HR: 09:15h
AN: T31A-05    [PDF]
TI: Possibility of existence of serpentinized material at the Izu-Bonin subduction plate boundary around 31N using Q structure by FDM-simulation
AU: * Kamimura, A
EM: kamimura@bosai.go.jp
AF: National Research Institute for Earth Sciences and Disaster Prevention, 3-1 Tennodai, Tsukuba, 305-0006 Japan
AU: Kasahara, J
EM: kasa2@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi Bunkyo, Tokyo, 113-0032 Japan
AB: At the Izu-Bonin subduction zone (IBSZ), there is a chain of serpentine seamounts at the forearc slope of trench axis, and few large earthquakes occurred at shallow depth ($<$100km) in spite of many large ones at greater depth ($>$400km). To elucidate these characteristics we carried out a seismic refraction-reflection study at the forearc slope of the IBSZ around 31N using 22 OBSs and chemical explosives and airguns as seismic sources in 1998. As the results of forward and travel-time inversion modeling of the study, P-wave velocity structures were obtained along E-W and N-S survey lines which is perpendicular to and parallel to the trench axis, respectively (Kamimura et al., 2002). The result of E-W line (transect a summit of serpentine seamount) suggests presence of a low velocity zone just above the subducting Pacific plate, and this zone connects to the Torishima Serpentine Forearc Seamount. The interpretation of the result was: dehydration of hydrated oceanic crust supplies water to the mantle wedge, and peridotites of the mantle wedge were serpentinized. The serpentinized peridotites have moved between the oceanic slab and the overriding island arc crust and were diapiring into the serpentine seamount. The serpentine on the plate boundary might act as a lubricant and decrease seismic activity along the subduction zone, and this can explain the characteristics of seismicity of IBSZ. In order to evaluate Q structures of the above low velocity zone on the subducting slab, we calculated synthetic waveforms using FDM (Finite Difference Method) with elastodynamic formulation (E3D code, developed by Dr. Shawn Larsen) and the P-wave velocity 2D structure of Kamimura et al. (2002). The E3D uses staggered grid, and 2nd order and 4th order approximation in time and space, respectively. Grid spacing of the calculation is 30 m in x and z, and 1.5 msec in time. Five-Hz and 0-phase Ricker wavelet_@pressure source was used. Several structure models are used for comparison. One model has no low-Q zone, another one has low-Q zone only just below the serpentine seamount. Other models have low-Q zones just below the serpentine seamount and above the subducting slab, horizontal width of the low-Q zone are different one another. Comparing synthetic waveforms and observed data, we can conclude that there must be a low-Q zone just below the serpentine seamount and on the subducting oceanic slab. The low-Q zone on the slab has ca. 80 km wide east to west and connects to the serpentine seamount. It is very important to understand where serpentinites of the seamounts came from to explain the characteristics of seismicity at the IBSZ. In this presentation we are going to explain an interpretation that serpentine moved through the plate boundary and reached just below the serpentine seamount, using an existence of the low-Q zone. Kamimura, A., Kasahara, J., Masanao S., Hino, R., Shiobara, H., Fujie, G., Kanazawa, T., 2002. Crustal structure study at the Izu-Bonin subduction zone around 31$\deg$N: implications of serpentinized materials along the subduction plate boundary, Physics of the Earth and Planetary Interiors, 132, 105-129.
DE: 7218 Lithosphere and upper mantle
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting