BackgroundBorehole Seismic Observatories
Aside from plugging an important gap in the global seismic array, the Site WP-1B observatory will produce high-quality digital seismic data. Tests with other borehole seismometers show the noise level for oceanic borehole instruments is much lower than most land counterparts (e.g., Stephen et al., 1999; Fig. 4). Recent studies that exploit high-quality digital seismic data obtained on land have shown exciting new phenomena on mantle flows. In the western Pacific, for example, Tanimoto (1988) demonstrated the existence of a strong l = 2 (angular order) pattern of deep (>550 km) high-velocity anomalies from waveform inversions of R2, G1, G2, X1, and X2 surface waves. This suggests a complex interaction of subducting slabs with the surrounding mantle, including the 670-km discontinuity in the region (Tanimoto, 1988). However, because of sparse global coverage by existing seismic stations, current seismic wave resolution is insufficient to image the actual interaction of the plates with the mantle. More recent studies show the potential of new mantle imaging techniques, with finer scale images having been obtained in certain locations where high-quality data are dense, such as the deep extension of velocity anomaly beneath ridges (Zhang and Tanimoto, 1992; Su et al., 1992) or the fate of subducted plates at the 670-km discontinuity (van der Hilst et al., 1991; Fukao et al., 1992). These detailed conclusions come from extraction of more information from existing seismograms. Such studies are limited by sparse data coverage, a barrier that new ocean bottom stations can help break.
Seismic Observatory Design
The WP-1B observatory will be equipped with two broadband seismometers (Guralp CMG-1) attached to a pipe hung from the reentry cone (Fig. 5, Fig. 6), which will position the seismometers near the bottom of the cored hole. Installation of two identical seismometers will add redundancy to the observatory. A back-up sensor (PMD2023) also will be included. However, a combination of one Guralp CMG-1 seismometer and an additional back-up sensor (PMD2023) is an option. Signals from the seismometers will pass uphole by wires and be recorded in a data control box with a multiple-access expandable gateway (MEG). The observatory will be powered for about 3 yr by four units of 6-Watt batteries (SWB 1200, Kornsburg Simrad) attached to a battery frame that sits on the reentry cone (Fig. 5, Fig. 6).
In September 1989, a feedback-type accelerometer capsule was installed in Hole 794D in the Japan Sea during Leg 128 (Ingle et al., 1990; Suyehiro et al., 1992, 1995). The instrument recorded a teleseismic event (body-wave magnitude [Mb] 5.4 at ~4000-km epicentral distance) that clearly showed a surface wave dispersion train (Kanazawa et al., 1992). In May, a comparison of seafloor and borehole (Hole 396B) sensors was made using a deep-sea submersible for installation and recovery (Montagner et al., 1994). Although, at this stage, there is no consensus as to how we should establish seafloor seismic observatories, it is becoming clearer that oceans can provide low noise environments. In August 1999, a seismometer and a strainmeter were cemented at Sites 1150 and 1151 in the deep-sea terrace of the Japan Trench during Leg 186 (Suyehiro, Sacks, Acton et al., in press). The tool was cemented in place to stop fluid motion around the sensors to lower the noise level and to record broadband seismic observations with high sensitivity. Because it is imperative that no fluid motion occur around the broadband seismometers at proposed Site WP-1B, the sensors will be cemented during Leg 195 as well. Once instruments are installed at the site, an ROV will activate the observatory by handling underwater mateable connectors (UMCs). In 2001, Kaiko, an ROV (Fig. 3) designed to operate in water depths up to 10,000 m by the Japanese Agency of Marine Science and Technology Center (JAMSTEC), will visit Site WP-1B to begin seismic observations.
Site WP-1B is located in the west Philippine Sea about 100 km west of the inactive Kyushu-Palau Ridge and 450 km north of the extinct Central Basin Fault (Fig. 2). Early interpretations of magnetic lineations (Hilde and Lee, 1984) indicate that the site lies on 49-Ma crust near Chron 21 and formed by northeast-southwest spreading on the Central Basin Fault. The spreading direction then changed to north-south at ~45 Ma and finally ceased at ~35 Ma. Because the earliest magnetic anomalies in the region predate the initiation of subduction at ~45 Ma along the Kyushu-Palau Ridge, Hilde and Lee considered that the Philippine Sea formed by entrapment of an older Pacific spreading ridge. More recent bathymetric and magnetic surveys (Okino et al., 1999) show that the site lies at the transition from well-defined anomalies south of the Oki-Daito Ridge to more complicated anomalies to the north, which implies that the crust to the north may have formed at a different spreading center.
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