Abstract
To study earthquake generation mechanisms, we carried out two seismic experiments in the Izu-Bonin Trench and the Japan Trench subduction zones using OBS and controlled sources in 1996 and 1998, respectively. In the Izu-Bonin subduction zone, we inferred the presence of serpentinized rocks along the top of the slab (Kamimura et al., 2002). Between 38-39° N along the Japan Trench forearc slope, we observed a good correlation between aseismicity and PP reflection phase from the top of the slab (the plate boundary) (Fujie et al., 2002).
To confirm the results of the survey in the Izu-Bonin Trench subduction zone, we calculated synthetic waveforms from the velocity structure obtained by travel-time tomography and seismic attenuation (Q model). A comparison of observed and synthetic waveforms suggests the existence of a low Qp ( 10) layer beneath the Torishima serpentine seamount and extending from the seamount 80 km from the west of the seamount along the top of the slab. This modeling supports the presence of serpentinized peridotite at the top of the slab. We suggest that such a layer may allow an aseismic slip during subduction of the oceanic plate, primarily due to the low frictional coefficient of the crysotile (a phase of low temperature serpentine).
In the Japan Trench subduction zone, we carried out a more extensive OBS-controlled source seismic experiment to confirm the results from 1996. We shot airguns along 7100 kmlong path that runs nearly parallel to the trench axis, and this paper presents the results from the western 5 lines. We observed very intense PP reflections from the top of the slab over the aseismic zone. By including a thin low velocity layer just above the slab, we obtained similar strong PP reflection phases in synthetic waveforms from the top of the slab. These observations and models suggest that a layer with a P velocity as low as 2-4 km/s and a thickness of 100-400 m is needed at the top of the slab to explain the observed PP reflection intensity. Such a layer can comprise aqueous fluid, clay, and/or serpentine-chlorite generated by hydration/dehydration of the subducting oceanic crust and the wedge mantle peridotite. The top of the slab at 18 a path of km exhibits stronger PP reflections than it does at 12 a depth of km depth, and it suggests upwelling of serpentinized and/or clay-rich material from the wedge mantle to the shallower plate boundary.
In the Izu-Bonin Trench and the Japan Trench regions, we conclude the presence of low P wave velocity and low Q materials along the top of the slab. This result strongly suggests aseismic slip during the subduction of the Pacific Plate in the shallow part of the Izu-Bonin Trench subduction zone and in the aseismic zone between 38-39° N over the Japan Trench forearc slope.
