Precise seabottom topography on the outer slope of the Japan Trench was analyzed from swath bathymetric data. Grid bathymetric data with spacing of 0.005 degrees (555 m by 438 m at 38 degree north) was synthesized from full covered multi-beam bathymetries on the outer slope of Japan Trench. Topographic maps and profiles were drawn by the grid bathymetry. The topographic profiles are in a direction of 296 degrees which is parallel to the plate motion of the Pacific plate at the area. Fault systems characterized by horst and graben structure are developed on the outer slope of Japan Trench. They are caused on the upper surface of the Pacific plate by downward bending of the plate subduction. The fault systems developed on the western margin of the outer rise, are emphasized with evolving near by the trench axis. West throw faults are dominant in every profile. On the assumption that the Pacific plate is homogeneous in terms of physical property and moves constantly, an average displacement rate of the fault system can be estimated from the distance between the fault system and the position where the fault system is initially recognized on the profile. Strain rates close to the trench axis are estimated as 9×10-8 yr (at northern Japan Trench), 7×10-8/yr (at southern Japan Trench) and 12×10-8/yr (at northern most of Izu-Ogasawara Trench). These are almost same as the extension rate of the crustal strain in the central mountain region in the Chubu district, those are highest in Japan.

Strong tidal currents at a narrow strait erode seafl oor and make caldrons. Tidal current changes the fl ow direction according to the ebb and fl ood, thus, caldrons are formed on both sides of the strait. There are strong tidal currents over 5 knots (2.5m/s) at the Hayasui-Seto, between Sagano-Seki, Kyushu and Sada-Misaki, Shikoku. The caldrons are well developed at the both side of the Hayasui-Seto. Maximum water depth of the caldron is 465m which is the deepest point of the Seto Naikai. Tidal currents become weak to the offi ng of the strait as well as sediments are deposited on the seafl oor.

We conducted swath bathymetry and gravity surveys the whole-length of the Yap Trench, lying on the southeastern boundary of the Philippine Sea Plate. These surveys provided a detailed morphology and substantial insight into the tectonics of this area subsequent the Caroline Ridge colliding with this trench. Horst and graben structures and other indications of normal faulting were observed in the sea-ward trench seafloor, suggesting bending of the subducting oceanic plate. Major two slope breaks were commonly observed in the arc-ward trench slope. The origin of these slope breaks is thought to be thrust faults and lithological boundaries. No flat lying layered sediments were found in the trench axis. These morphological characteristics suggest that the trench is tectonically active and that subduction is presently occurring. Negative peaks of Bouguer anomalies were observed over the arc-ward trench slope. This indicates that the crust is thickest beneath the arc-ward trench slope because the crustal layers on the convergent two plates overlap. Bouguer gravity anomalies over the northern portion of the Yap Arc are positive. These gravity signals show that the Yap Arc is uplifted by dynamic force, even though dense crustal layers underlie the arc. This overlying high density arc possibly forces the trench to have great water depths of nearly 9000 m. We propose a tectonic evolution of the trench. Subduction along the Yap Trench has continued with very slow rates of convergence, although the cessation of volcanism at the Yap Arc was contemporaneous with collision of the Caroline Ridge. The Yap Trench migrated westward with respect to the Philippine Sea Plate after collision, then consumption of the volcanic arc crust occurred, caused by tectonic erosion, and the distance between the arc and the trench consequently narrowed. Lower crustal sections of the Philippine Sea Plate were exposed on the arc-ward trench slope by overthrusting. Intense shearing caused deformation of the accumulated rocks, resulting in their metamorphism in the Yap Arc.

Two major marine surveys off northern Papua New Guinea (PNG) earlier this year now suggest, when survivors' reports are taken into account, that last summer's disastrous tsunami there was caused by a sediment slump 25 km offshore. The slump was probably the result of seabed shaking from an earthquake. Not only was a sediment slump, or submarine landslide, responsible for the tsunami, according to the data, but the magnitude and wave‐height distribution of the tsunami along the coast were the result of focusing by local seabed morphology.The conclusions are based on new off‐shore bathymetry, remote operated vehicle (ROV) dive investigations, the time delay between the source earthquake and when the tsunami struck, computer simulation models, and earthquake aftershock distribution. The most critical evidence is in survivors' accounts of the timing of the tsunami relative to the initially felt earthquake and aftershock [see Davies, 1998a].

Direct observation and sampling by research submersible Shinkai 6500 together with bathymetric and geophysical survey revealed that sequences of serpentinized peridotite, basalt and limestone are exposed on the landward slope of the Palau Trench. A huge block (larger than 3 km(2) in horizontal dimension) of coral-reef limestone was discovered at depths ranging from 4,900 m to 6,500 m. The result seems to indicate that extensive slope failure is occurring on the landward slope from which great amount of mass is wasted to the trench axis. Nevertheless, greater portion of the Palau Trench axis has V-shaped bottom with little sediment cover, regardless its very slow rates of plate convergence accompanied with no deep-focus earthquakes. Islands of Palau (Belau) are extraordinarily close to the trench axis and appear to be now uplifted. A hypothesis is postulated to explain this unique situation.

The distribution of Calyptogena phaseoliformis colonies in right-stepping en echelon patterns was observed by the Japanese submersible Shinkai 6500 at the foot of the landward escarpment of the northern Japan Trench at around 6437–6274 m depth. The north-south–trending Sanriku Escarpment has a thrust origin and is subparallel to the trench axis along which the Pacific plate is being subducted beneath the North America or Okhotsk plate at about 300° at a rate of about 7.8 to 8.3 cm/yr. The trends of colonies are concentrated at 250°, 300°, and 330°: each trend matches either an antithetic riedel shear, extension fracture, or synthetic riedel shear, respectively, within a left-lateral shear regime caused by the oblique subduction. Methane- and hydrogen sulfide–bearing fluid advection from depth occurs essentially along the thrust fault, but finally seeps along the fractures at the sea floor. This supplies energy to the food chain through bacteria utilizing hydrogen sulfide, then eventually sustains the Calyptogena colonies. Because the clams select the best places to survive, the geometric arrangement of the clam colonies provides a kinematic indicator of relative plate motions.

A detailed magnetic anomaly map around the central rift area in the North Fiji Basin was compiled from 6 starmer cruises. The data range is almost 800 km in length (between 22°S and 14°30′S), 100 km in width (between 173°E and 175°E). Detailed magnetic structure analysis was applied for the data around the rift axis, which revealed 5 or 6 segments along the 800 km long rift axis. Inversion trials were done on the grid data and good comparison with topographic features was obtained. The results of inversion indicate that the lineations which correspond to the Olduvai event were recognized in this area broadly, but not so common as those for the Gauss epoch. The central rift system seems to have started its spreading from around the end of the Gauss normal epoch or the beginning of the Matuyama reversed epoch (2.5 Ma).

The 800-km-long N-S spreading system in the North Fiji Basin consists of six contiguous fan-shaped rift segments each 100–200 km long with various structural styles, such as structural overprinting, triple junction, rift-propagation, voluminous magmatism, and transform or strike-slip fault influence. Deformation of the plate boundary system within the young, hot, weak lithosphere occurred under local stress conditions resulting in a unique rift segmentation distinct from that of midoceanic ridges. Spreading across a series of short and variably oriented segments produces series of fanning spreading centers whose rotation pole is located at the end of each segment. The surrounding seafloor basement has adjusted to this setting with non-rigid deformation or with fragmentation. Furthermore, the changing tectonic framework caused by the arc rotation probably makes it difficult to keep the same stress condition for a long period. Small-scale fan-shaped sea-floor spreading is short-lived and results in a rapidly evolving plate boundary geometry. This suggests a more complicated tectonic style for marginal basins than mid-ocean ridge spreading centers.

Two cruises of the French-Japanese starmer joint project carried out with the submersibles Nautile (1989) and Shinkai 6500 (1991), have been devoted to the in situ exploration of the North Fiji Basin ridge. Four areas, located near 16°20′, 17°, 18°50′ and 19°S have been explored during 42 dives. At 16°20′S, the axial zone trending N160° corresponds to a prominent graben with walls 1000 m high. Tectonic features and recent screes are present, and the hydrothermal activity is characterized by sporadic fluid discharge. At 17°S, present accretion is confined to a high dome less than 1900 m deep and cut in its axial part by a wide graben trending N15°. At this site, hydrothermal features of two types are observed: fossil chimneys of sulfides, and active vents, made of anhydrite and expelling water at 285°C. The site at 18°50′S is located on top of an axial dome, very similar to those of the East Pacific Rise. Small fissures and very fresh volcanic flows argue for a present-day activity, with weak evidence of tectonics. Hydrothermal emanations with animal colonies are found scattered over the whole area. The fourth site, at 19°S, shows old and viscous lavas and a very mature tectonic activity, compared to the preceding station which is located only 20 km northward. Large-scale variations have been detected between three segments. Different thermal regimes can explain the distinct morphologies observed. Decakilometric scale variations do occur between stations 14, with volcanic predominance, and “19°S”, where tectonic activity is dominating. Finally, small-scale variations are observed on sites 4 and 14, where volcanic features gradually give way to tectonic structures. The North Fiji Basin ridge—with an intermediate average spreading rate—shows tectonic, volcanic and hydrothermal variability, ranging from 1 km to tens of kilometres and related to a temporary fine scale organisation of the accretionary processes.

The aim of the Japanese-French Kaiyo 87 cruise was the study of the spreading axis in the North Fiji Basin (SW Pacific). A Seabeam and geophysical survey allowed us to define the detailed structure of the active NS spreading axis between 16° and 22° S and its relationships with the left lateral motion of the North Fiji Fracture Zone. Between 21° S and 18°10′ S, the spreading axis trends NS. From 18°10 S to 16°40 S the orientation of the spreading axis changes from NS to 015°. North of 16°40′ S the spreading axis trends 160°. These two 015° and 160° branches converge with the left lateral North Fiji fracture zone around 16°40′ S to define an RRFZ triple junction. Water sampling, dredging and photo TV deep towing give new information concerning the hydrothermal activity along the spreading axis. The discovery of hydrothermal deposits associated with living communities confirms this activity.