Borehole Seismic Observatories
The scientific importance of establishing long-term geophysical stations at deep ocean sites has been acknowledged by the Earth science and Ocean Drilling Program (ODP) communities and is expressed in various reports (COSOD II, JOI-ESF, 1987; Purdy and Dziewonski, 1988; JOI/USSAC, 1994; Montagner and Lancelot, 1995; JOIDES Long Range Plan, 1996). The objective is to understand the processes driving Earth's dynamical systems from a global to a regional scale by imaging the Earth's interior with seismic waves. Unfortunately, few seismometers are located on the 71% of the Earth's surface covered by oceans and this makes accurate imaging of some parts of the mantle impossible. New ocean-bottom sensors, the location of which have to be carefully selected to maximize results (Fig. 5A), are needed to accomplish the goals of the international geoscience programs that rely on earthquake data. Aside from Site WP 1B, which will be drilled and instrumented on Leg 195, several other western Pacific sites have been selected for instrumentation. Observatories at Sites 1150 and 1151, on the inner wall of the Japan Trench (JT on Fig. 5A), were installed during Leg 186 (Suyehiro, Sacks, and Acton, 2000). In addition, Site WP-2, located in the northwest Pacific Basin, was recently successfully drilled and instrumented during Leg 191.
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 that
the noise level for oceanic borehole instruments is much lower than for most land counterparts
(e.g., Stephen et al., 1999) (Fig. 8). 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 anomalies
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 Site WP-1B observatory will be equipped with two broadband seismometers (Guralp CMG 1) attached to a pipe hung from the reentry cone (Fig. 9B, Fig. 10), 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-W batteries (SWB 1200, Kornsburg Simrad) attached to a battery frame that sits on the reentry cone (Fig. 9A, Fig. 9B, Fig. 10).
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., 2000). 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. 7) designed to operate in water depths of 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.
South Chamorro Seamount
Site MAF-4B is located in the Mariana system, a nonaccretionary convergent margin with a pervasively faulted forearc. The Mariana system contains numerous large (30 km diameter, 2 km high) mud volcanoes (Fryer and Fryer, 1987; Fryer, 1992; 1996) (Fig. 3), which are composed principally of unconsolidated flows of serpentine muds with clasts of serpentinized mantle peridotite.
Only one active serpentine mud volcano (Conical Seamount) has ever been sampled by drilling and this was done during Leg 125 (Fryer, Pearce, Stokking et al., 1990). Little was then known of either the processes that formed such seamounts, their distribution, their relation to the tectonics of the forearc region, or of the potential for understanding the deeper forearc processes they reflect. Advances in the understanding of nonaccretionary forearcs over the last decade, such as the nature of geochemical cycling within them, their structure, tectonic evolution, and the various (thermal, hydrologic, metamorphic, biological) active processes involved in the formation of mud volcano seamounts, allow the planning of comprehensive studies of the intermediate-depth processes within the "subduction factory." We now know that serpentine mud volcanism in convergent margin settings is not merely a local curiosity of the Mariana system but occurs world-wide.
The South Chamorro Seamount (Fig. 4) is located on the southern Mariana forearc and exhibits the second strongest slab-fluid signal yet detected in the Mariana system. It is the only known site of active blueschist mud volcanism in the world and produced the only documented megafaunal assemblages associated with serpentine/blueschist mud volcanism.
West Philippine Sea
Site WP-1B, the site of the proposed seismometer installation, 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. 6). 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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