Figure 1A | Table of Contents


Figure 1. A. Location map of seismic station coverage in the northwest Pacific. B. Location of stations needed in the oceans worldwide. At least five major plates with consuming boundaries interact in the northwest Pacific, causing subduction, back-arc opening, slab collisions, terrane accretion, and island arc development. Solid circles (A) indicate land seismic stations, whereas open circles are current and proposed seafloor borehole observatories. Note that a few borehole stations effectively complement and expand the existing network. M = magnitude. YSS = Yuzhno Sakhalinsk, Russia, 46.9583°N, 142.7610°E; NMR = Nemuro, Japan, 36.1525°N, 145.7430°E; PHN = Pohang, Korea, 36.03°N, 129.36°E; HCH = Hachijo-shima, Japan, Reserved; OGS = Chichi-jima, Japan, 27.0570°N, 142.2030°; MCSJ = Minami-tori-shima, Japan, 24.290°N, 153.978°E; ISG = Ishigaki, Japan, 24.3793°N, 124.2347°E; PATS = Ponsei, Micronesia, 6.8367°N, 158.3125°E; PMG = Port Moresby, Papua New Guinea, -9.41°N, 147.16°E; TGY = Tagaytay, Philippines, 14.10°N, 120.94°E.

Figure 2. Location map showing proposed Leg 191 drill sites (squares) and pre-existing DSDP and ODP sites (circles) in the northwest Pacific.

Figure 3. Photograph of the Japan Marine Science and Technology Center's (JAMSTEC) ROV, the Kaiko. All seafloor assembly electrical connections, the data storage unit, and the data handling and control unit (see Fig. 6A and 6B) can be removed and replaced by such an ROV. The Kaiko is scheduled to visit Site WP-2 in November 2000 to activate the borehole observatory.

Figure 4. Leg 191 drill sites and their tectonic context. Thick lines show magnetic lineations and fracture zones; thin lines show 500-m bathymetry contours (from Sager et al., 1999; Nakanishi et al., 1999).

Figure 5. Noise spectra from the borehole seismometer at Site JT-1 (Leg 186) off Sanriku, Japan (from Suyehiro et al., 1999). The noise level is positioned at a satisfactory level between the High Noise Model and the Low Noise Model. The rise of noise around 0.01 Hz is known to be infragravity wave noise induced from a long-period surface wave in the ocean. At Site WP-2, the borehole seismometer will be installed in the basement so that such serious noise should be sufficiently suppressed. The seismometer in Hole 1150D (Leg 186) has a vertical (V) and two horizontal components that are perpendicular to each other. The direction of the horizontal components could not be determined during installation of the instrument; therefore, H1 and H2 denote the noise spectra from records of two horizontal components of the seismometer. dB=decibels. m**2 is fortran code for m2 and m**4 is fortran code for m4.

Figure 6. A. Schematic block diagram of the seismic observatory components. B. Schematic configurations of the instrument package for broadband seismometry. All the equipment in this assembly is accessible to an ROV (remote operating vehicle). Cables from the sensors grouted at ~500 mbsf terminate in a four-way underwater-mateable connector block. The data control unit (MEG [multiple-access expandable gateway]) plugs into this connector block. A single output from the top of this package is coupled (by ROV) to the battery/recorder unit (PAT) installed after the sensors are grouted. A data recording unit (SAM) can be retrieved by an ROV when required. PAT = Power supply access terminal. SAM = storage acquistion module.

Figure 7. Schematic of the seafloor assembly with expected lithologies extrapolated from Leg 185. PAT = Power supply access terminal. SAM = storage acquistion module.

Figure 8. Schematic diagram of the hammer-drill hard-rock reentry system.

Figure 9. Schematic diagram of the hammer-drill system showing bits and casings.

Figure 1A | Table of Contents