This proposal directly addresses the second of three initiatives outlined in the ODP Long Range Plan (JOIDES Long Range Plan, 1996): In situ monitoring of geological processes (pp. 49-51). It also represents an initial step in accomplishing the oceanic crustal component of the third initiative: Exploring the deep structure of continental margins and oceanic crust (pp. 52-54). The drilling is intimately tied to the use of seafloor observatories (p. 63) and represents the partnership of ODP with DEOS, British DEOS (B-DEOS), and ION. (Page numbers refer to pages in the Long Range Plan.)
Drilling at the proposed equatorial Pacific observatory site addresses both teleseismic, whole-Earth seismic, and regional studies. The site is located in a region on the Earth's surface ~2000 km from another continental or island seismic observatory. For uniform coverage of seismic stations on the surface of the planet, which is necessary for whole-Earth imaging using modern tomographic inverse methods, a seafloor seismic observatory is required. This site is one of three high-priority prototype observatories for the Ocean Seismic Network (OSN) (Purdy and Orcutt, 1995). Global seismic tomography 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, large-scale geoid anomalies, 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 South America, those that pose a hazard to structures, are only observed at regional distances on land stations in South and Central America and Global Seismic Network (GSN) stations on the Galapagos Islands and Easter Island, which restricts the azimuthal information to an arc spanning ~180°. Observation of these earthquakes at regional distances to the west and constraint of earthquake source mechanisms requires seafloor stations. Since the equatorial observatory data will be available in real time, data will be incorporated into focal mechanism and centroid moment tensor determinations within minutes of Central and South American earthquake events. Other problems that can be addressed with regional data 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 Pn and Sn, and pure-path oceanic surface wave studies.
In 1998 in the pilot experiment at the OSN-1 site established by ODP (Site 843) in seafloor west of Hawaii, three broadband seismometers were deployedone on the seafloor, one buried in the sediment, and one in the boreholeto compare the performance of different styles of installation. Figure F13 and Figure F14 summarize for vertical and horizontal component data, respectively, the improvement that we expect to see in ambient seismic noise by placing a sensor in basement rather than on or in the sediments. Above 0.3 Hz, the seafloor, buried, and borehole spectra at the OSN-1 site show the borehole installation to be 10 dB quieter on vertical components and 30 dB quieter on horizontal components (Stephen et al., 1999; Collins et al., 2001). Shear wave resonances within the thin sediments are the physical mechanism responsible for the higher noise levels in or on the sediment.
As noted in the Leg 200 Prospectus, there are no deep boreholes (>100 m) in the Pacific plate, the largest modern tectonic plate. Table T1 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, and no holes with >100 m penetration. Furthermore, there are no boreholes off axis in "very fast" spreading crust. Having a reference station in "normal" 12-Ma ocean crust will constrain geochemical and hydrothermal models of crustal evolution. 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 must 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).
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