves for determination of the discontinuity depths, in which receiver functions were stacked in a depth-velocity space. The Ps-waves from the mantle discontinuities were successfully detected at the most of the BBOBS stations, from which the discontinuity depths were determined with the Iasp91 velocity model. The 410-km discontinuity depths were estimated to be 403-431 km over the Superswell region, which are not substantially different from the global average considering the estimation error of 10 km. The 660-km discontinuity depths were also determined to be 654-674 km, close to the global average, at most of the stations. Data from a station near the Society hot spot, however, provide an anomalously shallow depth of 623 km, indicating a presence of a local hot anomaly at the bottom of the mantle transition zone beneath near the Society hot spot. Taking into consideration a possible effect of velocity anomalies on the depth estimation, the shallow anomaly is significant. The present result suggests that the thermal anomalies are not obvious in the Superswell-scale, but present locally beneath the Society hot spot.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005