No.310 研究課題 / Project
多摩川上流域の山地斜面における深層崩壊に関する地形・地質学的研究Geomorphological and geological study of deep-seated gravitational slope deformation and large-scale landslides on mountain slopes in the upper Tama River basin, west Tokyo| 学術研究 Academic Research |
No.310 |
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| 代表研究者 Principal Investigator |
苅谷 愛彦 Yoshihiko Kariya |
| 所属（採択当時） Affiliation |
専修大学文学部 教授 Professor, School of Letters, Senshu University |
| 研究内容要約 Research Summary |
堆積岩山地では深層崩壊（巨大崩壊）や，その準備過程として岩盤の重力変形（Deep-Seated Gravitational Slope Deformation：DSGSD）がしばしば生じる．多摩川上流地域も広く堆積岩が分布するが，深層崩壊やDSGSDに関する研究はほとんどなかった．本研究は空中写真判読（地形分類），GIS解析，野外地質調査，火山灰・14C編年等を統合した地形学・地質学的手法により，多摩川上流地域の深層崩壊とDSGSDの実態を解明したものである． DSGSDは北東向き斜面でよく発現し，座屈変形やトップリングを伴う．特に，奥多摩町倉戸山，小菅村三頭山，丹波山村保之瀬天平に好例が多い．深層崩壊は日原川右支樽沢や小菅川右支玉川などに限定される．これら諸現象の形成期は不明な点も多いが，保之瀬天平では9.5万年以前，倉戸山では1万年以前にDSGSD性の線状凹地が形成されていた．玉川の深層崩壊は鎌倉時代以前に生じ，河道堰き止めを引き起こした． 多摩川上流地域では，深層崩壊の発生地点は限定的であるが，DSGSDやそれに関連した地形（線状凹地）は各地で生じている．この事実は，この地域の地形発達に対してDSGSDが大きな影響を及ぼしてきた可能性を示唆する．本研究により明らかになった諸点をふまえ，多摩川上流山地の斜面発達史を再検討することが必要である．また深層崩壊の潜在発生域となりうるDSGSDの分布や実態が明らかになったことで，砂防や治山の面でも種々の検討課題が生じてくると予想される．一方，深層崩壊による河道堰き止めの堆積物や，線状凹地の埋積物質はテフラや植物化石を良好に保存することが確かめられた．一般に，山地は侵食作用が卓越するため，古環境研究に関する資料の獲得は難しいことが多いが，これらの堆積場を活用した当地域の第四紀研究が進展することも考えられる． Deep-Seated Gravitational Slope Deformation (DSGDS) as a precursor of landslide and large-scale bedrock landslide (LS) are often developed in mountain areas composed of sedimentary rocks. Although sedimentary rocks of the Chichibu and Shimanto Belts are widely present in the upper Tama River basin, little is known about DSGDSs and LSs. This study attempted to identify DSGSDs and LSs in the upper Tama River basin by using airphoto interpretation (geomorphological classification), GIS analysis, field geological survey and dating analysis with tephras and 14C measurements. DSGDSs often occur on the northeast facing valley side slopes with buckling and toppling. Especially, typical examples are found in Mount Kurato, Mount Mitou and Hounose Dendeiro Ridge. DSGSDs are confined to the specific river basins such as Tarusawa Valley of Nippara River and Tamagawa Valley of Kosuge River. Chronological data show that linear depressions related to DSGSD on Hounose Dendeiro and Mount Kurato were already present at 95 ka and 10 ka (or before), respectively. The LS in the Tamagawa Valley was caused in the Kamakura Period or before, forming with a small dammed pond. Although LSs are limited to occur in the upper Tama River basin, DSGSD and its related landforms (e.g., linear depression and valley side bulging) are common. This fact suggests that DSGSDs have affected on geomorphic development in this region. Based on the geomorphological and geological information obtained by this study, Quaternary landscape evolution connected to DSGSD and LS in the upper Tama River should be reevaluated. In addition, elucidation of spatial distribution and geologic deformation of DSGSD will help to new evaluation of erosion control and soil and water conservation problems. This study also revealed that fallout tephra grains and plant fossils are well preserved in the landslide-dammed lake deposits (e.g., Tamagawa Valley) and in the slope material filling a linear depression (e.g., Hounose Dendeiro Ridge). Erosion processes are more generally in mountain areas and thus it is difficult to obtain preferable materials for dating and paleoenvironment reconstruction. However, it seems that these depositional environments will provide good opportunities for Quaternary studies in the upper Tama River. |
| 共同研究者 Collaborators |
佐藤 剛 目代邦康 清水長正 Go SATO Kuniyasu MOKUDAI Chosei SHIMIZU |
| 研究全文 Full Text |
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