Most of irrigated water in paddy fields infiltrates underground. This process, however, has not been clarified. In the case that groundwater table is deep, the measurement of water content in unsaturated aquifer has not been reported. We measured water content in unsaturated aquifer in the Ashigara Alluvial Fan using neutron moisture meter. As a result, there was no difference in the water content between in irrigation period and in non-irrigation. It was revealed that the infiltrated water from the paddy fields goes down, not increasing water content in unsaturated aquifer in the Ashigara Alluvial Fan.

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本研究では，我が国における地下水・湧水中のCFCs濃度の実態を明らかにするとともに，CFCs濃度を用いた滞留時間推定の可能性について検討を行った．

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Water quality and stable isotopes in groundwater, river water and precipitation in the Ashigara Plain were analyzed to consider the groundwater flow-pattern in the plain. Regression analysis between isotope ratio in precipitation and those in groundwater both in summer and winter suggested that the groundwater was recharged mainly by summer precipitations.

The Ashigara Plain could be classified into four regions by the movement patterns of the unconfined groundwater. In the first area, the δ¹⁸O values in groundwater was almost equal to that in the upper part of the Sakawa River, showing that the groundwater is recharged by infiltration of the irrigation water derived from the upper part of the Sakawa River. In the second area, the groundwater was judged to be local water which was unaffected by the Sakawa River, but mostly supplied from precipitation, because δD and δ¹⁸O values in the groundwater were different from those in the river water of the Sakawa River, but were almost equal to the weighted mean of the values in the precipitation. In the third area, the groundwater was also judged to be unaffected by the Sakawa River, but isotope values in the groundwater was isotopically heavier than the weighted mean of those in the precipitation. In contrast, in the fourth region, the EC and water quality of the groundwater were different from those in the other regions, because the groundwater was supplied from the foot part of Mt. Hakone.

The plain was also classified into three confined groundwater-flow regions. The first region locates, in the center of the Ashigara Plain, where the δ¹⁸O values of groundwater were equal to that in the river water of the upper part of the Sakawa River. This suggests that groundwater recharge from the upper part of the Sakawa River exists in this region. In the second region, i.e., the Kari River Basin, the EC in the confined groundwater was smaller than those in the other two areas. From this result and the geologic column, the groundwater recharge from the foot part of Mt. Hakone can be expected in this region. In the third region, i.e., in the southeast part of the Ashigara Plain, the δ¹⁸O values of groundwater were isotopically lighter than that in the unconfined groundwater in the same region, suggesting that the isotopically light groundwater, which is affected by the altitude isotope effect, is supplied from the Ooiso-uplands.

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The Ashigara Plain is divided into three areas based on its topography: the upper area consists of the alluvial fans of the Sakawa and Kawaoto Rivers, the intermediate area in the alluvial plain characterized by a number of springs and flowing wells and the lower area in the coastal plain. The groundwater system of these areas can be explained by Tolman’s classification of the fan. The upper zone of the plain is a recharge area. The groundwater system composed of one thick aquifer is mainly recharged by both continuous influent flow from the Sakawa and Kawaoto Rivers and summer irrigation water for paddy fields. These effects on the groundwater are recognized by tracing groundwater temperature and seasonal fluctuations of groundwater level. Tracing cold and warm water movements in the aquifer, the trace is closely accord with the groundwater flow system estimated by the distribution of groundwater heads. The groundwater system along the intermediate and lower zones, which is hydrogeologically composed of two gravel aquifers and one silty aquitard, is divided into a shallow and a deep aquifer system. These groundwaters are mostly supplied by subsurface flow from the upper zone of the plain and the area surrounding the Hakone volcano. Groundwater in the deep aquifer, which is confined by the silty aquitard, flows out of artesian wells. Their distribution narrowed in the intermediate and lower zones due to increase in groundwater pumping rates during the 1960s. Artesian wells and their recharge system are well explained by the profile of groundwater heads, and the distribution of tritium and δ¹⁸O. It is clarified that the small tritium content below 5TU and the low temperature below 16℃ in the deep aquifer of the intermediate and lower zones indicate that groundwater recharge is derived from the Hakone volcano.

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Lateral heterogeneities of S-wave velocity structures in and around Ashigara valley in the west of Kanagawa prefecture, Japan were discussed using the phase velocity dispersion of Love waves. The phase velocities in the frequency range of 0.1 to 0.5Hz were determined using the strong motion records of over 40 in and around Ashigara valley from the east off Izu peninsula earthquake (M_j=5.7) of May 3, 1998. We applied a semblance analysis for determining phase velocities by dividing the observation area into several sub-arrays, which consist of 4 or 5 sites. A clear difference of phase velocity dispersions in a wide frequency range was found between the valley region and the mountain/hill sites that are surrounding the valley. S-wave velocity structure models beneath the sub-arrays were estimated by an inversion technique of the genetic algorithm. The structure models show a laterally complex heterogeneity and two distinctive features. The depth to the basement in the east side of Kouzu-Matsuda fault is shallower than that of the west side. The depth to the layer with S-wave velocity of 1.5km/s in the south of Ashigara valley is deeper than that in the north, however, it is opposite for the depth to the layer with S-wave velocity of 2.4km/s, that is, the south of the valley is shallower than that in the north.

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The North Matsuda fault in the Kozu-Matsuda fault zone is one of the active fault system, showing the largest vertical component of slip rate onshore Japanese islands. The Kozu-Matsuda fault system is considered as a part of the northern margin of the Philippine Sea plate. It is necessary to obtain the horizontal shortening component of slip rate of the North Matsuda fault for a better understanding of active tectonics in the northern margin of the Philippine Sea plate. To reveal the subsurface structure of the North Matsuda fault, high-resolution shallow seismic reflection survey with about 3.2 km line length was carried out across the fault, A kind of accelerated weight drop system named as Yuatsu Impactor (JGI Inc. ) was used as seismic source which has wide band frequency spectrum. Both the standard shot intervals and group intervals of geophones were 10 m. The Sakawa River near the center of the line obstructed both shooting and geophone setting, and the half of the line in the Matsudayama Mountains was severely crooked. Although such a bad survey conditions, the seismic section after careful data processing shows a drastic structural change and a north dipping thrust fault with 18 degrees near the Sakawa River, which is suggested as the North Matsuda fault. The fault trace revealed by a result of the seismic reflection survey is exactly located along the Sakawa River.

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酒匂川右岸地区では,新たなる田園環境づくりが行政と住民とのコンセンサスによって精力的に行われている。高齢化社会を迎えている今日,地域のコミュニケーションづくりに土地改良施設は大いに役立っている例を,開成町の「あじさい農道」で紹介する。さらに,歴史的に繰返された災害の経験の中から住民管理による国土保全思想が培われ,それが酒匂川水系保全対策協議会の発足に生かされ,の環境保全に貢献していることを取上げた。また,防災対策強化地域にご指定されているでは,地域防災のために文命用水が重要な係わりをもっていることを指摘しつつ,地域の活性化と保全に果たす農業用水の役割について考察した。

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Tectonics and geological environments in and around Ashigara valley are very complex, especially surrounding rock outcrop sites are not on the same geology. Therefore, it is our concern to find the best reference site and to determine the seismic response at a site in the valley. We had an opportunity to use the ground motion data sets from the very far (>700km) and large (>M7) events to compare the rock sites motion as well as the response of sediments site. Advantages of using far and large events are that the source and path effects will be common with a sufficient approximation and that the ground motions of wide frequency band content are expected. Selecting one rock site, seismic responses of the sediment sites and rock sites were determined in terms of frequency as well as time domain amplitudes applying band-pass filtering techniques. Results are as follows; (1) Spatial distribution of seismic responses as a function of frequency in the valley are displayed. The spatial distribution of amplitude in low frequency range under 0.2Hz is relatively simple and similar to Bouguer anomaly map. The characteristics of spatial distribution in high frequency are more complex. (2) Deviations of spectral amplitude at rock sites were determined by taking the spectral ratio of the site to the average spectra of 6 rock sites. Only lower frequency motion than 0.07Hz are the same and the deviations of spectral ratio are a factor of 2 in the frequency range of 0.1-10Hz. (3) The level of spectral ratios of very far earthquakes are more stable than that of near earthquakes in 0.1-10Hz.

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の開成町を包含する約500haの区域において流量測定を行った結果, 潅漑期の還流率は0.88と0.72となった。減水深換算では潅漑期は31mm/dと70mm/dとなり, 25-60mm/dが地下浸透量となっていることが推測された。還流の過程で, 潅漑期にはT-N, COD, BODとも濃度および負荷量は減少し, 非潅漑期には濃度および負荷量は増加していた。しかし, 濃度はCODとBODでは潅漑期の方が非潅漑期よりも高かった。農業用水の還流と集落保全機能の関係について調べた結果, 非潅漑期には潅漑期の流量のほぼ40%の通水が行われ, 市街地率の高いところほど冬期通水ヵ所率が高く, 集落への防火用水を中心とした通水管理の行われていることがわかった。

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After the eruption of Mt. Fuji in 1707, tremendous volumes of sediment flooded the right bank of the Sakawa River in the Ashigara Plains, and other areas. We conducted field studies of the right bank of the Sakawa River and other areas in order to determine the terrain formed by the scoria-flood flow flooding and the distribution, stratum thickness, properties, and other features of the sediment deposits. At the same time, we also determined the terrain before the Hoei eruption. In the upstream region, the scoria-flood flow flooding created the Madarame-Kanaishima Uphill, while in the midstream region, the grooved concave ground was formed by erosion. The scoria-flood flow covered an area of about 12.4 km², approximately 61% of the total area of the right bank. The maximum thickness of the flood deposits was 4 to 4.5 m. The total volume of soil deposited on the right bank and other areas was approximately 1.8 x 10⁷ m³.

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The South Fossa Magna in central Japan is thought to be a collisional zone between the Philippine Sea plate and the Asia plate. In this region, intense tectonic movements such as folding, faulting and other crustal movements have been proceeding since the early Quaternary. Several active faults which have an extremely high vertical slip rate are distributed along the north end of the Izu bar.
Three tectonic sub-regions can be distinguished on the basis of the sense of fault movement and other tectonic features. That is, the area of the southwestern foot of Mt. Fuji, the area of the south end of the Tanzawa Mts. and the Ashigara-Oiso area. The first area is located in the northern extension of the Suruga trough. This area is divided into three blocks by N-S trending active faults. The easternmost block shows distinct subsidence and the others upheave with a high rate. In the second area, an E-W trending high angle reverse fault bounds the south end of the Tanzawa Mts. The topographic feature of this fault is indistinct. The third area is located in the northwestern extension of the Sagami trough. The NW trending Kozu-Matsuda fault which has a sence of reverse and right lateral slip divides the area into the Oiso hills and the Ashigara plain. The former shows a distinct upheaval which occurred in recent geological ages and the latter suggests the decrease of its subsidence rate.
Studies of the crustal movements in each sub-region reveal the following common tectonic features. That is, each crustal block distant from the Izu bar has changed its movement style from the phase of intense subsidence to that of upheaval through the transitional phase. A newly created fault or a subsidence zone has always occurred in a block closer to the Izu bar. As a consequence, the latest tectonic zone in the South Fossa Magna seems to have migrated from a distant position closer to the Izu bar. Fig. 3 shows a concept of the tectonic process across the tectonic zone in the South Fossa Magna. This tectonic process is the process of accretion of the sediment to Honshu in the landward region of the Suruga and Sagami troughs. The active faults in this region are considered to be neither interplate faults nor branches from the main boundary fault but the imbricated thrust faults in the accretional sediments.

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