Niigata Plain Research Articles

The snow‐shadow effect of Sado Island on Niigata City and the coastal plain

AbstractJapan's Hokuriku region, near the Sea of Japan, typically experiences heavy snowfall; however, Niigata City, the largest city on the Sea of Japan side, experiences lower levels of snowfall than neighbouring cities. This study investigates the snow‐shadow effect of Sado Island on snowfall in Niigata City, located 45 km away leeward. Statistical analysis of long‐term radar data for 10 winters showed that snow‐shadow effects in the Niigata plain occurred in 151 (80%) of the 188 cases, during which a strong approaching wind reached the island. The location of this snow‐shadow effect was always downwind of Sado Island and depended on the wind direction. Numerical experiments using the Weather Research and Forecasting model predicted that snowfall over the Niigata Plain would be lighter with the island than without it. Additionally, the snow‐shadow effect occurs in areas more than 150 km downwind. The experiments showed that Sado Island reduces heat fluxes from the sea surface by weakening leeward winds. At the same time, the horizontal wind convergence downwind is weakened. Meanwhile, the orographic snowfall over Sado Island reduces the amount of water vapour, cloud water and cloud ice over the leeward sea. Therefore, Sado Island prevents cloud lines from redeveloping over the leeward sea and can further reduce snowfall over the leeward plain, including in Niigata City.

Industry-Academia-Government Collaboration for Visualization of Drainage Water in the Niigata Plain

Industry-Academia-Government Collaboration for Visualization of Drainage Water in the Niigata Plain

100. 新潟平野の地形発達史について(その4)

100. 新潟平野の地形発達史について(その4)

O-93 S波ランドストリーマー浅層反射法探査による新潟平野西縁断層近傍の表面地質構造(9. 第四紀末期における沖積低地の形成史と災害・環境地質の諸問題,口頭発表,一般講演)

O-93 S波ランドストリーマー浅層反射法探査による新潟平野西縁断層近傍の表面地質構造(9. 第四紀末期における沖積低地の形成史と災害・環境地質の諸問題,口頭発表,一般講演)

O-219 新潟平野の液状化現象 : 新潟県中越地震と新潟地震の事例から(24.環境地質,口頭およびポスター発表,一般講演)

O-219 新潟平野の液状化現象 : 新潟県中越地震と新潟地震の事例から(24.環境地質,口頭およびポスター発表,一般講演)

新潟平野東縁・村松地域の活断層 : 村松断層

新潟平野東縁・村松地域の活断層 : 村松断層

新潟平野,宍道低地帯における温泉水中のヒ素濃度 ―地下水ヒ素汚染問題におけるヒ素の供給源の検討 その2―

新潟平野,宍道低地帯における温泉水中のヒ素濃度 ―地下水ヒ素汚染問題におけるヒ素の供給源の検討 その2―

新潟平野における表層地質中のヒ素濃度分布 : 地下水ヒ素汚染問題におけるヒ素の供給源の検討 その1

新潟平野における表層地質中のヒ素濃度分布 : 地下水ヒ素汚染問題におけるヒ素の供給源の検討 その1

ボーリングコア解析による新潟平野西縁，角田・弥彦断層の完新世における活動度

ボーリングコア解析による新潟平野西縁，角田・弥彦断層の完新世における活動度

Millennial-scale reworking of tephra in alluvial to shallow marine settings: Distinguishing pseudo-isochrons from genuine ones

Primary tephra layers can provide isochrons in a stratigraphic sequence. Such isochrons are powerful tools in terms of correlation and chronology of strata, and they constrain a chronological framework for geology, geomorphology, archaeology and palaeo-environmental studies in the Neogene to Quaternary. However, reworking processes can re-distribute tephra materials in both time and space, so that a given tephra can form a new discrete layer later in the depositional sequence, or it can be distributed in low concentrations (no visible layers, i.e., crypto-tephras) over a range of different stratigraphic horizons. The present study, based on sedimentological, petrographical, and geochemical analyses of reworked tephras in Japan, evaluates the duration and persistence of reworking and the tephra mobilisation and delivery to watersheds. Furthermore, it discusses how a unique isochron can be constructed from tephra distribution in a sedimentary sequence. Examples presented here are from fluvial and deltaic sequences in the Japanese Quaternary, which contain several visible tephra layers and a certain amount of tephra materials from multiple sources at different stratigraphical horizons. In the Niigata Plain, 5.4ka Numazawako tephra material continued to be reworked for more than 4000 years after the eruption of Numazawa volcano, and was re-deposited as reworked pumice grains consisting of a “visible” tephra layer in delta front deposits. Mixed tephra materials originating from an earlier eruption of Numazawa volcano and eruptions from other calderas were also identified. In the Tsugaru Plain, post-2.5ka subsurface deposits have >10% glass shards content throughout the sequence. Glass shards were derived from several different tephra materials, such as those from the most recent eruption of Towada volcano (AD 915), and earlier caldera forming eruptions (30 ka, 15ka). Some of the glass shards probably originated from erosion of exposed Pliocene pyroclastic bedrock. Thus, tephra reworking processes in alluvial to shallow marine settings can continue long after an eruption has ended, and can persist even during inter-eruptive (i.e., background sedimentation) periods. As tephra reworking involves complicated processes, careful observation and interpretations are necessary to evaluate whether tephra materials can truly provide an isochron for the strata in which they are found to exist.

Classical Tsutsugamushi disease in Niigata Plain, Northeast Japan-Land condition of "Yuudokuchi"(Noxious area)-

Classical Tsutsugamushi disease in Niigata Plain, Northeast Japan-Land condition of "Yuudokuchi"(Noxious area)-

A STUDY ON MODELING OF DEEP SUBSURFACE STRUCTURE CONSIDERING RANDOM VELOCITY FLUCTUATION FOR ESTIMATION OF BROADBAND SITE AMPLIFICATION FACTOR

The site amplification factors in Niigata plain, Japan, are empirically estimated by the spectral inversion analysis from S-wave parts of strong motion data. The resultant characteristics of the site amplification are relative large in the frequency range of about 1.5-5 Hz, and decay significantly with the frequency increasing over about 5 Hz. In order to explain these features, we propose to consider random fluctuation in the modeling of deep S-wave velocity structure for site factor estimation. As a result, the site amplification in the frequency range higher than about 5Hz are filtered out by the effects of the velocity fluctuation similar to those observed in Niigata plain. We further demonstrate that the addition of the damping factor in subsurface modeling also has the same high-cut filtering effect on the site amplification factor estimation. We also discuss the relationship between the additional damping factor and the strength of random fluctuation.

Occurrence conditions of hyperpycnal flows, and their significance for organic‐matter sedimentation in a Holocene estuary, Niigata Plain, Central Japan

AbstractRiver floods influence sedimentary environments and ecosystems from the terrestrial to the deep‐marine. This study documents the occurrence conditions of hyperpycnal flows generated by river floods and related organic‐matter sedimentation for Holocene sediments of the Niigata Plain, Central Japan, based on detailed sedimentary facies, total sulfur and total organic carbon content, diatom assemblages and organic‐matter composition. Holocene sediments of the Niigata Plain consist of sand, mud and gravel that were deposited in estuarine and fluvial systems during a sea‐level rise (15 000–6800 years BP) and stillstand (after 6800 years BP) following the Last Glacial Maximum. Hyperpycnites are present in the upper part of the estuarine lagoon sediments. The depositional age is considered to be about 5000 years BP. The hyperpycnites comprise two successions of a top fining‐up unit and a basal coarsening‐up unit, and include abundant terrigenous organic matter and freshwater diatoms. A large volume of freshwater is inferred to have flowed into the lagoon during deposition of the upper part of the lagoon sediments. In consequence, hyperpycnal flows may have readily formed in the lagoon, because the halocline was weak. The hyperpycnal flows also produced a layer of concentrated terrigenous organic matter in the uppermost part of the hyperpycnites. The abundant organic matter on the estuarine floor is inferred to have produced anoxic bottom conditions owing to oxidative decomposition by benthic bacteria.

Seasonal Variations of Nitrogen Isotopic Ratios of Ammonium and Nitrate in Precipitations Collected in the Yahiko–Kakuda Mountains Area in Niigata Prefecture, Japan

Precipitation was collected from May 15, 2001 to November 18, 2002, at the mountain top (620 m a.s.l.) and mountain foot (47 m a.s.l.) of the Yahiko–Kakuda Mountains area, which is located in the western part of the Niigata Plain in central Japan. Major ion constituents and nitrogen isotopic ratios of ammonium (δ15N-NH4+) and nitrate (δ15N-NO3−) were measured in order to investigate the sources and behavior of NH4+ and NO3− in precipitations. The concentrations of sea salt constituents considerably increased in the cold season from November to March, and for the majority of the ion species, the concentrations at the mountain foot were higher than those at the mountain top. The precipitation weighed monthly and seasonal means of δ15N-NH4+ values in the warm season were lower than those in the cold season at both the mountain top and mountain foot. The δ15N-NH4+ values at the mountain foot were lower than those of mountain top in both seasons. This seasonal variation of 15N-NH4+ could be caused by the incorporation of ammonia gas (NH3) with lower δ15N values, emitted from agricultural activities. On the other hand, the monthly and seasonal means of δ15N-NO3− values showed noticeable seasonal difference with higher in the cold season at both the mountain top and mountain foot; however, the elevation difference was not observed either in the warm or cold season. The obtained δ15N-NH4+ and δ15N-NO3− values were regarded as intermediate in comparison with the reported values. No significant correlations were observed either between NH4+ concentrations and δ15N-NH4+ values or between NO3− concentrations and δ15N-NO3− values. These results suggest that different factors may affect the nitrogen isotopic variations and concentration variations of NH4+ and NO3− in precipitations collected at the two sites.

Breakout flood from an ignimbrite-dammed valley after the 5 ka Numazawako eruption, northeast Japan

Numazawa volcano in northeast Japan erupted most recently at ca. 5 ka, forming a 2-km-diameter caldera and emplacing at least 4 km 3 of valley-confined ignimbrite. The ignimbrite dammed the Tadami River to a depth of >100 m, temporarily impounding >1.6 km 3 of water. Overtopping of the barrier triggered breaching and catastrophic release of the dam lake. Pyroclastic material redeposited by the ensuing flood is widely distributed along the Tadami and Agano Rivers in deposits tens of meters thick as far as the coastal Niigata Plain >150 km downstream of the volcano. Evidence for damming and flooding found along the rivers includes (1) fine-grained, thinly laminated lacustrine deposits upstream of the pyroclastic blockage; (2) 30-m-thick, pumiceous debris-flow and hyperconcentrated-flow deposits that show continuous sedimentation with no major hiatus downstream of the volcano; (3) fine-grained slackwater deposits at tributary river mouths; and (4) large flood boulders that form lags on stripped ignimbrite surfaces and younger terrace surfaces, or cropping out within hyperconcentrated-flow deposits. Paleohydraulic estimation techniques indicate a peak discharge of 30,000–50,000 m 3 /s at the breach point. Burial of Neolithic Jomon settlements by flood and volcaniclastic aggradational deposits at distal locations indicates that such indirect volcanogenic hazards are significant, even where direct primary consequences of volcanic eruptions are minor or absent.

Statistical Analyses of Snowfall Distribution in the Niigata Area and its Relationship to the Wind Distribution

Analyses of precipitation amounts and surface winds observed at weather stations were conducted to investigate the effect of the topography on the snowfall distribution in Niigata. The topographic effect on the precipitation, particularly in the Niigata plain, has not been well discussed although Akiyama (1981a, 1981b) discussed the relationship between the snowfall distribution in Niigata and the synoptic scale condition. Using an empirical orthogonal function analysis, the winter precipitation distribution from 1988 to 2007 in Niigata was categorized as plain-type (P-type) and mountain-type (M-type). The P-type precipitation occurs under an upper trough over the Sea of Japan. The synoptic westerly wind, brought by the lower trough near the Japan Islands, and the deep convective mixed layer will be responsible for the precipitation in the plains of Niigata and Hokuriku. In addition, the Noto Peninsula and the mountain range to the east of the Hokuriku area seem to play an dynamical role in the precipitation in the Niigata plain. The M-type precipitation occurs under the upper trough over Japan. The synoptic scale condition brings northwesterly wind, which causes a topographic updraft at the mountain slope, resulting in precipitation dominated in the Niigata mountain range.

比抵抗と密度構造から推定される新潟堆積盆地東縁，村松断層周辺の基盤構造

In order to clarify the subsurface pre-Tertiary basement structure at the eastern margin of the Niigata plain, we carried out remote sensing, geological, electromagnetic and gravity surveys at Muramatsu town, Niigata Prefecture. The study area contains distinctive topographic features : a NNE-trending active reverse fault (the Muramatsu Fault) and an asymmetrical tectonic relief (the Atago hill) whose eastern margin is bounded by the Muramatsu Fault. Resistivity and gravity analyses reveal that the pre-Tertiary basement is characterized by resistive and high-density zone, and the overlying Tertiary to Quaternary sediments correspond to conductive and low-density zone. Based on the two-dimensional resistivity and density structures, a significant gap of the upper surface of the basement is inferred below the Atago hill, which is attributed to a west-dipping fault with a remarkable throw of about 1,400 m. The west-dipping fault is probably interpreted to have formed as a normal fault during rifting stage due to extensional stress. This subsurface normal fault seems to be connected to the surface active reverse fault (the Muramatsu Fault), indicating the reactivation of the former normal fault as a reverse fault under the present compressional stress. The tectonic relief of the Atago hill is likely to reflect the asymmetrical deformation of the Tertiary to Quaternary sediments due to the reactivation of the same fault.

新潟平野におけるイベント性堆積作用における堆積システムの変化(12.堆積作用・堆積過程)

新潟平野におけるイベント性堆積作用における堆積システムの変化(12.堆積作用・堆積過程)

洪水起源混濁流とエスチュアリーの底層環境変化 : 新潟平野完新統を例として(12.堆積作用・堆積過程)

洪水起源混濁流とエスチュアリーの底層環境変化 : 新潟平野完新統を例として(12.堆積作用・堆積過程)

Sedimentary Environment and Channel Change during Histric Times in the Southern Part of Niigata Plain

Sedimentary Environment and Channel Change during Histric Times in the Southern Part of Niigata Plain