Deep-sea Terrace Research Articles

Active sediment creep deformation on a deep-sea terrace in the Japan Trench

AbstractEighty-six new acoustic survey lines along and across the Japan Trench revealed active sediment creep deformation on a deep-sea terrace at water depths of 400–1200 m in an area of arcuate-shaped depressions that are probably associated with tectonic erosion. The most active region of creep is located on the top at the surface of the depression south of 38° N. The area of creep deformation is characterized by arcuate-shaped topographic lineaments with active folds and active normal faults stepping down trenchward. In contrast to the southern region, normal faults at the top of the depression north of 38° N cut a sedimentary sequence (Unit 1) that is acoustically transparent with continuous weak reflectors, and this is covered by the undeformed layered sediment sequence of Unit 2. Unit 2 corresponds to the period of rising sea level that extended from the latest Pleistocene to the early Holocene (14–6 ka). Thus, creep is ongoing at the top of the depression south of 38° N in the surface layer, whereas it stopped north of the depression between 14 and 6 ka. These observations might indicate that the active region jumped from north to south due to probably retrogressive sliding.

Principal horizontal stress orientations prior to the 2011 Mw9.0 Tohoku-Oki, Japan, earthquake in its source area

[1] We determined principal horizontal stress orientations as of 1999 at sites in the source area of the 2011 Mw 9.0 Tohoku-Oki earthquake, on the deep-sea terrace of the Japan Trench. The maximum principal horizontal stress orientation at site 1151 of Ocean Drilling Program Leg 186, located in an aseismic zone, was east-southeast, parallel with the plate convergence direction; and the stress orientation at site 1150 of Leg 186, in a seismically active zone, was south-southeast, suggestive of a local stress feature due to the influence of seismic activity. Aftershocks with normal faulting mechanisms, observed in the hanging wall of the plate interface that ruptured during the Tohoku-Oki earthquake, indicate a normal faulting postseismic stress regime, which clearly differs from the stress state prior to the earthquake. Consequently, the stress state may have changed from a reverse to a normal faulting stress regime during the Tohoku-Oki earthquake sequence.

Enrichment of adsorbed methane in authigenic carbonate concretions of the Japan Trench

Substantial amounts of adsorbed methane were detected in authigenic carbonate concretions recovered from sedimentary layers from depths between 245 and 1,108 m below seafloor during Ocean Drilling Program Leg 186 to ODP sites 1150 and 1151 on the deep-sea terrace of the Japan Trench. Methane contents were almost two orders of magnitude higher in the concretions (291–4,528 nmol/g wet wt) than in the surrounding bulk sediments (5–93 nmol/g wet wt), whereas methane/ethane ratios and stable carbon isotopic compositions were very similar. Carbonate content of surrounding bulk sediments (0.02–3.2 wet wt%) and methane content of the surrounding bulk sediments correlated positively. Extrapolation of the carbonate contents of bulk sediments suggests that 100 wt% carbonate would correspond to 1,886±732 nmol methane per g bulk sediment, which is similar to the average value observed in the carbonate concretions (1,321±1,067 nmol/g wet wt, n = 13). These data support the hypothesis that, in sediments, adsorbed hydrocarbon gases are strongly associated with authigenic carbonates.

Sediment Physical and Acoustic Properties at ODP Sites 1150 and 1151, Japan Trench

ABSTRACT Two sites (Sites 1150 and 1151) located in the deep-sea terrace on the landward side of the Japan Trench were drilled during the Ocean Drilling Program (ODP) Leg 186. Measurements of sediment physical and acoustic properties were made on whole-round core sections and on the least-disturbed portions of split-core sections. Compressional wave velocity generally increases with depth. But the data below 600 meters below seafloor (mbsf) is more scattered due to velocity variation caused by microstructures than that of the upper section (< 600 mbsf). Physical properties at Sites 1150 and 1151 match well each other, characterized by decreasing water content and porosity and increasing wet bulk density with depth, indicating variations in hydrological and mechanical conditions. The reverse trend below 100 mbsf is probably responsible for sampling disturbance and degassing cracks by gas expansion. In addition, it is most likely due to the underconsolidation caused by the rapid deposition (100 ∼ 200 m/m.y.) and impermedable nature of the sediment (clay and diatomaceous ooze rich) that prohibit dewatering. Correlations between physical and acoustic properties at Sites 1150 and 1151 show a nearly similar pattern, suggesting no evidence of change of sediment properties caused by frequent earthquakes. Therefore, sediment physical and acoustic properties are not likely to be related to sediment deformation caused by seismic activity occurring in the plate boundary.

Characterization of physical properties for sediments and sedimentary rocks from Japan Trench, ODP Leg 186

Two sites (Sites 1150 and 1151) located in the landward deep-sea terrace of the Japan Trench were drilled during the Ocean Drilling Program (ODP) Leg 186. Site 1150 is seismically active, whereas Site 1151, aseismic. Measurements of sediment physical properties were made on both whole-round core and split-core sections. Bulk density is relatively uniform with similar variation with depth, although its absolute values are slightly different. It is most likely attributable to coring disturbance. Magnetic susceptibility is characterized by the highest value at the lithologic unit I. Depth variation of water content and porosity shows gradual decreasing pattern, except for lithologic unit I, similar to that of natural Gamma radiation. The abnormal characteristics of unit I are likely related to the under-consolidation caused by rapid deposition (100–200 m/myr) as well as impermeable nature of the sediment preventing dewatering. Grain density is higher in the unit I, but below which is relatively constant with depth. P-wave velocity gradually increases from 1500 to 2000 m/s with depth, caused by sediment compaction and/or consolidation. Sediment physical properties of both sites are very similar, although p-wave velocity is slightly higher (≈20 m/s on the average) at Site 1151. This indicates that sediment source, mineral composition and sediment diagenesis are not significantly different at both sites, implying that seismic activities such as earthquake do not likely cause marked changes of sediment physical properties.

日本列島周辺の海底地質 その３． 南海舟状海盆（トラフ）及びその付近

The Nankai Trough is regarded as a trench from the structural point of view, in spite of its shallowness and flatness. The topography of the sea floor of the Nankai Trough and its vicinity shows typical features consisting of the continental shelf, the upper slope (shelf slope), the deep-sea terrace, the lower slope (landward wall of the trough), the trough floor, and the ocean basin. The upper slope is comparatively monotonic. The deep-sea terrace is a large structural basin lying at the foot of the upper slope, behind the outer ridge rising on the continental slope. The lower slope from the outer margin of the deep-sea terrace to the trough floor consists of accretions which form ridges and troughs. This is divided into smaller scale units by the structure of faults and folds. Sediments on the sea bed of the Nankai Trough have been deformed in conformity with the formation of the hollow of the trough. The Nankai Trough and its related topographies, namely the upper slope, the outer ridge shifting landward on the whole, and the ridge-and-trough zone, were formed by the subduction of the Philippine Sea plate. Topographic trends of the Nankai Trough area are deviated largely at the boundary, which can be estimated by distribution of hypocenters, between sub-slabs in the subducting Philippine Sea plate. The age of formation of the major topographies of the Nankai Trough and its vicinity falls within the post-Pliocene.

Crustal structure and seismicity beneath the forearc off northeastern Japan

Marine seismological and other data in the Japan Trench area (north of 38.5°N) were used to infer the relation of subducting plate structure, seismicity, and focal mechanisms especially at the aseismic to seismogenic zone along the interface between the two interacting plates. Previous crustal models from ocean bottom seismographic refraction (OBS) surveys were improved by taking into account the relative seismic amplitude characteristics and constraining the shallow structure using multichannel seismic data. A wave speed discontinuity in these models is interpreted to be the contact zone of the crust of the overriding plate and the subducting Pacific plate crust. Its dip angle increases to about 7°, 110 km landward of the trench axis. A large increase beneath the deep sea terrace is required to reach the well‐defined angle of 25° beneath the Tohoku east coast. A large wave speed gradient within layer 2, commonly observed under normal oceans, seems to vanish beneath the inner trench slope at about 10‐km depth. Within the overriding plate, apparently brittle material with a P wave speed of ∼6 km/s can be found as near as 35 km from the trench axis. The upper limit of the seismogenic zone of interplate low‐angle thrust events is about 15‐km depth from OBS seismicity and large‐event analyses. Both the strength of the crust of the overriding plate and the characteristics of subducting sediments must be investigated to define the seismic coupling of interacting plates.

日本列島周辺の海底地質 その１ 日本海溝及びその付近

The topography and morphology of the Japan Trench and its vicinity are compiled in Figures 1 and 2, based on the results of submarine topographic and geological surveys, including multinarrow beam soundings. A typical feature of the Japan Trench and its vicinity from land to sea, consists of the continental shelf, the continental slope, the trench itself (trench landward slope, trench axis basin and trench seaward slope), and the marginal swell between the trench and the typical ocean floor. On the continental slope, generally on its outer margin, elevations of basement are often found. The structural basin on the back side of elevations is usually filled up with younger sediments and forms the deep-sea terrace. Benches are recognized on the trench landward slope. They repesent probably a hinge zone between whole uplifting of the Tohoku Arc and sinking of the lower part of the trench landward slope originated from the subduction around the trench axis since the late Miocene. Topographies of drainages, hollows and mounds caused by submarine landslides are developed along the foot of the trench slope, and in some places the trench-slope apron stretches from the foot of the trench landward slope to the trench axis. The trench axis consists of a series of the elongated basins arranged weakly in echelon. From the trench seaward slope to the marginal swell are found many narrow but long depressions, showing the horst and graben structure. These depressions develop in parallel with formation of the marginal swell under the tensile stress. The subducting angle of the oceanic plate beneath the Japan Trench is 14° at 110km west of the trench axis, based on the analysis of geomagnetic data.

Nature and development of Late Mesozoic and early cenozoic sedimentary basins in southwest Japan

The southwestern part of the Japanese Islands is a typical example of an active continental margin characterized by backarc, intra-arc and forearc sedimentary basins. The back-arc basins provide evidence of tectonic continuity with the Asian continent. The basins are genetically classified as half-graben, ridge and forearc basins. The half-graben and ridge basins are controlled by strike-slip faulting, although the half-graben basins are pull-apart basins on a much larger scale than the ridge basins and are in many instances closely associated with igneous activity. Forearc basins occupy the area from the deep-sea terrace to the trench floor. Forearc basin-fill sediments form accretionary prisms as a result of subduction processes. Good examples of these basins are well developed in the Cretaceous and Paleogene sediments of southwest Japan.

海洋地質の諸問題について

The cause of sharp boundary between sand layer and mud layer amomg sand and mud alternation is explained from the hydrodynamical view point. In nature, gravel, sand, and mud are being separated by the difference of settling velosity and threshold velosity. This process causes the sharp boundary between sand and mud layers among sediment. Furthermore, ocean floor spreading theory, plate tectonics theory, accretionary wedge, the discovery of Oyashio ancient landmass beneath the deep sea terrace of japan Thrench and their significance are introduced.

Small‐scale deformation structures and physical properties related to convergence in Japan Trench slope sediments

Diatomaceous mudstones, cored on DSDP legs 56/57 off Japan, contain numerous postdepositional veins, healed fractures, and microfaults. Beneath the forearc basin, these secondary structures occur only at depths greater than 620 m (Miocene) in a zone of normal faulting. Beneath the landward slope of the trench, however, they occur at depths as shallow as 250 m (Pliocene) in hemipelagic sediments which are inferred to blanket tectonically accreted deposits. The intensity of deformation in the trench slope deposits increases in a seaward direction. Bulk densities indicate that the trench slope deposits have not dewatered normally but are, above 400 m, more consolidated than sediment deposited on the deep sea terrace. A decline in sediment bulk density with increasing depth suggests that open fractures may exist in situ, perhaps induced and/or maintained by excess pore pressure. Below about 550 m, a normal pattern of increasing density with increasing depth may reflect fracture closure and consolidation under lithostatic load. We suggest that tectonic stress related to plate convergence has been communicated to the sediment on the landward Japan Trench slope, causing tectonic dewatering within the subduction complex and inducing pervasive but small‐scale veining, fracturing, and faulting in near‐surface (≤700 m) hemipelagic deposits. This deformation occurs in sediment which has not been subject to large tectonic movement and does not eradicate larger (50–100 m thick) structural/depositional units, which are apparent on seismic reflection records. The brecciation induced by deformation may reflect natural hydrofracturing under abnormally high pore pressures or opening of fractures formed earlier (at shallower depths) during the dewatering process. The overpressures may, in turn, locally reduce the sediment shear strength at shallow levels in the sediment column and induce downslope mass sediment movement.

Seismic refraction and reflection measurements around Hokkaido. 2. Crustal structure of the continental slope off Hidaka.

Refraction measurements by the two ship method were conducted in July 1969 in the deep sea terrace of Hidaka, Hokkaido. The general features of the crustal structures derived are similar to those in the adjacent oceanic areas. Thick sediments cover the whole area, especially its northernmost part. The layer with a P-wave velocity of 6km/s is found in all profiles. That is, the crustal structure in this area is continental and similar to that off the Sanriku area.

Submarine Topography off the Southern Part of Sanriku District

In this paper are summarized the findings of the bathymetric survey carried out off the Pacific coast of the southern part of Tohoku (Sanriku) District of Honshu, Japan.Significant topographic features of the continental shelf, continental slope, deep sea terrace and the Japan Trench are illustrated on Profiles I to XII.

Seismic Refraction and Reflection Measurements around Hokkaido.

Seismic refraction and reflection measurements were made across the continental slope off the southeastern coast of Hokkaido and the Kuril-Kamchatka trench. The whole slope is covered by thick low-velocity sediments. Layers with velocities of 5.4 and 5.8 km/sec are found beneath the upper slope. Under the lower deep-sea terrace (1400-2000 m) an unconformity is observed by profiler technique which may show a presence of a large tectonic movement in this area in the past. A structural change is remarkable at water depth between about 4000 and 4500 m, and beneath the land-side slope of the trench a troughlike structure is buried by materials with velocities 4.2 and 2.0 km/sec. Beneath the oceanside slope of the Kuril-Kamchatka trench the crust of the oceanic type is obtained. Many clear faults are observed on this slope by seismic profiler measurements as same as in the Japan and the Izu-Ogasawara trenches.

A Seismic Refraction Exploration of Tosa Deep-Sea Terrace off Sikoku

A seismic refraction measurement was made in the Tosa deep-sea terrace south of Sikoku (Shikoku). Thick sediments consisting of 6 layers with velocities of 1.70, 2.04, 2.68, 3.42, 4.07 and 4.58km/sec respectively were found and the total thickness reaches 11km. The 6.89km/sec layer under the sediments could correspond to the oceanic crust in the Sikoku basin.