SCIENTIFIC OBJECTIVESThe Leg 186 Scientific Party sailed out to investigate the dynamic properties of one of the world's most active plate subduction zones, the Japan Trench, where the oldest oceanic plate (>100 Ma) is subducting at a high rate (~100 km/m.y.). The drill sites were located about midway within the Japan Trench zone, which is ~650 km long.
The prime objective of this leg was to establish two geophysical observatories that monitor strain, tilt, and seismic waves to further our understanding of subduction dynamics. Coring and logging were aimed at gathering information about past and present tectonic and paleoceanographic conditions.
Dynamic Sliding of the Subducting Plate and Earthquake Process
The seismic coupling efficiency of the subduction zone off Tohoku appears to be as low as 25%. This means that of the total Pacific plate motion expected, only one-quarter is seen as stick slip motion leading to thrust-type earthquakes. One possibility is that three-quarters of the motion is released as slow earthquakes, which are not recorded on normal seismographs. In the past, sparse observations suggest that the slow strain release may consist of multiple episodes in which each event is rather small. For this reason, installation of an instrument of the highest achievable sensitivity is required. Any data leading to better understanding of the partitioning of strain release into damaging "fast" events and slower events will be extremely valuable and may lend further insight into the whole earthquake process.
The plate boundary off northeast Japan fulfills three important conditions for a long-term geophysical observatory:
The borehole geophysical observatories at Sites 1150 and 1151 will greatly improve source location (particularly depth), and focal mechanism and rupture process determinations of the earthquakes near the Japan Trench (Nishizawa et al., 1990, 1992; Suyehiro and Nishizawa, 1994; Hino et al., 1996). Near-field data, obtained from these observatories with the aid of ocean bottom seismographs, will particularly improve the resolution of source mechanisms of very slow rupture events such as tsunami earthquakes. These earthquakes may occur seaward of the updip end of normal thrust earthquakes (e.g., Tanioka and Satake, 1996).
High-Resolution Geometry of the Plate Boundary
The two stations at Sites 1150 and 1151 will be linked to the network of broadband and/or very broadband seismometers on the main Japanese islands and will make a dense seismic network that is 50 km in scale. The observations of various phases of body waves from the many shallow to deep earthquakes within the network will provide sufficient data to improve the structural image of the plate boundaryin particular, the changes in physical properties associated with tectonic erosion and seismogenesis.
Miocene and Younger Volcanic Ash Stratigraphy in the Western Pacific
The cores will represent an important reference section near Japan to compare with the remote ash deposits already cored to the east. They will also 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 (J. Natland, pers. comm., 1997).
During Leg 132, a number of rhyolitic to dacitic volcanic ash beds were recovered on Shatsky Rise, east of Japan. Comparison with ash stratigraphy at DSDP Sites 578-580, about halfway between Shatsky Rise and Japan, indicates that the Shatsky ash beds were derived either from Japan or the Kurile-Kamchatka arc systems and that they were carried far to the east on the high speed polar and sub-tropical jet streams (Natland, 1993). A summary appraisal is that 25-40 eruptions in each of the past 3 m.y. produced ash that reached one or more of those sites, with ~10% of these reaching Shatsky Rise in the form of discrete ash beds or pumice. Some of the eruptions were extremely large, resulting in deposits 5 to 15 cm thick, even on Shatsky Rise. The last drilling in this region was during DSDP Legs 56 and 57, before the advent of hydraulic piston coring. An important, although seriously incomplete and at times highly disturbed, ash record was recovered in Holes 438A and 440B (e.g., Cadet and Fujioka, 1980).
Subsidence History Across the Continental Slope to Constrain the Processes of Tectonic
Quantitative estimates of the tectonic erosion process were made for the Neogene history of the Japan Trench region based on drilling and seismic records (von Huene and Lallemand, 1990; von Huene et al., 1994). Key evidence came from DSDP Site 439. Evidence collected from additional coring at ODP Sites 1150 and 1151, which are ~200 km south of Site 439, will further constrain the timing and erosion volumes in relation to backarc opening and the style of convergence. The comparison of results between 38°N and 41°N will delineate relative changes along the axis.
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