Landslide - yamakuzure

A phenomenon in which rocks or soil that make up the slope of a mountain suddenly collapse. Cliff collapses are also a type of landslide. The most common cause is heavy rainfall, which occurs throughout Japan during the rainy season and typhoon season, causing damage to human lives, houses, forests, and cultivated land along with mudslides. They can also be caused by earthquakes and volcanic eruptions. Landslides can occur anywhere on a slope with an angle of 30 degrees or more, but there are some places where they are more likely to occur and some places where they are less likely to occur due to topography and geology. In landslides caused by heavy rainfall, infiltration groundwater flow plays an important role. The increase in moisture due to infiltration of precipitation reduces adhesion, while the generation of pore water pressure creates buoyancy, reducing vertical stress and reducing internal friction, causing landslides where the effect of gravity becomes greater than the resistance to sliding. Landslides are more likely to occur in places where groundwater collects due to topography and geology, or near fault lines or fracture zones where there is a large supply of groundwater. Easily weathered rocks such as granite and agglomerate weather from the surface to deep below, causing the degree of solidification of the rock particles to decrease. When a large amount of rainwater seeps in, a boundary is formed between the weathered layer and the fresh rock surface below, causing it to lose stability and making the upper part more susceptible to collapse. In terms of the shape of the slope, collapses often occur on concave slopes with a gradient of 35 to 45 degrees. Major earthquakes and volcanic eruptions cause them less frequently, but they can be large in scale. Unlike those caused by rain, landslides caused by earthquakes tend to occur more frequently on convex, steeper upper parts of slopes. Landslides themselves can cause disasters, and are also a source of sediment that flows into rivers, causing various effects on downstream areas.

Large-scale collapses generally become mudslides or debris flows, which are extremely fluid and can flow far (up to 10 times their height), completely changing the topography downstream and causing major damage. Although they do not occur very frequently, they are a phenomenon that deserves special attention in terms of natural disasters.

[Kazuo Ashida and Takahisa Mizuyama]

"Landslides and landslides: current status and countermeasures" by Shinichi Yamaguchi (1990, Daimeido) "Earthquakes, high tides, and landslides: an introduction to natural disasters" by Osamu Shiota (1998, Shinpusha)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

山地の斜面をつくっている岩石や土壌の一部が突発的に崩壊する現象。崖(がけ)崩れも山崩れの一種である。もっとも多い原因は豪雨であり、梅雨期や台風期に日本の各地で発生し、土石流とともに人命や家屋、山林、耕地などに被害をもたらしている。そのほか、地震や火山爆発によって発生することもある。山崩れは傾斜角がほぼ30度以上の斜面ではどこでもおこる可能性があるが、地形的、地質的におこりやすい所とおこりにくい所とがある。豪雨による山崩れでは浸透地下水流が重要な役割を演ずる。降水の浸透による水分の増加によって粘着性は減少し、一方、間隙(かんげき)水圧の発生のため浮力を生じて垂直応力を減少して内部摩擦が小さくなり、滑りに対する抵抗力より重力の作用が大きくなった所で山崩れがおこる。地形的、地質的に地下水の集水する所や、断層線や破砕帯の近くで地下水の供給の多い所におこりやすい。花崗岩(かこうがん)や集塊岩のような風化しやすい岩石は地表から深い所まで風化して岩石の粒子の固結度が小さくなり、雨水が多量に浸透すると風化層とその下の新鮮な岩石面との間に境を生じて安定を失い、その上部が崩れやすい。また、斜面の形についてみると、勾配(こうばい)が35～45度で凹形の斜面に崩壊が多く発生している。大地震や火山爆発による頻度は少ないが、規模が大きい場合がある。地震による山崩れは雨によるものと異なり、凸形でより急勾配の斜面上部に多く発生する傾向がある。山崩れはそれ自体災害の原因になるとともに、河川への流出土砂の生産源となり、下流地域へ種々の影響を与える。

　大規模崩壊は、一般に土石流あるいは岩屑(がんせつ)流となり、きわめて流動性が高く、遠く（高さの10倍程度）まで流出し、下流の地形を一変させて大災害を与えるので、頻度はそれほど高くはないが、自然災害上きわめて注目すべき現象である。

［芦田和男・水山高久］

『山口真一著『地すべり・山崩れ　実態と対策』（1990・大明堂）』▽『塩田修著『地震・高潮・山崩れ　自然災害入門』（1998・新風舎）』

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