The Ocean Seismic Network
Drilling at the H2O area would address both teleseismic, whole-Earth seismic studies, and regional studies. The site is located in a region on the Earth's surface where there is no land in a 2000-km² area. For uniform coverage of seismic stations on the surface of the planet, which is necessary for whole-Earth tomographic studies, a seafloor seismic observatory is required. This site is one of three high-priority prototype observatories for the OSN (Butler, 1995a; Butler, 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 restricts the azimuthal information to an arc spanning about 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 Site H2O data will be available in real time, data will 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, the variability of elastic and anelastic structure in the Pacific lithosphere from P_o and S_o and pure-path oceanic surface wave studies, and improved locations for Juan de Fuca Ridge earthquakes from T-phase arrivals (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 different styles of installation (Fig. 14). Figure 15 and Figure 16 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 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 H2O, we expect to eliminate these high ambient noise levels.

Basement Drilling on the Pacific Plate
There are no deep boreholes (>100 m) in the Pacific plate, the largest modern tectonic plate. Table 1 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 because they would be affected by the mid-Cretaceous super plume (Pringle et al., 1993). In 30 years of deep ocean drilling and more than 1000 holes world wide, there have been only 12 holes with >10 m penetration into "normal" igneous Pacific plate: only one hole during ODP, no holes with >100 m penetration, and no holes in crust with ages between 29 and 72 Ma. Furthermore, there are no boreholes off axis in "very fast" spreading crust. At the latitude and age of the H2O area, the spreading rate was 140 mm/yr (full rate). Having a reference station in "normal" 45- to 50-Ma ocean crust will constrain geochemical and hydrothermal models of crustal evolution.

Although fast-spreading ridges represent only about 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 created by fast seafloor 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 (Dick and Mével 1996).