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:
Earthquake Source Studies
Stations at proposed Sites JT-1C and JT-2G will greatly improve source location (particularly depth), 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 the stations at Sites JT-1C and JT-2G will particularly improve the resolution of the source mechanisms of very slow rupture events such as tsunami earthquakes.
High-Resolution Geometry of the Plate Boundary
The two stations at proposed Sites JT-1C and JT-2G will be linked to the network of the broadband and/or very broadband seismometers on the main Japanese islands and will become part of a dense seismic network roughly 50-km in scale. The observations of various phases of body waves from many shallow to deep earthquakes within the network will provide sufficient data to improve the structural image of the plate boundary, particularly the changes in physical properties associated with tectonic erosion and seismogenesis.
Miocene and Younger Volcanic Ash Stratigraphy in the Western Pacific (Site JT-1C)
The cores should contain an important reference section from 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 on Shatsky Rise, east of Japan were recovered (Fig. 1). Comparison with ash stratigraphy at Deep Sea Drilling Project (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 subtropical jet streams (Natland, 1993). A summary appraisal is that 25-40 eruptions produced ash that reached one or more of those sites in each of the past 3 m.y., with ~10% of these reaching Shatsky Rise in the form of discrete ash beds or pumice drops. 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, but seriously incomplete and at times highly disturbed, ash record was recovered in Holes 438A and 440B (e.g., Cadet and Fujioka, 1980). Fluctuations in accumulated ash thickness through time over the 15° of latitude represented by the DSDP Leg 86 sites indicates that both the position and velocity of the jets have changed during the past 3 m.y., during the period of pronounced climatic change since the early Pliocene.
Subsidence History across the Continental Slope to Constrain the Processes of Tectonic Erosion
Quantitative estimates of the tectonic erosion process were made for the Neogene history of the Japan Trench region based on drilling and seismic records (Fig. 4; von Huene and Lallemand, 1990; von Huene et al., 1994). Key evidence came from Site 439. Evidence collected from additional drilling will further constrain the timing and erosion volumes in relation to backarc opening and the style of convergence.
Age and Nature of the Cretaceous Basement
Only one hole (Hole 439) touched the Cretaceous basement during DSDP Leg 57 (Shipboard Scientific Party, 1980). This Cretaceous unit is unconformably overlain at the drill site by a 48-m-thick breccia conglomerate, which contains 24 Ma hypabyssal dacitic to rhyolitic boulders (von Huene et al., 1994). Although, the likelihood of reaching the basement is low, if it is achieved, the age determination and areal extent of the basement could confirm the prevailing hypothesis that the now-subsided Oyashio landmass was previously above sea level (Shipboard Scientific Party, 1980).
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