著者
小山 真人
出版者
国立大学法人 静岡大学理学部地球科学教室
雑誌
静岡大学地球科学研究報告 (ISSN:03886298)
巻号頁・発行日
vol.50, pp.115-142, 2023-08-02 (Released:2023-08-08)

Based on field surveys and terrain analysis using open data of aerial LiDAR point cloud (VIRTUAL SHIZUOKA), we analyzed the characteristics and causes of flooding in the lower basin of the Tomoe River caused by the heavy rains associated with Typhoon 15 in 2022. The water level of the main river was diverted into the Oya drainage channel and dropped once, but the level rose again and overflowed where the tributaries merged. The levee itself was not damaged or breached. Tributaries that had not undergone sufficient river improvement, and thus still had many meanders and shallow channels, flooded significantly, while tributaries that had undergone advanced river improvement also flooded due to increased water levels before joining the main river. Tributaries often overflowed in front of the insufficient cross-sectional area of the river channel where they passed under roads and railroads. Comparison of the flood depth distribution with existing hazard maps showed that the main channel flooding was generally on the planned scale. Some of the tributary flooding corresponded to the assumed maximum magnitude, while others fell far short of it. The high resolution of the topographical map based on point cloud data is considered effective for understanding flood risk and raising public awareness because it realistically reconstructs the flooded urban landscape.
著者
北村 晃寿 矢永 誠人 岡嵜 颯太 片桐 悟 中西 利典 森 英樹
出版者
国立大学法人 静岡大学理学部地球科学教室
雑誌
静岡大学地球科学研究報告 (ISSN:03886298)
巻号頁・発行日
vol.49, pp.87-95, 2022-07-31 (Released:2022-07-31)

On July 3, 2021, a debris flow caused by a landslide from an embankment occurred along the Aizome River in the Izusan area of Atami City, Shizuoka Prefecture, central Japan, and buried sabo dam which was located approximately 450 m downstream of the embankment. Shizuoka Prefectural Government sampled one 13.1-m long core of deposits buried in the sabo dam in late August 2021. In this paper, radiocesium concentration was measured and grain compositions was investigated for this core sample. As a result, 134Cs and 137Cs released from the Fukushima Daiichi Nuclear Power Plant in mid-March 2011 were detected, and these markers specified the depositional surface at mid-March 2011 at 3.95-3.85 m core depth. In addition, the occurrence of fragments of both mudstone and chert in the overlying sediments indicates that the 3.74 m-thick portion was definitely the debris flow deposit.
著者
荒井 章司
出版者
静岡大学地球科学教室
雑誌
静岡大学地球科学研究報告 (ISSN:03886298)
巻号頁・発行日
no.20, pp.p175-185, 1994-03
被引用文献数
4

The Circum-Izu Massif Serpentine Belt is a fossil transcurrent plate boundary of the Oligocene-Miocene time beween the Philippine Sea and the Eurasia plates. The Circum-Izu Massif Serpentine Belt encircles the northern end of the Izu Massif, almost parallel to a trace of the present plate boundary, the Sagami Trough and the Suruga Trough. The Circum-Izu Massif serpentinite had been mainly emplaced into or protruded onto the Oligocene-Miocene sediments. The Circum-Izu Massif peridotite, mainly harzburgite, is characterized by the mineral assemblage, olivine (Fo_<90-92>) + opx + cpx + chromian spinel (Cr#, ca. 0.5) +/- plagioclase (An_<90>), and is a low-pressure (ca. 5 kb) restite with or without melt impregnation. It may represent the uppermost mantle of the Shikoku Basin, which had begun opening during Oligo-Miocene, protruded along the transcurrent plate boundary at the northern end of the Philippine Sea plate ( the Shikoku Basin ) , by analogy with the protrusion of abyssal peridotites along the oceanic fracture zone. It had been accreted to the Eurasia plate by the ocean-ward bending of the central Honshu arc during the opening of the Sea of Japan in the Miocene. Detrital chromian spinel grains in the Oligocene-Miocene sediments in the Circum-Izu Massif Serpentine Belt memorize some ancestral peridotites with arc-mantle characteristics which had been protruded in the Belt before the present Circum-Izu Massif serpentinite (peridotite). The ancestral peridotites and dioritic rocks frequently associated with the sepentinites were derived from the deep parts of the Paleo-Izu-Bonin arc, which had been split and disrupted by the Shikoku Basin opening. Picritic basalts of intra-plate type closely associated with the Circum-Izu Massif serpentinite were possibly derived from the mantle plume which had caused the opening..
著者
狩野 謙一 小田原 啓 山本 玄珠 伊藤 谷生
出版者
国立大学法人 静岡大学理学部地球科学教室
雑誌
静岡大学地球科学研究報告 (ISSN:03886298)
巻号頁・発行日
vol.46, pp.19-49, 2019-07-31 (Released:2021-11-18)

We have examined the surface geology and tectonic landforms of the Hoshiyama Hills and its surrounding areas in the Fujikawa-kako Fault Zone (FKFZ), central Japan. The FKFZ is generally regarded as a collision zone between the Honshu Arc, mainly composed of the pre-Neogene accretionary and the Neogene collisional complexes of South Fossa Magna region, and the Neogene Izu-Bonin Volcanic Arc on the Philippine Sea Plate. This zone is believed to be the most active and dangerous area of Japan in association with violent earthquakes. One of the keys to understand the activities of this zone since the Pleistocene is to reveal the geologic structures of the Hills, as well as the characteristics of the Omiya and Iriyamase Faults along the northeastern and southeastern margin of this Hills, respectively. Our surface geological survey reveals that the upper Lower-lower Middle Pleistocene Ihara Group, main constituent of the basement of the Hoshiyama Hills, has complicated structures including several-hundreds meters scale steeply-dipping beds without distinctive preferred orientations. The structures also include chevron-shaped anticlines and flat synclines, suggesting that the E-W horizontal shortening due to fault-related foldings was the main cause of their formations. The flexure-landform associated with the Omiya Fault clearly suggests that the Fault, previously believed to be a high-angled normal fault dipping toward NE, is a reverse fault dipping toward SW. However, the landform around the Iriyamase Fault, also believed to be a NW dipping reverse fault, show no evidence of its existence. Finally, we have summarized the tectonic and volcanic events in and around the FKFZ since about 1 Ma. These results suggest that the Quaternary tectonics and seismic activities of the FKFZ should be necessary to re-evaluate based not only on the surface geological and landform data but also on the subsurface geological structures now being poorly known.
著者
北村 晃寿 平田 将也
出版者
国立大学法人 静岡大学理学部地球科学教室
雑誌
静岡大学地球科学研究報告 (ISSN:03886298)
巻号頁・発行日
vol.48, pp.23-26, 2021-07-31 (Released:2021-11-18)

The present study detected pumice grains of Kawagodaira Pumice, which was erupted at 3160‒3137 cal BP, from the >0.063 mm size fraction of cored deposits of the Upper Holocene at site 3 of Nakazato, Yaizu City. We inferred the initial stratigraphic position of the pumice at a core depth of 6.51 m (elevation −3.70 m).
著者
Wada Hideki Ohashi Fumio Fujioka Kantaro
出版者
Shizuoka University. Institute of Geosciences
雑誌
Geoscience reports of Shizuoka University (ISSN:03886298)
巻号頁・発行日
vol.20, pp.167-173, 1994-03

Massive magnesite deposits in the Setogawa Group in Shizuoka were formed by an extensive carbonation of serpentinite in an ancient accretionary prism. Stable isotopic investigations of the origin of massive magnesite deposits revealed that a homogeneous deep-seated magmatic carbon mixing with marine limestones may be the main source of the carbon. Many calcite and dolomite veins occur in the serpentines, basaltic and pelitic rocks and based upon carbon and oxygen isotopes are supposed to be of similar origin to carbonate veins found in modern serpentine diapirs of the Izu-Bonin and Mariana is-land arc. In contrast, massive magnesite deposits were formed during the subduction of an active volcano and/or an active ridge, with circulation of magmatic carbon providing the carbon source for the serpentine diapirs.
著者
中原 竜二 海野 進 渡辺 一樹
出版者
静岡大学地球科学教室
雑誌
静岡大学地球科学研究報告 (ISSN:03886298)
巻号頁・発行日
no.18, pp.p25-57, 1992-07
被引用文献数
2

伊豆半島と伊豆大島の間の海底には40ケ所以上の地形的高まりが存在して, そのほとんどは海底火山体と考えられている。1991年4月27日から5月1日にかけて海上保安庁水路部によって, この海域に存在する海山の頂上, 約30ポイントから約60個の火山岩, 砂岩, 火山噴出物と砂などから成る集塊岩などがドレッジされた。Na_2O+K_2O-FeO^*(全FeO)-MgO図(AFM図)で示される全岩化学組成から, ドレッジされた火山岩は, よりアルカリ元素に富み, 低いFe/Mg比をもつ局アルカリソレアイト系列と, よりアルカリ元素に乏しく, 高いFe/Mg比をもつ低アルカリソレアイト系列とに分かれる。両系列はその地球化学的, 岩石学的特徴が, それぞれ東伊豆単成火山群, 伊豆大島火山と似ている。これらの高アルカリソレアイト系列の岩石と東伊豆単成火山群から得られる火山岩は低アルカリソレアイト系列の岩石及び伊豆大島火山から得られる火山岩よりもより高いZr/Y比をもっている。マントルの部分溶融の程度が高いほどZr/Y比は低くなる。したがって高アルカリソレアイト系列の岩石と東伊豆単成火山群の本源マグマは低アルカリソレアイト系列の岩石と伊研大島火山よりもマントルの部分溶融の程度の低いマグマに由来していると思われしる。また, 高アルカリソレアイト系列の岩石と東伊豆単成火山群の岩石のZr/Y比はかなり広い組成領域をもち, マントルの部分溶融の程度にもかなりの差がある。この違いはN-type MORBのソースマントルが4〜10%分別部分溶解を起こすことによって説明できる。Zr/Y比の地域変化を見ると, 東伊豆単成火山群(海底火山も含む)は系統的な分布を示さず, 部分溶解の程度も場所によってさまざまであると思われる。したがって東伊豆単成火山群が同一の部分溶解したマントルダイアピールに由来するというモデルは成立しない。
著者
中原 竜二 海野 進 渡辺 一樹
出版者
静岡大学地球科学教室
雑誌
静岡大学地球科学研究報告 (ISSN:03886298)
巻号頁・発行日
vol.18, pp.25-57, 1992-07

伊豆半島と伊豆大島の間の海底には40ケ所以上の地形的高まりが存在して, そのほとんどは海底火山体と考えられている。1991年4月27日から5月1日にかけて海上保安庁水路部によって, この海域に存在する海山の頂上, 約30ポイントから約60個の火山岩, 砂岩, 火山噴出物と砂などから成る集塊岩などがドレッジされた。Na_2O+K_2O-FeO^*(全FeO)-MgO図(AFM図)で示される全岩化学組成から, ドレッジされた火山岩は, よりアルカリ元素に富み, 低いFe/Mg比をもつ局アルカリソレアイト系列と, よりアルカリ元素に乏しく, 高いFe/Mg比をもつ低アルカリソレアイト系列とに分かれる。両系列はその地球化学的, 岩石学的特徴が, それぞれ東伊豆単成火山群, 伊豆大島火山と似ている。これらの高アルカリソレアイト系列の岩石と東伊豆単成火山群から得られる火山岩は低アルカリソレアイト系列の岩石及び伊豆大島火山から得られる火山岩よりもより高いZr/Y比をもっている。マントルの部分溶融の程度が高いほどZr/Y比は低くなる。したがって高アルカリソレアイト系列の岩石と東伊豆単成火山群の本源マグマは低アルカリソレアイト系列の岩石と伊研大島火山よりもマントルの部分溶融の程度の低いマグマに由来していると思われしる。また, 高アルカリソレアイト系列の岩石と東伊豆単成火山群の岩石のZr/Y比はかなり広い組成領域をもち, マントルの部分溶融の程度にもかなりの差がある。この違いはN-type MORBのソースマントルが4〜10%分別部分溶解を起こすことによって説明できる。Zr/Y比の地域変化を見ると, 東伊豆単成火山群(海底火山も含む)は系統的な分布を示さず, 部分溶解の程度も場所によってさまざまであると思われる。したがって東伊豆単成火山群が同一の部分溶解したマントルダイアピールに由来するというモデルは成立しない。