ABSTRACT
Two borehole geophysical observatories were installed ~1100 m below the seafloor on the deep-sea terrace of the Japan Trench during Ocean Drilling Program Leg 186. Site 1150 (39°11'N, 143°20'E) and Site 1151 (38°45'N, 143°20'E) are located in areas with contrasting seismic characteristics. The northern site is within a seismically active zone where microearthquakes are frequent and M7 earthquakes recur. The southern site is within an aseismic zone where no earthquakes are observed. These features coexist within the seismogenic zone of the Japan Trench plate subduction zone, where the >100-Ma portion of the Pacific plate is subducting at a fast rate (~8 cm/yr) beneath northern Japan causing major earthquakes along the trench. Such a dynamic nature of the subduction seismogenic zone remains unexplained because no geodetic and few seismic stations exist on the seafloor that give us hard evidence in the vicinity of the fault (décollement) zone. Leg 186 is the first scientific venture to succeed in installing state-of-the-art strain, tilt, and seismic sensors for long-term operation in seafloor boreholes. The borehole instruments were installed only 10 km above the gently dipping (< 5°) plate boundary. The systems will start collecting data in September 1999 and will be serviced by a remotely operated vehicle (ROV) at least once a year to recover continuous high sampling rate and wide dynamic range data. These stations will make invaluable additions to the existing geophysical network over the western Pacific. This type of multiple-sensor seismo-geodetic observatory can now be emplaced by the JOIDES Resolution at many other areas where active processes await to be monitored.Previous drilling in the area took place during Deep Sea Drilling Project (DSDP) Legs 56, 57, and 87, which transected the Japan Trench at ~39.8°N-40.7°N. These legs established the concept of tectonic erosion along a subduction zone. The Neogene subsidence history of the forearc was documented and numerous ash records were obtained that span the past 9 m.y. The coring and logging data obtained from Sites 1150 (2681-m water depth) and 1151 (2182-m water depth) provide additional observations to further our understanding of the tectonics of this area with better recovery and higher resolution than was available from previous drilling along the Japan Trench. The ages of the recovered sediments are 10-0 Ma at Site 1150 and 16-0 Ma at Site 1151. The sedimentation rates significantly differ before ~7 Ma between the two sites, but are similar since then, and are similar to the rates at DSDP Leg 57 Site 438 and Leg 87 Site 584.
Estimations on the deformational history from observed microfractures and microfaults and logging data are consistent with a general east-west extensional stress field in which normal faulting dominates. It will be of great interest to compare these structural data and additional mechanical properties measurements to be made on whole-round core samples with data from the observatories, where the current tectonic stress field is actually east-west compressional across the Japan arc to the Sea of Japan.
Recovering detailed ash records was one of the highlighted drilling objectives. As with previous drilling results, a general increase from ~9 Ma and a peak in the past 0.5-4 Ma are observed at the two sites. Postcruise studies will examine the details of the ash record, which was more completely recovered on Leg 186 than on previous cruises.
Inorganic geochemical analysis confirmed that a large decrease in chlorinity and salinity with depth exists in the Japan Trench region. This was first observed at DSDP Sites 438 and 439 but not at other sites of Legs 56 and 57. The character of the anomalies varies also between the two ODP Leg 186 sites. Overall, the magnitude of decrease seems much larger than other subduction environments such as at Nankai or Barbados.
To further our understanding of the plate subduction dynamics, near-site multiple disciplinary investigations are clearly needed. In particular, geological "hysteresis" concealed in present-day dynamics needs to be better understood to construct physical models by linking geological/geochemical and geophysical studies. Leg 186 is one such investigative attempt to link current and past dynamics by establishing borehole observatories and by obtaining core and logging data at the seismogenic zone of the Japan Trench.