Tokamachi City Research Articles

Geological Structure and Groundwater Geochemistry around Mud Volcanoes in the Kamou Area, Tokamachi City, Niigata Prefecture

Mud volcanoes are defined as conical geometric high mounds composed of erupted mud that originates deep underground, and are induced by abnormal pore water pressure. It is well known that a serious problem occurred due to swelling mudstone during tunnel construction work in the Tertiary sedimentary basin in Japan. The swelling mudstone is partly caused by the activity of mud volcanoes. However, it has not been demonstrated in the field yet. Boreholes with a depth of 120 m were drilled at an altitude of 329 m inside a depression in the Kamou area, Tokamachi City, Niigata Prefecture where two active mud volcanoes and an inactive mud volcano are distributed and the Nabetachiyama Tunnel with swelling mudstone is located. Geologic structure and geochemical properties of groundwater around the mud volcanoes were studied by core logging and geochemical analysis of pore water squeezed from cores. Humus soil occurs from ground surface to a depth of 2.1 m. Erupted deposits composed of mudstone fragments and clay are located in the interval from 2.1 m to 5.8 m in depth. Intact massive mudstone is located from 5.8 m to about 50 m in depth. Mud breccia composed of mudstone fragments and scaly soft clay is located from fifty meters in depth to the bottom of a borehole. Mud breccia is classified into two types based on its textural and structural characteristics. The distribution of each type of mud breccia suggests that mud breccia was formed by hydro-fracturing caused by the activity of a mud volcano mainly at depths greater than fifty meters. As a result of an XRD analysis of clay minerals and XRF analysis of rock, Ca-type smectite is found to be dominant and CaO content is rich from the ground surface to 55 m in depth, whereas Na-type smectite is dominant and Na2O content is rich at depths greater than 50 m. Groundwater squeezed from cores greater than fifty meters in depth is highly saline. Therefore, the cation exchange from Ca2+ to Na+ occurred due to the highly saline groundwater. In summary, saline pressurized groundwater ascended with hydro-fracturing to a depth of fifty meters and replaced the fresh groundwater. Variation of stable isotope ratios of pore water with depth supports the assumption above.

Underground Structure of Mud Volcanoes in Tokamachi City, Niigata Prefecture Determined by Electromagnetic Exploration, and Geographical and Geological Surveys

In Tokamachi City, Niigata Prefecture, where Tertiary sedimentary rocks are distributed, excavation of the Mt. Nabetachiyama tunnel took a long time to complete due to the presence of a swelling mudstone zone. This swelling zone is distributed at a depth of 180m under the mud volcanoes. Geological and geographical surveys as well as electromagnetic surveys applying the Controlled Source Audio-frequency Magneto-Telluric Method (CSAMT), were carried out to clarify the geological structure under the mud volcanoes and the relationship between geographical features and the swelling zone. The laser scanner survey revealed that the geological structure around the topographic depression is intensely disturbed and the geological survey showed that the mud volcanoes and eruption points of groundwater and natural gas are located mainly along the outside edge of the geologically disturbed zone. This suggests that the area was active in the past due to the uplift and eruption of groundwater. The extremely low resistivity zone (ELR) at a depth of 400m with a diameter of 500m was detected below the geologically disturbed zone by the CSAMT survey. The ELR is estimated to correspond to a mud chamber filled with saline groundwater and mud, based on laboratory tests measuring the electrical properties of several rock samples obtained from the survey area. The low resistivity zone (LR) was also detected; it continues from the ELR to the mud volcano at the ground surface, indicating that saline groundwater, mud, and natural gas may be ascending via a concentric path to the mud volcanoes. On the basis of all the results obtained, it is interpreted that the swelling zone in the tunnel corresponds to the path of mobilized mud and saline groundwater in the fractured mudstone with abnormal pore water pressure.

Origin of Materials Erupting from Mud Volcano in Tokamachi City, Niigata Prefecture, Central Japan

Mud volcanoes are structures formed as a result of the emissions on a land surface or the sea floor of argillaceous material, which is composed of erupting remobilized mud, petroliferous or magmatic gases, and high-salinity water. Recently, large constructions have been planned deep underground besed on the expectation of geological stability. Therefore, it is important to study the origin of erupted mud and groundwater and the depths from which they ascend when evaluating long-term stability. Three active mud volcanoes and a passive mud volcano are found in the Tertiary Shiiya Formation distributed in Tokamachi City, southern part of Niigata Prefecture. Detailed descriptions of the mud volcanoes are provided by Shinya and Tanaka (2005). However, the origin of erupted mud and the formation mechanism of abnormal pore water pressure have not yet been identified. The authors measured the oxygen and hydrogen isotopic ratio of groundwater and vitrinite reflectance of coal fragments separated from erupted mud of an active mud volcano to investigate the origin of erupted mud, particularly the depth of the origin, and the formation mechanism of abnormal pore water pressure. As a result, δ18O and δD values of erupted water are 1.2‰, -5‰ respectively, showing good agreement with those of the Nanatani Formation distributed at a depth of 3400 m in depth in the studied area. Vitrinite reflectance (Ro) shows a bimodal distribution (i.e., 0.3-1.2% and 1.5-1.8%). Ro value of coal fragments sampled from the Shiiya Formation at the outcrop in the studied area are 0.3-0.45%. High Ro (1.5-1.8%) values of coal fragments are obtained in core samples at a depth of 4000 m in the Gimyo SK-1 oil well, which was excavated 2 km NW from the mud volcano. As a result of an investigation of erupted materials at the mud volcano, they were found to have originated at depths of from 3400 m to 4000 m in the studied area. Geothermal temperature of underground at depth of 3400 m to 4000 m in the in the studied area is estimated to be about 120°C to 150°C. Estimated temperature is high enough to cause diagenetic transition from smectite to illite. Transition from smectite to illite results in the release of a large volume of pore water into the sediment. It is concluded that dehydration due to mineral transition might be the major reason for abnormal pore water pressure formation at depths of 3500 m to 4000 m in the study area.

新潟地域の大規模地すべりと異常高圧熱水系

High salinity Na-Cl-type geothermal waters have often been found in the anticlinal hilly terrains and landslide-prone areas of Niigata Prefecture by boring to depths of more than 1,000 meters. These fluid pressure gradients are much higher than the hydrostatic pressure gradient and approach the lithostatic pressure gradient with increasing depth. The geothermal waters in the Matsunoyama area, Tokamachi City, have the highest orifice temperature in Niigata Prefecture. Eight geothermal water wells in this area were drilled along the anticlinal axis of a nearby hilltop or higher breast of the Matsunoyama dome and ranged in depth from 170 to 1,170 meters and in temperature from 35 to 95°C. For example, the Takanoyu-1 geothermal water well is only 170 meters in depth but has an orifice temperature of 90°C. These waters show geyser action associated with methane gas, and typically have very high salinity with considerable amounts of chloride. Hydrogen and oxygen isotope values and chloride concentration of approximately 9,000mg/L suggest that the origin of the waters is altered fossil seawater trapped in organic-matter-bearing sedimentary rocks. The temperature and depth of the primary reservoir are estimated to be 139°C and 3,000 to 4,000 meters, respectively, using a Li-Mg geothermometer and the mean geothermal gradient of 30-40°C/km in the Niigata sedimentary basin. Na-Cl-type groundwaters emerged from several landslides located in the Higashi-kubiki and Naka-kubiki areas including the Matsunoyama area. Well loggings were carried out to profile the electric conductivity and hydrochemistry of groundwater in the Utsunomata landslide in the neighborhood of the Matsunoyama area. Profiles in the active landslide mass showed sharp increases of the electric conductivity and the NaCl components at around the depth of the sliding surface. In contrast, no significant variations of the profiles were recognized in the inactive landslide mass. Na-Cl-type groundwaters are formed by mixing deep Na-Cl-type geothermal waters with meteoric groundwaters. These phenomena suggest that the geothermal water injection into shallow aquifers in landslide mass generates a partially high pore water pressure around the sliding surface and causes landslides.

Pictorial 2: Results of the Laser Scanner and the CSAMT Surveys Carried out in the Kamou Area, Tokamachi City, Niigata Prefecture

Pictorial 2: Results of the Laser Scanner and the CSAMT Surveys Carried out in the Kamou Area, Tokamachi City, Niigata Prefecture

表面波探査による松代室野地区の泥火山の浅部Ｓ波速度構造調査

We investigated the geological structure below the Matsudai-Murono mud volcano, Tokamachi city, Niigata prefecture, Japan, using vertical distributions of S-wave velocities, estimated from surface wave inversion. From the surface to a depth of several meters, the S-wave velocity first increases, then decreases, and finally increases again. Below a depth of 10 m, there are two areas with high and low velocities in the horizontal direction. We found that the subsurface low-velocity layer is a mud layer produced by an eruption, and the deep low-velocity zone is a mudstone zone, some parts of which are transformed into soft clay by muddy water that drifts upward. The planar distribution of the S-wave velocity indicates that the deep low-velocity zone may be subdivided. The zone in which the velocity is particularly high corresponds to the area without surface uplift indicated by GPS; hence, this area is not influenced by the mud volcano. This study shows that it is possible to conduct subsurface geophysical investigations to clarify the eruption mechanism of the mud volcano.

新潟県の魚沼層群と平層から発見された１．７Ｍａ頃の津池火山灰単層

The Tsuike volcanic ash bed (TsA)(newly defined) is found in the marine silt-sand beds of the Uonuma Group over 24 km in the Uonuma Hill. The Tsuike volcanic ash bed is 6 to 35 cm thick, fine sand to silt sized, mainly composed of vesiculated thin glass shards with a few crystals of plagioclase, hornblende and biotite. It is intercalated about 25 m above the Tsujimatagawa volcanic ash bed (1.75 Ma), and about 70 m below the SK110 volcanic ash bed (1.61 Ma) at Tagawa, Tokamachi city and the age of the Tsuike volcanic ash bed was estimated to be about 1.7 Ma. The ash bed is also identified in the Niitsu Hill at Jojo, Kamo City (4 cm thick) and Sugenosawa, Gosen City (5 cm thick). At Sugenosawa in central Niitsu Hill, it is intercalated about 5 m above the basal part of the marine mud of the Taira Formation. The Tsuike volcanic ash bed is thus useful marker tephra for correlation of the 1.7 Ma marine beds widely interbedded in the Uonuma Group and Taira Formation in Niigata Prefecture.

屋根融雪装置の能力設計に関する提案

Both of effective design and efficient operation of snow-melting system on roof is needed to reduce its initial and running cost. This paper presents a simulation method to estimate remaining amount of snow, considering the snow-melting rate and the threshold value to run the system, using daily air temperature and precipitation data. Simulation was done using the data at Nagaoka City and Tokamachi City in 2 snow abundance winters. It was shown the minimum melting rates to keep building safe are 11.6 and 17.8 kg/(m2d) for Nagaoka and Tokamachi, respectively in recent 30 years. The values are remarkably smaller than the existing system. It was also shown the greater threshold value to run the system brings lower running cost, and the appropriate operation brings less running cost than the cost of manual removing operation.

A Method for Estimating the Weight of Snow Cover on the Ground and Roofs using only Items of AMeDAS's Data

This research has the important objective of finding a method for estimating the weight of snow cover on the ground and roofs over large areas and on a long term for the Tohoku region using only the meteorological factors of AMeDAS observed at many places of snowy regions in Japan.The weight of the snow cover was obtained by adding variable parameters obtained by deducting the daily quantity of melting snow from the daily quantity of precipitation continuously from the first snowfall of the winter season. The daily quantity of melting snow was defined as the differ-ence of the weight of snow cover measured serially every the two days without rainfall and snowfall during the winter season, and was obtained as a function of average atmospheric tempera-ture, using 1979 to 1989 data from Tokamachi Forest Testing Station in Niigata prefecture.In order to study the relation between snow cover on the ground and roofs, two small testing houses were constructed to observe the accumulation and melting processes of snow cover, one in Aomori city in Aomori prefecture and one in Tokamachi city in Niigata prefecture. Both of the areas are typical heavily snowy regions in Japan. The following data were measured around and on the houses during the winter season of 1990: atmospheric temperature, wind speed and direction, daily quantity of snowfall, snow depth on the ground and on roofs, and densities of snow cover.By substituting these data into a estimation formula, the values of the weight of snow cover were computed on the ground and on roofs. The estimates obtained from the model were correlated with the actual measured values.From the data it was determined that the quantity of accumulation of newly fallen snow on a roof per day was apt to be influenced by wind speed in comparison with accumulation on the ground, and the tendency for accumulation on the roof decreased with increasing wind speed. Therefore, the weight of snow cover on roofs was recognized as 70-80% of the accumulated snow on the ground at cach place.Consequently, if the daily data of atmospheric temperature, quantity of precipitation, wind speed and direction can be collected, then the weight of the snow cover on the ground and roofs can be estimated with confidence.

Avalanche Protective Effect of Young Broad-leaved Forests

During the winter of 1968-1977 the relation between the growth of young broad-leaved forests and the snow glide was investigted on mountain slopes in Tokamachi city, Niigata Prefecture. It was observed that snow glide was greatly affected by the total number of trees, the tips of which appeared on the surface of snow cover of 2-3m in depth and those which were buried under the snow cover; the stems were over 6 cm in diameter. Avalanches tended to occur in the forests in which the number of trees was less than 3, 000/ha. When the number of trees increased to more than 4, 500/ha, the snow glide decreased to 2-3m in a winter and the snow cover became stable. It appeared that the snow cover on the southern slopes became stable in 10-15 years after regeneration felling and the same was observed on the northern slopes in 20-25 years.

The observations of the snow cover and its sliding down on the galvanized iron roofs

Recently, many houses built in heavy snow districts have been roofed with galvanized iron sheets which have less friction with snow cover on them. It is convenient that the snow cover on the roofs falls down naturally but its sudden falling is very dangerous. In spite of a wide use of the roof, the condition of the sliding down of the snow on the galvanized iron roof have not as yet been fully investigated.Since 1974, the snow cover on galvanized iron roofs of 76 houses and its sliding down have been observed in Nagaoka city, Tokamachi city and Tsunan town, Niigata prefecture.Following results were obtained from the observation.(1) A large amount of snow was accumulated on the roof, during heavy snow periods from January 9, 1975 to January 13 and from January 18, 1976 to January 24 in the Niigata district. The amount of the snow did not depend upon the gradient of the roof and this leads us to the conclusion that the freezing between the roof surface and snow cover causes such a large amount of snow accumulation on the roof.(2) The sudden sliding of the snow cover occured when the air temperature rose after the heavy snows. In a certain case, the snow fell 6 m far from a house.(3) The coefficient of the friction between the roof surface and the snow cover was estimated from the sudden falling down of the snow cover occured in Tokamachi city and its value is 0.1.

Pollen Stratigraphical Study of the Uonuma Group

In the present article the authors intended to make clear the geologic age of the Chuetsu and Uonuma Groups, through the studies on the stratigraphy and the pollen floral change of these two groups. The results obtained are summerized below.1) The geology of the Chuetsu and Uonuma Groups in the western area of Tokamachi city is shown in Fig. 1.2) The results of the pollen analysis are shown in Figs. 2 and 3. The prosperity and decline of Taxodiaceae are well shown in these figures, and the floral zonation in Fig. 4 is established based on the occurrence of Taxodiaceae.3) The curve of the paleo-temperature in Fig. 4 was drawn upon the ecological data of the pollen assemblages.4) In Fig. 5, the Uonuma Group is compared with the Osaka Group which shows the most representative Quarternary succession in Japan, mainly based on the pollen floral change.5) The occurrence of the genera of Taxodiaceae is very characteristic: namely they change from (1) inferiority of Metasequoia, to (2) flourish of Metasequoia, to (3) inferiority of Metasequoia with Cryptomeria, to (4) flourish of Cryptomeria ascendingly. This phenomenon is samely seen in the Osaka Goup.6) The disappearance of the Tertiary elements and the appearance of the Quarternary elements in the Uonuma Group are earlier than in the Osaka Group.7) If we permit the presumption that the horizon of so-called Olduvai normal events is the boundary of Plio-Pleistocene, the boundary concerned in the Uonuma Group may be in the neighbourhood of Mf-3.