Kanto Plain Research Articles

関東平野・濃尾平野周辺域における第四紀後期の海水準・地殻変動と侵食・堆積作用による地形発達史

関東平野・濃尾平野周辺域における第四紀後期の海水準・地殻変動と侵食・堆積作用による地形発達史

関東平野北西部，埼玉県寄居町用土付近の完新統～上部更新統(MIS3)の花粉分析

関東平野北西部，埼玉県寄居町用土付近の完新統～上部更新統(MIS3)の花粉分析

関東平野南部，武蔵野台地地下に伏在する上総層群の層序・編年に関する最近の研究

関東平野南部，武蔵野台地地下に伏在する上総層群の層序・編年に関する最近の研究

Morphological description of four species belonging to the genus Nigrobaetis (Ephemeroptera: Baetidae) from Japan

We associated imagoes and nymphs of four Nigrobaetis species from Japan and described their morphological characters. We found that N. sp. P comprised two Nigrobaetis species in Kanto Plain and in Ishigaki Island, which we described as N. apterus Fujitani n. sp. and N. ishigakiensis Fujitani n. sp., respectively. We also described N. sp. I as N. latus Fujitani n. sp. We recorded N. acinaciger (Kluge), the nymph of which had been confused with N. chocoratus (Gose), from Japan for the first time. We provide identification keys for eight Nigrobaetis species of Japan.

Observations on seismically induced liquefactionfluidization in reclaimed land at Hinode, Honshu, Japan

As construction and reclamation activities become larger and more widespread the distribution of man-made strata is rapidly extending especially in developed countries. Such strata are often more likely to suffer geohazards or to contain harmful substances than natural strata. In particular, geological disasters are often associated with the unconformity between artificial and natural strata (known in Japan as the Jinji Unconformity) with highly consolidated strata beneath and less consolidated above. The flow of liquids within man-made strata is often less simple than in natural strata and a specific flow system tends to develop at the unconformity. The Great East Japan Earthquake on March 11, 2011 (Magnitude: M7.9, Epicenter: longitude 38°06.22 and latitude 142°51.62 and 24km in depth) affected the Kanto Plain near Tokyo. Large-scale liquefaction fluidization caused by the seismic vibrations was essentially confined to areas with man-made strata. The present study focused on land reclaimed by hydraulic filling at Hinode, a suburb of Itako City, which had been used subsequently for urban development and suffered significant damage. Cored boreholes and observation wells were made to assess evidence of liquefaction. Continuous cores were halved longitudinally and lacquer peels were made and examined using X-ray photograms. Samples taken from liquefaction zones, liquefaction-fluidization zones and non-liquefaction zones were subjected to grain size analysis. A large peel was made from the outcrop in a dewatered trench section. In the Hinode area, the Jinji Unconformity is relatively deep in the centre thinning towards each margin of the reclaimed area, essentially forming an irregular basin shape into which the man-made strata were emplaced. The reclaimed area is dominated by fine sand and silt which is generally more susceptible to liquefaction and fluidization. Therefore it is not surprising that sand eruptions caused by the earthquake of 11 th March 2011 demonstrate widespread liquefaction-fluidization. In the northern and eastern parts of the reclaimed area, where man-made strata, directly overlie the natural Sarawa Sand and Mud Formation and in areas underlain only by natural strata there were relatively few sand eruptions. Although a higher density of ground investigation and sampling points, particularly in a ring about half way between the central area and margins, would be needed to refine these observations and conclusions but it is already clear that the variations laterally and vertically in the distribution of the mobilized layers occurs at a scale of less than 1 meter. This calls into question current Japanese practice for assessing liquefaction and fluidization potential which relies on N-value SPT tests at 1 meter intervals. This needs to be reconsidered.

Settlement behavior in the Kanto Plain during the Japanese Paleolithic based on lithic raw material procurement and consumption

This study examines settlement system changes before and during the Last Glacial Maximum in central Japan. Human mobility is estimated from raw material procurement to infer how Upper Paleolithic hunter–gatherers acquired necessary materials. Seventy-four sites are investigated with the methodological assumption that the lithic raw materials were obtained through an embedded strategy.Changes in settlement patterns through time are reconstructed in the following ways. First, around 30,000 14Cyrs BP, hunter–gatherer territories formed between high-quality lithic raw material sources approximately 200 km apart, constrained by the distribution of lithic raw material sources and the area's terrain and suggesting that their subsistence activities were also conducted in those prime areas. Second, during the Last Glacial Maximum, hunter–gatherers still acquired lithic raw materials from distant sources around 200 km apart, while the area of hunting activities using spear points decreased to an 80 km radius. As territories shrank, large manufacturing sites for making spear points appeared in the vicinity of high-quality raw material sources in this period.In sum, the embedded lithic procurement system in Japan during this period was largely changed by ecological deterioration. Patterns transitioned from general resource exploitation in the early period to seasonal movement between isolated resource-rich areas during the Last Glacial Maximum, resulting in significant change from the mobile hunter–gather society at the end of the Late Pleistocene to the sedentary society of the Holocene.

The Tornadic Supercell on the Kanto Plain on 6 May 2012: Polarimetric Radar and Surface Data Assimilation with EnKF and Ensemble-Based Sensitivity Analysis

A tornadic supercell and associated low-level mesocyclone (LMC) observed on the Kanto Plain, Japan, on 6 May 2012 were predicted with a nonhydrostatic mesoscale model with a horizontal resolution of 350 m through assimilation of surface meteorological data (horizontal wind, temperature, and relative humidity) of high spatial density and C-band Doppler radar data (radial velocity and rainwater estimated from reflectivity and specific differential phase) with a local ensemble transform Kalman filter. With assimilation of both surface and radar data, a strong LMC was successfully predicted near the path of the actual tornado. When either surface or radar data were not assimilated, however, the LMC was not predicted. Therefore, both surface and radar data were essential for successful LMC forecasts. The factors controlling the strength of the predicted LMC, defined as a low-level maximum vertical vorticity, were clarified by an ensemble-based sensitivity analysis (ESA), which is a new approach for analyzing LMC intensification. The ESA showed that the strength of the LMC was sensitive to low-level convergence forward of the storm and to low-level relative humidity in the rear of the storm. Therefore, the correction of these low-level variables by assimilation of dense observations was found to be particularly important for forecasting and monitoring the LMC in the present case.

Tephrochronology of the Kazusa Group in southern Kanto Plain, central Japan

三重会合点の北西数百kmに位置する関東平野には，かつての前弧海盆とその周辺域に堆積した上総層群が分布する．鮮新世後期から第四紀前〜中期に堆積した同層群中には多数のテフラが含まれている．これらを用いた火山灰編年研究により，日本列島最大規模の面積をもつ関東平野の地形発達・古地理，地殻変動，そして上総層群にテフラを供給した爆発的火山噴火の復元が可能となる．こうした研究を進めるには関東平野の広大な地下に埋もれたテフラも解明する必要がある．現在関東平野において，ボーリングで確認された地下のテフラや，地表に露出するテフラの対比・認定が進められつつある．本コースでは，関東平野南部，とくに狭山丘陵と多摩丘陵において前期更新世テフラを観察し，同時代のテフロクロノロジーとその応用研究の現状を紹介する．

Notes on the Reproductive Ecology and Description of the Preimaginal Morphology of Elaphrus sugai Nakane, the Most Endangered Species of Elaphrus Fabricius (Coleoptera: Carabidae) Ground Beetle Worldwide.

Elucidating the basic life-history of endangered species is the first important step in the conservation of such species. This study examined the reproductive ecology and the preimaginal morphology of the endangered ground beetle Elaphrus sugai Nakane (Coleoptera: Carabidae); currently, the Watarase wetland of the central Kanto Plain, Japan is the only confirmed locality of this beetle species. Laboratory rearing of reproductive adults collected in early April revealed that females can lay more than 131 eggs. Eggs were laid in mud, without an egg chamber. Larvae reached adulthood when fed a diet of mealworms, indicating that E. sugai larvae are insect larvae feeders. An earthworm diet, the optimal diet for larvae of a congeneric species (E. punctatus Motschulsky), was lethal to E. sugai larvae. The egg stage was 3–4 days in duration under a 16L8D cycle (22°C). The duration from hatching to adult eclosion was 23–42 days at various temperatures simulating those of the reproductive period. Larval morphology was similar to that of consubgeneric species described previously. The pupa is unusual, in that the setae on the abdominal tergites are long (twice as long as those of the abdominal segment) and have somewhat “coiled” apices. Finally, the current endangered status of E. sugai was compared to that of E. viridis Horn, which has been regarded as the most endangered species of the genus worldwide.

数値解析手法のベンチマークテスト―2004年紀伊半島南東沖地震前震を対象とした関東平野の結果―

関東平野を対象とした既往の数値解析手法のベンチマークテストでは、差分格子の間隔により後続表面波がチームにより異なるという問題点が指摘された。2013年度のベンチマークテストは2004年紀伊半島南東沖地震前震を用いて行われ、複数のモデルについて計算結果の比較検討を行った。差分格子を統一した均質・2層地盤モデルを用いたケースでは各チーム間で高い整合性が得られたが、モデル境界面からの反射波の処理の違いによる結果の差異が見られ、Q値の設定の違いが反射波に影響を与え、その設定方法に注意する必要があるという新たな課題が明らかになった。差分格子を統一せず、23層地盤モデルを用いたケースでは、差分格子の違いにより地盤構造の差異が生まれ、震源から遠方になるほど計算結果に影響を与えることが示唆された。

埼玉県加須低地のS波速度構造調査

埼玉県加須低地のS波速度構造調査

Extent of use of wetlands and surrounding habitat by over-wintering raptors on the Kanto Plain

Extent of use of wetlands and surrounding habitat by over-wintering raptors on the Kanto Plain

PS干渉SAR解析を用いた関東平野北部の地盤沈下域の推定

PS干渉SAR解析を用いた関東平野北部の地盤沈下域の推定

関東平野北部における地下温度の高温域の構造と変化

関東平野北部の群馬県館林周辺には，従来から高地下水温域の存在が指摘されていたが，筆者らの先行研究（宮越ほか，2003）により，高温域の変化が示唆された。そこで本研究では，地下温度分布の現状を把握し，高温域の経年的な変化とその要因を検討した。高温域は館林の西方と東方に分かれて分布し，これらに挟まれた地域に低温域が確認された。本研究（測定年：2010年）と既存研究による過去の地下温度情報（同：1950～1970年代および2000年）の比較から，地下浅部には温度上昇が確認され，都市化等に起因した地表面付近の温度上昇や浅部の地下水流動の影響が考えられた。これに対して，地下深部では館林周辺地域において温度低下が確認され，その分布は低水理水頭部と概ね一致した。これらの結果から，館林周辺においては，地下水開発の影響によって地下温度が長期的に低下していると考えられた。

関東平野北西部，高崎台地から井野川低地帯にかけての地下地質

There are two geomorphic surfaces (terraces) in the northwestern Kanto Plain: Takasaki Upland (Takasaki Surface) and Inokawa Lowland (Ino Surface). It is considered that about 11,000 years ago the Inokawa Mudflow traveled down from the west, forming the Takasaki Upland as a depositional surface. However, opinions are divided and another theory is that the Inokawa Lowland is also the depositional surface of the Inokawa Mudflow. Therefore, this study examines the subsurface geology around the Inokawa Lowland, based on an expanded dataset of existing borehole data obtained from our portable hand-operated drilling survey. The results clearly show that the Inokawa Lowland is basically a depositional surface of the Inokawa Mudflow. Reflecting upon this conclusion, how the terrace scarp between the two surfaces was formed becomes problematic. There are two possible causes to be clarified by future investigations: (1) Mantle bedding of the Inokawa Mudflow on the pre-existing fluvial scarp, (2) As-yet-unrecognized vertical faulting after full deposition of Inokawa Mudflow deposits in this region, probably related to the Fukaya fault system.

Ensemble experiments using a nested LETKF system to reproduce intense vortices associated with tornadoes of 6 May 2012 in Japan

Experiments simulating intense vortices associated with tornadoes that occurred on 6 May 2012 on the Kanto Plain, Japan, were performed with a nested local ensemble transform Kalman filter (LETKF) system. Intense vortices were reproduced by downscale experiments with a 12-member ensemble in which the initial conditions were obtained from the nested LETKF system analyses. The downscale experiments successfully generated intense vortices in three regions similar to the observed vortices, whereas only one tornado was reproduced by a deterministic forecast. The intense vorticity of the strongest tornado, which was observed in the southernmost region, was successfully reproduced by 10 of the 12 ensemble members. An examination of the results of the ensemble downscale experiments showed that the duration of intense vorticities tended to be longer when the vertical shear of the horizontal wind was larger and the lower airflow was more humid. Overall, the study results show that ensemble forecasts have the following merits: (1) probabilistic forecasts of the outbreak of intense vortices associated with tornadoes are possible; (2) the miss rate of outbreaks should decrease; and (3) environmental factors favoring outbreaks can be obtained by comparing the multiple possible scenarios of the ensemble forecasts.

Groundwater management based on monitoring of land subsidence and groundwater levels in the Kanto Groundwater Basin, Central Japan

Abstract. Over 40 million people live on and exploit the groundwater resources of the Kanto Plain. The Plain encompasses metropolitan Tokyo and much of Chiba Prefecture. Useable groundwater extends to the base of the Kanto Plain, some 2500 to 3000 m below sea level. Much of the Kanto Plain surface is at sea level. By the early 1970s, with increasing urbanization and industrial expansion, local overdraft of groundwater resources caused major ground subsidence and damage to commercial and residential structures as well as to local and regional infrastructure. Parts of the lowlands around Tokyo subsided to 4.0 m below sea level; particularly affected were the suburbs of Funabashi and Gyotoku in western Chiba. In the southern Kanto Plain, regulations, mainly by local government and later by regional agencies, led to installation of about 500 monitoring wells and almost 5000 bench marks by the 1990's. Many of them are still working with new monitoring system. Long-term monitoring is important. The monitoring systems are costly, but the resulting data provide continuous measurement of the "health" of the Kanto Groundwater Basin, and thus permit sustainable use of the groundwater resource.

Reconstruction of Holocene relative sea-level change and residual uplift in the Lake Inba area, Japan

We collected and analyzed fossil diatoms and volcanic ash and determined 14C ages in core samples from the lowlands around Lake Inba in the eastern Kanto Plain, central Japan, an area where late Quaternary sea-level change, fluvial erosion, and tectonic uplift have affected a region of archeological interest. We inferred Holocene paleoenvironmental changes and relative sea levels on the basis of indicators, such as high tide levels, derived from fossil diatom assemblages as well as the stratigraphic positions of 14C-dated samples and the K–Ah tephra. Our results lead to estimates of the upper half tidal range during 6800–7000cal yr BP of 1.7m. The Holocene highstand took place at approximately 6400–6500cal yr BP, and mean sea level reached an elevation of 1.9m. However, the timing of this sea-level rise is earlier in the Lake Inba area than documented in previous studies, and it is suggested that the timing of the Holocene highstand may appear at least 1000years earlier as a result of Holocene residual uplift. After this, sea-level fell abruptly around 4000cal yr BP. We also recognized a sea-level fall corresponding to the Yayoi regression at about 2600cal yr BP, during which the mean relative sea level may have been as low as −2.5m after compensating for uplift, subsidence or geomorphologic effects in this area.

Integration of the rice paddy water management into a coupled surface-subsurface water flow model in the Sakuragawa River watershed (Japan)

Rice paddy water management was integrated into a distributed three-dimensional surface and subsurface coupling hydrological model of the Sakuragawa River watershed. This watershed is located in the Kanto Plain in Japan and includes the hillside of Mt. Tsukuba. Therefore, this watershed includes both steep mountainous areas and rice paddy-dominated flat land. Thus, water management of rice paddies is important and was calculated separately using a paddy model. The use of groundwater for rice paddy irrigation was considered as well as a water supply from outside of the watershed (Kasumigaura Lake). The model parameters were calibrated and validated with reference to the predictability of river water flow and the groundwater level. Using the calibrated model, three-dimensional streamlines, water travel time distributions, and water balance in some grids were clarified. The developed model will facilitate sustainable water resource management in the watershed.

Meteorological Characteristics of Local Heavy Rainfall in the Fukuoka Plain

Heavy local rainfall has been increasingly observed in urban Fukuoka on fine summer afternoons in recent years. Such rainfall tends to occur suddenly on calm afternoons and is considered to be caused by local wind conditions influenced by local topography rather than by weather fronts or typhoons. This local rainfall is considered to be caused by a mechanism different from similar rainfalls occurring on fine Kanto plain afternoons. We set up 14 rain gauges in urban Fukuoka in this study to clarify and confirm actual local rainfall conditions there. Maximum local rain is about 64 km2lasting 10 to 30 minutes. The maximum 10-minute rainfall was 13.8 mm. The average surface air temperature on days with local rainfall differs 2°–3°C from that on fine days. Upper atmosphere humidity distribution differs greatly between fine days and those with heavy local rain. Accordingly, heavy local rain is more likely to occur if surface air temperature and humidity in upper atmosphere rise above a certain level. Some difference is seen between days of heavy local rainfall and fine day in terms of the K index (KI), a measure of atmospheric stability. We confirmed that the atmospheric state becomes more unstable on days with heavy local rainfall than on fine days. Heavy local rainfall often begins in either the eastern or western inland Fukuoka plain and moves toward the coast. That is, based on numerical simulation using the meteorological mesoscale weather research and forecasting (WRF) model, wind blowing opposite to the sea wind blows in the upper atmosphere, moving cumulonimbus clouds causing heavy local rainfall toward the coast. We also confirmed that heavy local rainfall tends to occur in eastern inland areas with wind from the west, but tends to occur in western areas with wind from the east. We therefore assumed that heavy local rainfall in urban Fukuoka was triggered by updrafts generated when wind struck the inland Fukuoka plain mountain system.