HR: 1330h
AN: T52B-0257    [PDF]
TI: Low Intensity Characteristic of Plate-Boundary S-S Reflections Within a Region of Strong Plate-Boundary P-P Reflections and low Seismicity Along the Japan Trench Subduction Zone
AU: * Mochizuki, K
EM: kimi@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kasahara, J
EM: kasa2@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Hino, R
EM: hino@aob.geophys.tohoku.ac.jp
AF: Tohoku Univ., Aoba-ku, Sendai, 980-8578 Japan
AU: Nishino, M
EM: nishino@aob.geophys.tohoku.ac.jp
AF: Tohoku Univ., Aoba-ku, Sendai, 980-8578 Japan
AU: Yamada, T
EM: yamada@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Shinohara, M
EM: mshino@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kanazawa, T
EM: kanazawa@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: It has been pointed out that the epicenters of the microearthquakes along the forearc slope of the Japan Trench are not uniformly distributed but clustered in seismically active zones that are oriented perpendicular to the trench axis. One of the clear seismic-aseismic boundaries of such seismic clusters can be identified in latitude $39\deg$N. A seismic survey was conducted in 1996 with one profile running across the boundary and parallel to the trench axis, and a P-wave velocity structure model was obtained by travel-time inversion (Fujie, 1999). A strong anti-correlation between the seismicity and the intensity of the plate-boundary P-P reflected waves was found: strong plate-boundary P-P reflected waves were observed in a region where seismicity is quite low, and vice versa (Fujie et al., 2002). They discussed that a thin layer of low P-wave velocities (3~4 km/s) with its thickness up to a few hundred meters along the plate boundary could explain the intensity of the reflections. Results of finite-difference waveform calculations support this estimation (Moghaddam, 2002). Another seismic survey was carried out in 2001 with 7 trench-parallel profiles in the same region as the 1996 survey in order to map and verify the strong anti-correlation. The strong anti-correlation was observed over the seismic-aseismic boundary region, and it was inferred that a thin layer with low P-wave velocities along the plate boundary exists beneath the aseismic zone in the region. Understanding the characteristics of plate-boundary S-S reflections in addition to those of P-P reflections would greatly help put better constraints on the physical properties along the plate boundary. Substantial P-to-S conversion at the base of the sedimentary layers was observed. S-wave velocities, especially those of the sedimentary layers, should be precisely determined in order to have good estimates of the arrival times of the plate-boundary S-S reflected waves. The S-wave velocities of the sedimentary layers were obtained by the tau-p analysis of the OBS horizontal-component waveforms. The Vp/Vs ratios within the sediment layers were estimated to be 5.2 and 2.3 from top to bottom. The Vp/Vs ratios within the deeper structure were assumed to be 1.8 to 1.74 as the Vp varies from 4.5 km/s to 8.0 km/s. The calculated travel times explain well the observed travel times of P-S-converted refraction arrivals. Expected arrival times of the plate-boundary S-S reflected waves were calculated with respect to the obtained Vs structure. Such intense amplitude as observed for the plate-boundary P-P reflections was not identified for the S-S reflections. Irregularity of the interfaces within the structure or differences in the reflectivity coefficients between P-P and S-S reflections at the plate boundary does not explain the opposite appearances of those reflections. Although the strong plate-boundary P-P reflections could be explained by putting a thin low-velocity layer, some additional features for S-S reflections along the plate boundary may be required, such as very low Q values (high attenuation) for the S-wave propagation.
DE: 3025 Marine seismics (0935)
DE: 7230 Seismicity and seismotectonics
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting