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 57, 58, 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.
Site 1150 was drilled at common midpoint (CMP) 2337 on seismic line KH96-3, Line 1 (Fig. F2). We estimated that as much as 1400 m of drilling would be necessary to set the sensor strings, based on the seismic record and the core and logging data at Site 438. A compromise was made to drill to 1000 m, given the available length of time and space on the drillship. Site 438 results show that the P-wave velocity reached 2 km/s and the bulk density reached 1.8 g/cm3 at 920 meters below seafloor (mbsf) in the lower Miocene (Scientific Party, 1980). We estimated the reflectors at 5.0 and 5.3 s in two-way traveltime as Oligocene and Cretaceous boundaries found at Site 438, so the aimed depth of ~1000 m would be just above the Oligocene boundary.
From a recent multichannel reflection seismic survey, the plate boundary immediately below Site 1150 is estimated to be at ~15 km depth from the sea surface and, therefore, only ~11 km beneath the sensor bottom (Tsuru et al., 1999). The distance to microearthquakes that occur within the hanging wall would be only ~5 km. This location provides probably the most favorable observatory site to study the plate subduction dynamics. The plate convergence rate is high here, and the seismic activity is high and relatively well coupled.
The geophysical observatory is designed to record strain, tilt, seismic signals, and temperature change. Little is known about how the dynamic sliding of the subducting plate occurs in relation to earthquakes on and off the plate boundary, since such data have been unavailable.
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). Further geological hard evidence to constrain the style of convergence and the forearc subsidence history will be sought. 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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