Toyama Trough Research Articles

Effect of a current trapped by a continental slope on the pathway of a coastal current crossing Toyama Trough, Japan

To clarify the Tsushima Warm Current (TWC) system around a downstream region of Noto Peninsula (NP) where Toyama Trough (TT) causes a discontinuity of the along shelf-current, numerical experiments were performed using two-layered ocean model with simplified bottom topography. When a current trapped by the continental slope representing an offshore branch of TWC (OB) encounters the NP and TT, the OB was trapped by the coast facing the TT. A clockwise lee-eddy developed between a current axis and a coastline over the TT, which results in a current path transition offshore (CPT). The OB finally adjusted to a current path which crosses TT by shifting from the discontinuity point of the continental slope. If a coastal current representing a coastal branch of the TWC (CB) developed under this situation, the CB also formed lee-eddy within the identical region of the OB’s eddy, resulting in CPT. Period for the CPT of the CB was shorter with increase of the OB volume transport. These results indicate that the OB acted to accelerate CPT of the CB within the TT. To support these results, observational data comprising mooring current, tidal, and CTD measurements obtained in and around the TT, along with the output from high-resolution data-assimilated ocean model DR_C were analyzed. Although the lee-eddy of the CBs was generated in every summer, the CPT events did not fully develop when the OB was absent over the continental slope because of the slow growth rate of the lee-eddy.

Neotectonics in and around Toyama Trough, Japan Sea

日本海東縁の富山トラフおよび周辺海域について，海底地形観測ならびに地殻構造探査の成果をコンパイルした．その結果，つぎの知見を得た．

Geologic structures and basin formation tectonics in and around Toyama Trough, the Sea of Japan

Geologic structures and basin formation tectonics in and around Toyama Trough, the Sea of Japan

富山トラフ南東部，上越埋没海底谷群および海鷹海脚周辺のチャネル・斜面崩壊堆積物の三次元サイスミック地形学的解析

富山トラフ南東部，上越埋没海底谷群および海鷹海脚周辺のチャネル・斜面崩壊堆積物の三次元サイスミック地形学的解析

Submarine topography in and around the Toyama Trough

Submarine topography in and around the Toyama Trough

地震探査から見た富山トラフの地殻構造の特徴

地震探査から見た富山トラフの地殻構造の特徴

Transition of the Tsushima Warm Current Path Observed over Toyama Trough, Japan

AbstractMooring, CTD, and ADCP observations were made in 2012 in and around the Toyama Trough (TT) cutting across a continental shelf along the Japanese coast of the Japan Sea between Noto Peninsula (NP) and Sado Island (SI) to investigate spatiotemporal characteristics of path transition of the coastal branch of the Tsushima Warm Current (CBTWC). Around SI, downstream of the TT boundary, a wavelike alongshore current perturbation, accompanied by sea level rise, was observed. This perturbation occurred after the seasonal amplification of the CBTWC around the NP on the upstream boundary of the TT. This process was delineated by the results of numerical experiments performed with a two-layer model using idealized topography. The model showed that a current path of the CBTWC shifted from alongshore mode to offshore mode bridged over the TT in association with the lee eddy development behind the NP toward the SI over the TT. This lee eddy is generated by positive vorticity induced over topographic discontinuity between the continental shelf off the northern coast of the NP and deeper region of the TT. The model indicated the period of eddy formation is 60–90 days if the volume transport is 1 Sv (1 Sv ≡ 106 m3 s−1), whereas the observations showed the formation period was only 47 days at 1.2 Sv of volume transport. To explain this discrepancy, temporal variation of the CBTWC, vortex supply from preexisting eddies, or eddies caused by the scattering of coastal-trapped waves were suggested as new processes that accelerate the growth rate of the lee eddy.

Geometry and slip rates of active blind thrusts in a reactivated back-arc rift using shallow seismic imaging: Toyama basin, central Japan

Active blind thrust faults, which can be a major seismic hazard in urbanized areas, are commonly difficult to image with seismic reflection surveys. To address these challenges in coastal plains, we collected about 8km-long onshore high-resolution two-dimensional (2D) seismic reflection data using a dense array of 800 geophones across compressionally reactivated normal faults within a failed rift system located along the southwestern extension of the Toyama trough in the Sea of Japan. The processing of the seismic reflection data illuminated their detailed subsurface structures to depths of about 3km. The interpreted depth-converted section, correlated with nearby Neogene stratigraphy, indicated the presence of and along-strike variation of previously unrecognized complex thrust-related structures composed of active fault-bend folds coupled with pairs of flexural slip faults within the forelimb and newly identified frontal active blind thrusts beneath the alluvial plain. In addition, growth strata and fold scarps that deform lower to upper Pleistocene units record the recent history of their structural growth and fault activity. This case shows that shallow seismic reflection imaging with densely spaced seismic recorders is a useful tool in defining locations, recent fault activity, and complex geometry of otherwise inaccessible active blind thrust faults.

Structures and active tectonics of compressionally reactivated back-arc failed rift across the Toyama trough in the Sea of Japan, revealed by multiscale seismic profiling

Compressional reactivation of back-arc failed rifts in the Sea of Japan and its mechanism that mobilizes active tectonics based on the crustal structures remains poorly understood. Here, we show new results and interpretations of onshore-offshore deep seismic reflection profiles across the Toyama trough and its southern extension, located in the Sea of Japan back-arc rift, to reveal their deep to shallow structures of the crust and reactivated active faults. We observed (1) approximately 5km thick, syn- and post-rift Neogene sediments underlain by pre-Neogene basement rocks characterized by high velocity anomalies, and (2) crustal thrust wedges adjacent to the rift axis. Shallower high-resolution seismic reflection profiles also suggest Quaternary structural growth of the reactivated thrusts and thrust wedges, consistent with tectonic geomorphology of the middle to late Pleistocene marine and fluvial terraces. These findings demonstrate that the mechanical contrasts between the crustal thrust wedges, primarily composed of the pre-rift continental crust and high velocity lower crust caused by mafic intrusions in the rifting stage, have fundamentally controlled the styles of active deformation during the post-rift Quaternary compressional stress regime. These results also contribute to the construction of the seismic source fault models for tsunamis and seismic hazard estimations of this region. In addition, we emphasize that combining multi-scale seismic reflection profiles with tectonic geomorphology sheds light on complicated structural characteristics and recent activities of reactivated normal faults that are mostly blind because of the dynamic subsidence of failed rift structures.

Sediment supply system from Neogene Niigata-Shin'etsu basin to the Toyama Trough inferred from sand composition

Sediment supply system from Neogene Niigata-Shin'etsu basin to the Toyama Trough inferred from sand composition

Deep to shallow crustarl structure and active tectonics of the Toyama trough and its surrounding region, based on multi-scale seismic reflection profiling

Deep to shallow crustarl structure and active tectonics of the Toyama trough and its surrounding region, based on multi-scale seismic reflection profiling

日本海東縁海底下に分布する砂層の孔隙特性と年代 : MD179航海

In 2010 the MD179 project was undertaken aiming at recovery of deep seated gas hydrate, methane induced carbonate, and deep sediments older than 300 ka. Sediment samples were obtained in the Umitaka Spur, Joetsu Channel, Toyama Trough, Japan Basin, Nishi Tsugaru and Okushiri Ridge areas. Small amounts of sandy sediment have been retrieved as thin intercalations in Pleistocene and Holocene muddy layers, where trace fossils and strong bioturbations are commonly observed.Those sandy sediments consist of very fine- to fine-grained sand grains, and are sometimes tuffaceous. Pore-size distribution measurements and thin-section observations of these sands were undertaken, which indicated that porosities of muddy sediments are around 50 % but those of arenites range from 42 to 52%. Mean pore sizes and permeabilities of those arenites are larger than those of mudstones. While the presence of gas hydrate in intergranular pores of sands has not been confirmed, the soupy occurrence in recovered sediments may strongly indicate the presence of hydrate filling the intergranular pore system of sands. Such arenites have been recognized till now in the Mallik, NW Territory Canada, as well as in the Nankai Trough areas, which are expected common even in the subsurface sandy sediments at the eastern margin of Japan Sea. Concentration of gas hydrate may need primary intergranular pores large enough for gas hydrate to occur within host sediments likely deposited in the sedimentary environment such as deep sea channels. Small amounts of sandy sediment were retrieved as thin intercalations in Pleistocene silty layers, because supplying sediments may not be abundant due to sea level fluctuation during Pleistocene ice age. As time of deposition of coarse-grained sediments can be recognized by the thermoluminescence (TL) dating method, sandy sediments are usually tuffaceous and contain a small amount of quartz grains, and TL dating has been completed only for seven samples, which indicate 48 to 980 ka in age. This study was performed as a part of the MH21 Research Consortium.

MD179調査より得られた日本海東縁域海底表層堆積土の物理・力学的性質

Gas hydrates are attracting attention as a next-generation energy source. On the other hands, Methane gas contained in the natural gas hydrates has approximately 20 times the greenhouse effect of CO2. There are concerns that dissociation of methane gas from the gas hydrates distributed in submarine surface layers (shallow type gas hydrates) by rising ocean temperatures or by vaporization at recovery of the hydrates for energy may contribute to global warming, which in turn may raise the sea level and causing climatic instability. In addition, many gas-hydrate-bearing areas are distributed near the boundaries of tectonic plates, as shown in the figure. Gas hydrates in the surface layer of seafloor may dissociate when seismic activities cause seafloor landslides that in turn cause gas hydrate-bearing layers to fail. There have been concerns over the environmental effects of shallow type gas hydrates, but surveys for shallow type hydrates have been few.The purpose of this study is to understand the soil properties of the sea-bottom sediment obtained from the Eastern Margin of Japan Sea. The samples were collected from 5 regions (Umitaka Spur, Joetsu Ridge, Toyama Trough, West of Tsugaru and west of Okushiri Ridge), and the physical (soil density and water content) and mechanical (cone penetration ratio and vane shear strength) tests were performed by on-board used for the sea-bottom sediments.From the test results, it is found that the physical properties of the sediment samples obtained in these areas have no marked difference regardless of the locations or depth of sampling. However, the soil strength of sea-bottom sediments obtained gas hydrates was lower than those of other sediments. It would seem that this is because the effect of methane hydrates dissociation and vaporization of dissolved gas in the pore water at sampling.

Characteristics of deformation structure around the 2007 Niigata-ken Chuetsu-oki earthquake detected by multi-channel seismic reflection imaging

Abstract A multi-channel seismic reflection (MCS) survey was conducted to investigate the tectonic structure off Niigata, which caused the 2007 Niigata-ken Chuetsu-oki earthquake, using the research vessel (R/V) KAIREI of the Japan Agency for Marine-Earth Science and Technology. Based on the results of data processing and interpretation of available data, three areas are identified according to seismic characteristics. The most deformed area is located on the continental shelf near the source region of the 2007 Niigata-ken Chuetsu-oki earthquake, i.e., the area east of the Yoneyama-Ogi Uplifts. A remarkable growth of folds, including fault-related folds, and a strong reflector dipping east is identifiable by localized strain concentrations. The second area is located between the Yoneyama-Ogi Uplifts and the Jouetsu Knoll in the Toyama Trough. Although the deformation of deposits in the second area was smaller than in the first area, folds are identified. The third area is located toward the west of the Jouetsu Knoll in the Toyama Trough. No significant deformed structures developed in this area. Based on the interpretation of stratigraphy obtained in previous studies, seismic characteristics, and well data, the development of an anticline was initiated by a compression field after about 3.6 Ma. In particular, the deformation of sedimentary layers by the compression field occurred rapidly after about 1.3 Ma. Folds have grown larger toward the east after about 1.3 Ma. In addition, subsidence of about 0.2 s in sedimentary layers can be seen at the western margin of the Yoneyama-Ogi Uplifts, suggesting that tectonic movement related to reverse faulting has advanced there very recently. From aftershock distribution on a depth section on line S-2 near the hypocentral region, most hypocenters were determined to be below the strong reflector. This result suggests that fold growth has accompanied past large earthquakes, such as the 2007 Niigata-ken Chuetsu-oki earthquake.

基礎試錐「佐渡南西沖」の掘削結果に基づいた佐渡〜富山湾海域における石油システムの考察

The petroleum system in the deep water of the Toyama Trough to the southwest offshore of the Sado Island in the Japan Sea was considered based on the analytical results of METI Sado Nansei Oki S and D drillings and their preliminary surveys.The middle Miocene Lower Teradomari and Nanatani Formations indicate good source rock potential (0.90-2.98% TOC). The kerogen type is a mixture of Type II and III. The bitumens include organic matter from both marine and terrestrial sources, notably characterized by very abundant oleanane like the case of oils in the Kubiki area onshore Niigata.The oil sample collected from the Shiiya Formation of METI Sado Nansei Oki S is characterized by its high oleanane content. A seeped oil sample on the sea bottom, although heavily degraded, also indicates similar oleanane content and carbon isotope characteristics that can be correlated to the oil.The source rock of the oil is thought to be the mudstone of the Lower Teradomari and Nanatani Formations from the analytical results of the petroleum source potential, carbon isotopes and biomarker indices, especially the ratio of oleanane to norhopane content. The contribution of marine organic matter to the oil seems to be slightly higher than that to the mudstone in the Lower Teradomari and Nanatani Formations of the well. It suggests that the oil was derived from the source rocks of the same horizon that deposited in the basin center in comparison with the well location.The methane hydrate samples taken by preliminary sea bottom survey are referred to be thermogenic origin from the carbon isotope ratio. It can be isotopically correlated to headspace gas samples collected at 1900m and deeper zone of METI Sado Nansei Oki D well. This means that the deeper gas migrated to the surface very quickly without the appreciable input of shallow biogenic gas.Fluid inclusion analysis implies that the generated hydrocarbon was once trapped in the lower Teradomari Formation at METI Sado Nansei Oki D, then it migrated upward to the Shiiya sandstones through large faults and finally accumulated in the structural crest of the Shiiya horizon (METI Sado Nansei Oki S). Part of the hydrocarbons leaked from the Shiiya Formation through a number of minor faults to form methane hydrates and many Direct Hydrocarbon Indicators (DHI) that are observed with three dimensional (3D) seismic data.The kitchen area of the oil is estimated to be a syncline that locates west of the wells and is buried deeply along the Toyama Trough. It is suggested that the kitchen area provided hydrocarbons to deep water large structures and possibly to the onshore areas such as Kubiki Oil Field. The conditions required for oil accumulation in the Shiiya sandstone include the presence of deep faults for migration pathways and the absence of shallow fault that may leak hydrocarbons. Therefore 3D seismic is necessary to evaluate such geological condition.

基礎試錐「佐渡南西沖」の椎谷層砂岩への新たな物理検層解析法の適用

METI Sado Nansei Oki Wells, drilled in the deep water Toyama Trough in the Sea of Japan offshore Niigata Prefecture, Japan, encountered the Pliocene Shiiya Formation that contains 15 meters of oil column. Sandstones of the Shiiya Formation are turbidite origin and with varieties of lithology. Fine grained and poorly sorted lithic arenite is dominated in the upper part, and fine grained and poorly sorted lithic wacke is dominated in the lower part of the formation. Due to fine grained and poorly sorted nature, and tuffaceous or muddy matrix, pore size in the wacke varies substantially.Combinable Nuclear Magnetic Resonance (CMR) logging, which measures three different sizes of porosities, namely clay bound water porosity (pore occupied by clay-bound water), capillary bound fluid porosity (pore occupied by capillary-bound fluid) and free fluid porosity (pore in the center of medium to large size pores where fluid can flow), is very useful to characterize the sandstone in the Shiiya Formation.Complicated lithology of the Shiiya sandstone renders the formation evaluation a challenging task at METI Sodo Nansei Oki S Well. The resistivity of the oil reservoir wacke interval is very low, which is almost same with the arenite interval with no oil shows at all.Formation evaluation using CMR (CMR ELAN) revealed that the oil reservoir wacke contains up to 15% of capillary bound fluid porosity while free fluid porosity is 8∼14% at best. Although oil saturation is not high enough for the total porosity, the effective oil saturation is quite high if all oil is assumed to be confined in the free fluid pore. High oil saturation within free fluid pore with a large amount of clay bound water and capillary bound fluid is a reasonable explanation for the oil bearing but very low resistive wacke interval.

Basement-involved tectonics in North Fossa Magna, central Japan: The significance of the northern Itoigawa-Shizuoka Tectonic Line

The Toyama Trough and Fossa Magna basin are major transverse tectonic depressions located between northeast Japan (NEJ) and southwest Japan (SWJ). Itoigawa-Shizuoka Tectonic Line (ISTL), the western margin of Fossa Magna is a boundary between two tectonic provinces along the inner side of NEJ and SWJ. Abrupt lateral variations in thickness of the Middle to Upper Miocene strata are quite significant among these provinces in the North Fossa Magna. The development of the thrust/fold belt is attributed not only to horizontal compression but also to vertical block movements as basement-involved tectonics. In response to the Pliocene and later compression regime, the old fault-block boundaries were reactivated and produced differential block movement such as the tilting of the Central Upheaval Zone and uplifting of the Nishikubiki Belt. One possible model for the deeper geologic structure is presented that high-angled block faults on the surface among tectonic provinces originate in vertical weak zones in the deep seismogenic zone under the sedimentary layer. The folded Neogene system comprises the present-day thrust-fault province of the North Fossa Magna and the stress regime of strikeslip faulting occupies the basement, as inferred from focal mechanism solutions for small events. In order to account for the apparent discrepancy, a duplex stress field is possible for the active tectonics in the region.

Fault‐related folds and an imbricate thrust system on the northwestern margin of the northern Fossa Magna region, central Japan

Abstract The Nishikubiki Mountains, which are located on the northwestern margin of the northern Fossa Magna region, central Japan, and the area offshore to the north of the mountains are underlain by folded and faulted Neogene and Quaternary sequences. The folds are composed of open, symmetric anticlines or tight, asymmetric anticlines trending north 20–70° east. On the basis of the geometry of the anticlines and growth strata, the symmetric and asymmetric anticlines are interpreted as fault‐bend folds and fault‐propagation folds, respectively. The formation of the anticlines is attributed to the growth of an imbricate thrust system composed of three thrust sheets that developed, from southeast to northwest, mainly in the late Pliocene, early Pleistocene, and middle Pleistocene–Holocene. The horizontal component of the northwestern‐most sheet was estimated to be approximately 1.2 km on the basis of the width of the growth triangle, and the thickness of the sheet at its southeast margin was estimated to be 8.5 km on the basis of area balancing along one of the seismic profiles. The thrust is inferred to extend to a depth of more than 10 km toward the southwest. The three thrust sheets are probably connected by a detachment zone along the boundary between the upper and lower crusts. The anticlines are bounded by the Itoigawa–Shizuoka Tectonic Line (ISTL) to the west and by lateral ramps or tip lines to the northeast. The ISTL possibly continues northward offshore into the Toyama Trough. The structural model proposed in this paper suggests that similar thrust systems are wide spread in the northern Fossa Magna region and that active deformation zones have migrated and switched during the past 2–3 million years along the fold belt.

Channel-levee complexes, terminal deep-sea fan and sediment wave fields associated with the Toyama Deep-Sea channel system in the Japan Sea

The Toyama Deep-Sea Channel (TDSC) in the Japan Sea is one of the most prominent deep-sea channels in rifted margins. This study revealed the course and morphology of the entire TDSC system for the first time, based on high-resolution and air-gun seismic reflection profiles, and sediment cores. The channel starts from Toyama Bay and extends for 750 km through the Toyama Trough, a Miocene rift, to the Miocene Yamato and Japan back-arc basins. The TDSC system is fed with sediment from the Northern Japan Alps, through tributary canyons on the narrow shelf, which are directly connected with rivers, even during sea-level highstands. Levee complexes border the channel in the Toyama Trough and Yamato Basin, and in the Japan Basin, the channel feeds the terminal Toyama Fan. Levees in the confines of the Toyama Trough are accompanied by an over-bank turbidite plain, whereas levees in the Yamato Basin are characterized by prominent sediment waves. Toyama Fan consists of a channel-levee complex with sediment waves in the upstream part and of lobes at the channel end. The course and morphology of the channel-fan system are primarily controlled by basin morphology. Thick, sheet-like sediments, deposited from ponded turbidity currents, have accumulated in the narrow, elongate Toyama Trough, whereas extensive levees and the Toyama Fan have formed in the more open basins. Distribution of sediments and consequent morphology of the channel-levee complexes are also controlled by Coriolis and centrifugal forces. Preferential development of the levees on the right-hand side is attributed to Coriolis-force tilt effects in the Northern Hemisphere. Centrifugal forces at the meander loops or bends of the channel result in flow stripping, causing levees to build on the outer bends. The distribution, form and orientation of sediment waves are consistent with the extent and direction of inferred spill-over turbidity currents, and with consequent levee growth. Fluvial-like features such as meanders, terraces, levee slumps and a crevasse splay are developed along the TDSC. Unlike other submarine channels, sinuosity seems to be controlled by bedrock structures rather than by valley slopes. Channel avulsion has not been recorded in the TDSC system. Active clastic deposition on the uppermost lobe during the past 1 ka suggests recent active sediment transportation through the channel. Sediment transportation, however, may have ceased during the Holocene in the cut- and fill-tributaries developed in the Quaternary succession on the slope to the trough, where a relatively wide shelf separates canyons from rivers in the eastern margin of the drainage area.

やや長周期表面波の群速度の逆解析による地下構造の推定と地震動評価への応用

It is important to estimate effects of deep subsurface structures to long-period ground motion. In this study, the mean subsurface structures between a observatory at Kashiwazaki, Niigata prefecture and epicenters are estimated as a multi-layered model by an inversion of the group velocities using a genetic algorithm. The estimated models for the events located around Niigata have deep pre-Neogene basement and the depth of basement estimated from the epicenter near Toyama trough is especially deep. These features of the models agree with the regional variation of geological data and gravity anomaly. Furthermore, long-period ground motions at the observatory are evaluated by normal mode theory using the estimated models and existing fault parameters. Over whole features of the records were reconstructed by synthetics. It is concluded that the subsurface structure estimated from single station data is so useful to evaluate long-period ground motions.