Kohki Yoshida

Landslide susceptibility mapping along Bhalubang — Shiwapur area of mid-Western Nepal using frequency ratio and conditional probability models

Roads constructed in fragile Siwaliks are prone to large number of instabilities. Bhalubang-Shiwapur section of Mahendra Highway lying in Western Nepal is one of them. To understand the landslide causative factor and to predict future occurrence of the landslides, landslide susceptibility mapping (LSM) of this region was carried out using frequency ratio (FR) and weights-of-evidence (W of E) models. These models are easy to apply and give good results. For this, landslide inventory map of the area was prepared based on the aerial photo interpretation, from previously published/unpublished reposts, and detailed field survey using GPS. About 332 landslides were identified and mapped, among which 226 (70%) were randomly selected for model training and the remaining 106 (30%) were used for validation purpose. A spatial database was constructed from topographic, geological, and land cover maps. The reclassified maps based on the weight values of frequency ratio and weights-of-evidence were applied to get final susceptibility maps. The resultant landslide susceptibility maps were verified and compared with the training data, as well as with the validation data. From the analysis, it is seen that both the models were equally capable of predicting landslide susceptibility of the region (W of E model (success rate = 83.39%, prediction rate = 79.59%); FR model (success rate = 83.31%, prediction rate = 78.58%)). In addition, it was observed that the distance from highway and lithology, followed by distance from drainage, slope curvature, and slope gradient played major role in the formation of landsides. The landslide susceptibility maps thus produced can serve as basic tools for planners and engineers to carry out further development works in this landslide prone area.

The relationship between geology and rock weathering on the rock instability along Mugling–Narayanghat road corridor, Central Nepal Himalaya

The present study was conducted along the Mugling–Narayanghat road section and its surrounding region that is most affected by landslide and related mass-movement phenomena. The main rock types in the study area are limestone, dolomite, slate, phyllite, quartzite and amphibolites of Lesser Himalaya, sandstone, mudstone and conglomerates of Siwaliks and Holocene Deposits. Due to the important role of geology and rock weathering in the instabilities, an attempt has been made to understand the relationship between these phenomena. Consequently, landslides of the road section and its surrounding region have been assessed using remote sensing, Geographical information systems and multiple field visits. A landslide inventory map was prepared and comprising 275 landslides. Nine landslides representing the whole area were selected for detailed studies. Field surveys, integrated with laboratory tests, were used as the main criteria for determining the weathering zones in the landslide area. From the overall study, it is seen that large and complex landslides are related to deep rock weathering followed by the intervention of geological structures as faults, joints and fractures. Rotational types of landslides are observed in highly weathered rocks, where the dip direction of the foliation plane together with the rock weathering plays a principle role. Shallow landslides are developed in the slope covered by residual soil or colluviums. The rock is rather fresh below these covers. Some shallow landslides (rock topples) are related to the attitude of the foliation plane and are generally observed in fresh rocks. Debris slides and debris flows occur in colluviums or residual soil-covered slopes. In few instances, they are also related to the rock fall occurring at higher slopes. The materials from the rock fall are mixed with the colluviums and other materials lying on the slope downhill and flow as debris flow. Rock falls are mainly related to the joint pattern and the slope angle. They are found in less-weathered rocks. From all these, it is concluded that the rock weathering followed by geological structures has prominent role in the rock slope instability along Mugling–Narayanghat road section and its surrounding regions.

Weathering and mineralogical variation in gneissic rocks and their effect in Sangrumba Landslide, East Nepal

Sangrumba landslide is one of the largest and the most active landslides in Nepal Himalaya. Geologically the landslide belongs to the Higher Himalaya and consists of Pre-Cambrian biotite–garnet and sillimanite gneiss with some quartzite. The present paper aims at describing various degrees of rock weathering and their effect in Sangrumba landslide. Field study followed by mineralogical, geochemical and geotechnical analyses of the collected rock and soil samples from the landslide zone were used in characterizing weathering degree. The gneisses are intensely weathered while quartzite is unweathered. Petrographical and X-ray diffraction analyses showed that the rocks in the landslide zone had undergone weathering process with the formation of different types of clay minerals as kaolinite, vermiculite, smectite and chlorite. This was further confirmed by the Scan Electron Microscope and Energy Dispersive X-ray analyses. These clay minerals drastically reduced the rock strength facilitating the extensive failure of the Sangrumba landslide. The major and trace element composition of the rock and soil samples was calculated from the XRF analyses. The geochemical analyses and weathering indexes of rocks showed that they are significantly weathered and had a major influence in the formation of the Sangrumba landslide. In addition, mechanical strength measurement of rock/soil showed that the strength drastically decreases as the weathering intensity increases. Rainfall followed by the rock type are the most dominant parameters influencing the weathering process which leads to the formation of large landslide as the present one. These findings can be used in other areas with similar geological and topographical conditions.

Orbitolina-bearing limestone pebbles from the lowermost part of the Lower Yezo Group (Lower Cretaceous) in the Nakagawa area, northern Hokkaido, Japan and its significance

り下部蝦夷層群,中部蝦夷層群,上部蝦夷層群,函淵層群に 区分されている(Okada, 1983).このうち夕張山地周辺の下 部蝦夷層群中部には,“オルビトリナ石灰岩”と呼ばれる石 灰岩体が古くから知られている(矢部, 1901).この石灰岩体 は下部蝦夷層群中の異地性岩体,すなわちオリストリスであ ると解釈されている(高嶋ほか, 1997など).“オルビトリナ 石灰岩”には大型有孔虫 Orbitolina をはじめ,厚歯二枚 貝,ネリネア,造礁サンゴ,石灰藻など白亜紀当時の熱帯~ 亜熱帯地域に特有な化石群集が含まれている(橋本, 1936; 佐 野, 1995).このことから“オルビトリナ石灰岩”の分布は, 北西太平洋における白亜紀の古気候,古生物地理を考える上 で重要であると考えられる.また,“オルビトリナ石灰岩” を含むオリストストロームは単調な岩相を示す下部蝦夷層群 において鍵層として有効である.さらに Orbitolina属は, 種のレンジが短いため地層の年代決定にも用いられている. これまで“オルビトリナ石灰岩”は,夕張山地を中心に南 は じ め に 北約 30 kmという限られた範囲に知られ,その分布の北限 は北海道中央部の比布地域であった(鈴木, 1957).筆者らは 今回,それよりも約 100 km北方に位置する北海道北部中川 地域の下部蝦夷層群最下部の礫岩層から“オルビトリナ石灰 岩”に類似する石灰岩礫を多数発見した.礫岩層とその中に 含まれる石灰岩礫の産状および石灰岩礫の微岩相の観察,石 灰岩礫に含まれる大型有孔虫 Orbitolinaの分類学的検討を 行った結果,この地域の下部蝦夷層群最下部の地質年代,お よび下位の空知層群との時間間隙について新知見を得た.ま た,下部蝦夷層群堆積時における後期 Aptianの浅海域での 石灰岩の形成とそれを起源とする堆積物の深海域への供給が 広範囲にわたる現象であったことが確かめられたので,ここ に報告する.

Preliminary study for the Middle Miocene Kawabata Formation, Hobetsu district, central Hokkaido, Japan; special reference to the sedimentary system and the provenance.

北海道中央部, 穂別地域に分布する中部中新統川端層について堆積相解析を行った.その結果, 川端層はタービダイトと関連粗粒重力流堆積物からなり, 構成岩相の累重様式・水平分布は, 堆積盆東縁に発達した小規模あるいは短命のチャネルやスロープエプロンなどによる斜面基部～堆積盆底環境での形成を示唆する.また, その堆積の後期には, 側方の陸域からの供給が大きく寄与する側方流卓越型のシステムが成立していた可能性があり, 川端層の堆積に際する軸流と側方流の寄与は地域あるいは層準によって大きく異なっていたと推定される.また川端層砂岩には蛇紋岩片等の不安定な岩片や, 片麻岩起源のざくろ石などが含まれる.そのため, 川端層の供給源としては東に隣接する枝幸-日高帯南部地域があげられ, この時期にはすでに枝幸-日高帯南部が陸化していたことが示唆される.このことは従来の古地理の復元や日高帯の上昇時期について再考を促すものである.