The two drilling sites of Leg 186 are located at the deep-sea terrace of the Japan Trench (Fig. F1). Although sharing similar geological goals of the previous Deep Sea Drilling Project (DSDP) Legs 56, 57, and 87 in this area (Scientific Party, 1980; Kagami, Karig, Coulbourn, et al., 1986), the main objective of this leg was to establish borehole geophysical observatories to monitor active processes in a plate subduction zone. This site being only 48 km south of Site 1150 and along the strike of the Japan Trench, the general geological background is the same as that of Site 1150 (see "Background and Objectives" in the "Site 1150" chapter).
Site 1151 was drilled about 200 m north of shotpoint 4556 on seismic Line KH90-1 (Fig. F2). The seismic characteristics are similar to those at Site 1150 (Fig. F2). Along this line, the seismic stratigraphic features remain constant with a prominent reflector about 1.5 s below seafloor in two-way traveltime. This reflector most likely marks the Cretaceous unconformity and can be traced west to the end of the seismic line at the continental slope. Seismic Line KH96-3, Line 2, shows that the undulation of sedimentary layering has a much shorter wavelength in the north-south direction in accordance with the bathymetry change (Fig. F3). For the same reason as at Site 1150, we planned to drill to a depth of about 1000 m, given the available length of time and space for casing on the drillship.
In contrast to Site 1150, which is situated in a seismically active zone, Site 1151 is located in a seismically quiescent area. This means that there is no historical record that a significant earthquake that caused either large tremors or tsunamis to the coastal area of northern Japan occurred within this area. Also, few microearthquakes have been located within the area by the land seismic network, in operation since the 1970s. Except for this prominent seismic feature, no significant differences in tectonics, geology, or structure have been identified between the two areas. The present seismic observational data can provide resolution in space of less than a few kilometers and less than a few tenths of a kilometer per second in velocity structure in the vicinity of the plate boundary. We are yet to identify the key parameter that is governing this difference.
The geophysical observatory is designed to record strain, tilt, seismic signals, and temperature change. The objectives are the same as for Site 1150. By comparing the data from two sites, it is further expected that the observatory will provide data of sufficiently high resolution to infer the key differences in static and/or dynamic features at the plate boundary.
Previous drilling results and seismic reflection surveys show that a tectonic erosion process is taking place in the Neogene at the Japan Trench region (Scientific Party, 1980; Kagami, Karig, Coulbourn, et al., 1986; von Huene et al., 1994). Together with Site 1150 results, the style of convergence and the forearc subsidence history will be detailed. Analyses of faults and fractures in the cores will be compared with the present-day deformation as the observatory data become available.
The ash records from this site are expected to become an important reference section near Japan. A more detailed analysis of ash layers that are observed to increase from near the end of the upper Miocene into the lower Pliocene is expected to provide important information about eruptive processes, volcanic hazards, and aspects of climate such as response to wind, sand, and volcanogenic input of greenhouse and related gases.
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