The use of submarine cables provides a tremendous opportunity for real-time data acquisition from permanent broadband seismometers on the seafloor. Programs to use retired submarine cables for this purpose have been initiated in the United States (e.g., Butler et al., 1995a) and Japan (e.g., Kasahara et al., 1998).
The Hawaii-2 submarine cable system is a retired AT&T telephone cable system between San Luis Obispo, California, and Makaha, on Oahu, Hawaii (Fig. F1). The cable system was originally laid in 1964. Incorporated Research Institutions for Seismology (IRIS) installed a long-term seafloor observatory about halfway along the cable (~140°W, 28°N). The cable was cut and terminated with a seafloor junction box (Fig. F2). The location of the junction box on the seafloor defines the location of the Hawaii-2 Observatory (H2O), which was named after the original AT&T cable.
The junction box has eight underwater make-break connections. About 500 W of power is available from the junction box, and there is ample capacity for two-way, real-time communications with seafloor instruments. Data channels from the seafloor can be monitored continuously via the Oahu end of the cable to any lab in the world. The California end of the cable cannot be used because it was cut and removed from the continental shelf.
There is a shallow buried broadband seismometer operating at the site that monitored noise from the JOIDES Resolution during our cruise. The sensor consists of a modified Guralp CMG-3T broadband seismometer and a conventional 1-Hz three-component geophone and it is buried in a caisson ~1 m below the seafloor (mbsf) (Duennebier et al., 2000, in press). This sensor has been transmitting seismic data to shore continuously and in real time for over 2 yr. The seismic data are forwarded to the IRIS Data Management Center in Seattle and are included in the Global Seismic Network database for use in global and regional earthquake studies. Other seafloor observatories, such as a geomagnetic observatory (Chave et al., 1995), a hydrothermal observatory (Davis et al., 1992; Foucher et al., 1995), or a broadband borehole seismic observatory (Orcutt and Stephen, 1993), can be installed at the site as funding becomes available.
Within the Ocean Drilling Program (ODP) and marine geology and geophysics communities, there has been considerable interest in the past few years in long-term seafloor observatories that include a borehole installation. Prototype long-term borehole and seafloor experiments almost exclusively use battery power and internal recording. The data are only available after a recovery cruise. One exception to this is the Columbia-Point Arena ocean bottom seismic station (OBSS), which was deployed on an offshore cable by Sutton and others in the 1960s (Sutton et al., 1965; Sutton and Barstow, 1990). For the foreseeable future, the most practical method for acquiring real-time, continuous data from the seafloor will be over cables (Chave et al., 1990). The H2O project provides this opportunity.
The Hawaii-2 cable runs south of the Moonless Mountains between the Murray and Molokai Fracture Zones (Fig. F1) (Mammerickx, 1989). Between 140°W and 143°W, water depths along the cable track are typical for the deep ocean (4250–5000 m), the crustal age varies from 45 to 50 Ma (Eocene), and the sediment thickness to within the available resolution is ~100 m or less. Prior to the cable survey cruise in August 1997 (Stephen et al., 1997), sediment thickness was not well resolved along the track (Winterer, 1989).
Tectonically, the cable runs across the "disturbed zone" south of the Murray Fracture Zone, between magnetic isochrons 13 and 19 (Atwater, 1989; Atwater and Severinghaus, 1989). In the disturbed zone, substantial pieces of the Farallon plate were captured by the Pacific plate in three discrete ridge jumps and several propagating rifts. To avoid this tectonically complicated region and to be well away from the fracture zone to the south of the disturbed zone, the H2O was situated west of isochron 20 (45 Ma) at ~140°W. The crust west of 140°W was formed between the Pacific and Farallon plates under "normal" spreading conditions at a "fast" half-rate of ~7.1 cm/yr (Atwater, 1989; Cande and Kent, 1992). At the time this crust was formed, the Farallon plate had not split into the Cocos and Nazca plates, and the ridge that formed this crust was the same as the present-day East Pacific Rise.
Between 140°W and 143°W, the Hawaii-2 cable lies in the pelagic clay province of the North Pacific (Leinen, 1989). The sediments in this part of the Pacific are eolian in origin, consisting primarily of dust blown eastward from the arid regions of central Asia. This region of the Pacific is below the calcite compensation depth (~3500 m), and little or no biogenic calcite is thought to reach the seafloor (Leinen, 1989). Siliceous biogenic material is rapidly dissolved by the silica-poor bottom waters. The sediments are unfossiliferous red clays.
The H2O site lies in a smooth abyssal plane environment. The drill site, identified as Site H2O-5 during planning and now identified as ODP Site 1224, is on the same crustal block as the H2O junction box (Table T1; Fig. F3, Fig. F4).
Drilling at the H2O site was proposed to accomplish two main objectives:
Establishing a borehole seismometer in the H2O area is valuable for addressing both teleseismic (whole Earth) and regional seismic studies. For uniform coverage of seismic stations on the surface of the planet, which is necessary for whole-Earth tomographic studies, seafloor seismic observatories are required. This site, where there is no land in a 2000-km2 area, is one of three high-priority prototype observatories for the Ocean Seismic Network (OSN) (Butler, 1995a, 1995b; Purdy, 1995). Global seismic tomography (GST) provides three-dimensional images of the lateral heterogeneity in the mantle and is essential in addressing fundamental problems in subdisciplines of geodynamics such as mantle convection, mineral physics, long-wavelength gravimetry, geochemistry of ridge systems, geomagnetism, and geodesy. Specific problems include the characteristic spectrum of lateral heterogeneity as a function of depth, the anisotropy of the inner core, the structure of the core/mantle boundary, the role of oceanic plates and plumes in deep mantle circulation, and the source rupture processes of Southern Hemisphere earthquakes, which are among the world's largest (Forsyth et al., 1995).
The culturally important earthquakes in California are only observed at regional distances on land stations in North America, which restrict the azimuthal information to an arc spanning ~180°. To observe California earthquakes at regional distances to the west requires seafloor stations. Regional observations are used in constraining earthquake source mechanisms. Since the H2O data will be available in real time, data could be incorporated into focal mechanism determinations within minutes of California earthquake events. Other problems that can be addressed with regional data from Californian and Hawaiian earthquakes are the structure of the 400-, 525-, and 670-km discontinuities in the northeastern Pacific and the variability of elastic and anelastic structure in the Pacific lithosphere from Po and So (Butler 1995a, 1995b).
In 1998 at the OSN pilot experiment site established in seafloor west of Hawaii, we deployed seafloor, buried, and borehole broadband seismometers to compare the performance of three different styles of installation. Figures F5 and F6 summarize for vertical and horizontal component data, respectively, the improvement that we expect to see in ambient seismic noise on placing a sensor in basement rather than on or in the sediments. Above the microseism peak at 0.3 Hz, the seafloor, buried, and borehole spectra at the OSN-1 site show the borehole to be 10 dB quieter on vertical components and 30 dB quieter on horizontal components (Collins et al., 2001). Shear wave resonances (or Scholte modes) are the physical mechanism responsible for the higher noise levels in or on the sediment. The resonance peaks are particularly distinct and strong at the H2O site. Note the 15-dB peak on the vertical component and the 35-dB peak on the horizontal components near 1 Hz on the H2O spectra. By placing a borehole seismometer in basement at the H2O, we expect to eliminate these high ambient noise levels.
In over 30 yr of deep ocean drilling prior to ODP Leg 200 at more than 1200 sites worldwide, there have been only 13 holes with >10 m penetration into "normal" igneous Pacific plate (only one hole during ODP), only one hole with >100 m penetration, and no holes in crust with ages between 29 and 72 Ma. Table T2 summarizes the boreholes drilled on "normal" crust on the Pacific plate that have >10 m of basement penetration and crustal ages <100 Ma. Holes in seamounts, plateaus, aseismic ridges, and fracture zones were not included. Holes with crustal ages >100 Ma are not included since they would be affected by the mid-Cretaceous super plume (Pringle et al., 1993).
Besides the general sparsity of sampling of oceanic crust, there are no boreholes off axis in "very fast" spreading crust. Although fast-spreading ridges represent only ~20% of the global ridge system, they produce more than half of the ocean crust on the surface of the planet, almost all of it along the East Pacific Rise. Most ocean crust currently being recycled back into the mantle at subduction zones was produced at a fast-spreading ridge. If we wish to understand the Wilson cycle in its most typical and geodynamically significant form, we need to examine ocean crust produced at fast-spreading ridges. We have also known for more than 40 yr that crust generated by fast spreading is both simple and uniform, certainly so in terms of seismic structure (Raitt, 1963; Menard, 1964). Successful deep drilling of such crust at any single location is thus likely to provide fundamental information that can be extrapolated to a significant fraction of the Earth's surface. Seafloor spreading that generated the ~45 Ma crust at the H2O was fast, with the full rate averaging 142 mm/yr. Thus, one objective of Leg 200 was to provide a reference station in "normal," fast-spreading ocean crust for use in constraining geochemical and hydrothermal models of crustal evolution.