Introduction
Tomographic studies using earthquake waves propagating through the Earth's interior have revolutionized our understanding of mantle structure and dynamics. A great limitation on existing tomographic images of the Earth's interior is the uneven distribution of seismic stations, especially the lack of stations in large expanses of ocean such as the Pacific. The International Ocean Network (ION) project, an international consortium of seismologists, has identified gaps in the global seismic observation network and is attempting to install digital seismometers in many of those locations. A high priority for ION has been to install a station beneath the deep seafloor of the northwest Pacific (Fig. 1) to gain a better understanding of regional earthquake patterns (Fig. 2) and to enhance tomographic images of the Earth's interior.
Installing a borehole seismometer at Site 1179 (proposed site WP-2) in the northwest Pacific was one of two primary goals for Leg 191. That goal was accomplished at Site 1179, located on abyssal seafloor between Japan and Shatsky Rise (Fig. 3). The seismometer augments a regional network consisting of land stations in eastern Asia, Japan, and the western Pacific islands and borehole seismometers installed during Ocean Drilling Program (ODP) Leg 186 (Suyehiro, Sacks, Acton, et al., 2000) and planned for ODP Leg 195. Owing to its location, the Site 1179 seismometer will provide critical seismic observations from the seaward side of the Japan Trench (Fig. 1, Fig. 2).
Site 1179 is also important because it provides samples representative of the northwest Pacific Cretaceous oceanic crust and its sedimentary cover. Results from this site will augment those from Leg 185, which characterized material being subducted into the Mariana and Izu-Bonin Trenches (Plank, Ludden, Escutia, et al., 2000), in addition to results from prior Deep Sea Drilling Project (DSDP) and ODP drilling in the region. Results from this site will also add to our knowledge of ocean crust structure, geochemistry, plate tectonics, magnetic lineations, sedimentation, and microbiology.
The second primary goal of Leg 191 was to test the hard rock reentry system (HRRS), also known as the "hammer-drill," on a basaltic outcrop. The system uses a hydraulically actuated hammer that drives a drill bit into the ocean floor. Attached to the drill bit is a casing string that stabilizes the borehole walls and allows reentry into the hole with conventional drilling and coring tools. The HRRS is important to long-term ODP science goals as a tool to be used for starting holes in difficult lithologies that would have previously prevented spudding the drill string. In particular, the HRRS is viewed as a tool that can start holes on bare, fractured igneous outcrops where drilling would otherwise be impossible. Testing of the HRRS was also undertaken during ODP Leg 179 but with limited success owing to delays caused by misdirected freight shipments, bad weather, and tool failure brought about by large heave during the test (Pettigrew, Casey, Miller, et al., 1999).
HRRS testing was planned for proposed site SR-1, on a basaltic ridge atop the ocean plateau known as Shatsky Rise (Fig. 4). This site was chosen because it affords a bare basalt outcrop at a relatively shallow depth in a location convenient to Site 1179 and somewhat out of the main western Pacific typhoon track for late summer. Because of a medical emergency and problems with the ship's drawworks, testing was moved from proposed site SR-1 to an alternate location on a volcano a short distance from Guam.
Despite difficulties encountered with delays forced by the weather, the medical emergency, and the broken drawworks, the seismometer was emplaced successfully, a good set of cores was recovered from Site 1179, and a partial HRRS test was accomplished. Consequently, Leg 191 goals were mostly met and the leg can be considered successful.