FIGURE CAPTIONS
Figure 1. Map of western Pacific area where at least five major plates with consuming boundaries interact. Dark gray squares are primary drill sites, light gray triangles are alternate drill sites, and gray circles are DSDP sites. Bathymetry is in meters.
Figure 2. Swath bathymetry of the Japan Trench area. The proposed sites are located on a deep sea terrace.
Figure 3. Tectonic subsidence history (from von Huene and Lallemand, 1990).
Figure 4. Map of Japan Trench area with seismicity (R. Hino, pers. comm. 1998). The locations of proposed Sites JT-1C and JT-2G are shown. Focal depth symbols: open circle = 0-10 km, open square = 10-20 km, open triangle = 20-30 km, closed circle = 30-40 km, closed square = 40-50 km, closed triangle = 50+ km.
Figure 5. A. Schematic configuration of the instrument package with multisensors for crustal strain and broadband seismometry. B. Strainmeter installation schemetic for Sites JT-1C and JT 2G with lithologies extrapolated from Leg 57.
Figure 6. Site survey track lines and proposed sites. Contour interval is 100 m.
Figure 7. Multichannel seismic (MCS) record section across Japan Trench near JT-2G in an east west direction. The data is from the KH-90-1 cruise of ORI R/V Hakuho-maru. The location of the proposed observatories relative to the subduction geometry is shown. The strong reflector ~1.3 s below seafloor at the western edge continuing on east past the observatory site is interpreted to be Cretaceous basement. See Figure 6 for track line.
Figure 8. Seismic velocity structure across Japan Trench at about 40°N. Hatched zone is earthquake zone.
Figure 9. Prototype bottom-hole assembly about to be installed in a 200-m-deep water-filled hole south of Tokyo, Japan. The lowermost sensor is the strainmeter, followed by the Guralp broad band seismometer, an Applied Geomechanics tiltmeter, and a PMD broad-band seismometer. The tube linking the sensors is used to transport cement from the drill pipe through the strainmeter body and up around the whole assembly, thus anchoring it firmly to bare rock at the bottom of the hole.
Figure 10. Schematic of the seafloor assembly. All the equipment in this assembly is accessible to an ROV such as the one shown in Figure 11. Cables from the sensors grouted in ~1000 mbsf terminate in a 4-way underwater-mateable connector block. The data handling and instrument control package, marked “G,” plugs into this connector block. A single output from the top of this package is coupled (by ROV) to the multiyear battery installed after the sensors are grouted. A data storage unit can be retrieved by an ROV when required.
Figure 11. Photograph of the Japan Marine Science and Technology Center's (JAMSTEC) ROV, the DOLPHIN 3K. All seafloor assembly electrical connections, the data storage unit, and the data handling and control unit (“G” in Fig. 10) can be removed and replaced by such an ROV. The top of the battery unit shown in Figure 10 is the landing base for the ROV.