Mud Slides

Reuters reported on July 10, 1997

HARIHARA, Japan - Tons of mud and rocks swept over this sleeping village in southern Japan Thursday, killing 19 people when the side of a mountain loosened by recent earthquakes collapsed in torrential rain.

Excerpts from Physical Geology, Exploring the Earth, by James S. Monroe
: All slopes are in a dynamic state of equilibrium, which means that they constantly adjust in response to new conditions. ... Many factors cause mass wasting: slope gradient, weakening of material by weathering, increased water content, changes in vegetation cover, and overloading. ...; Slope gradient is probably the major cause of mass wasting. Generally speaking, the steeper the slope, the less stable it is. ...; Mass wasting is more likely to occur in loose or poorly consolidated slope material than in solid bedrock. ...; Large quantities of water from melting snow or heavy storms greatly increase the likelihood of slope failure. The additional weight that water adds to a slope can be enough to cause mass movement. Furthermore, water percolating through a slope's material helps to decrease friction between grains, contributing to a loss of cohesion. For example, slopes composed of dray clay are usually quite stable, but when wetted, they quickly lose cohesiveness and internal friction and become an unstable slurry. This occurs because clay, which can hold large quantities of water, consists of platy particles that easily slide over each other when wet. ...; By absorbing the water from a rainstorm, vegetation decreases water saturation of a slope's material ... The removal of vegetation by either natural or human activity is a major cause of many mass movements. ...; Overloading is almost always the result of human activity and typically results from dumping, filling, or pilling up of material. ...; If the rocks underlying a slope dip in the same direction as the slope, mass wasting is more likely to occur than if the rocks are horizontal or dip in the opposite direction. When the rocks dip in the same direction as the slope, water can percolate along the various bedding planes and decrease the cohesiveness and friction between adjacent rock units. This is particularly true when there are interbedded clay layers because clay becomes very slippery when wet. ...; The most common triggering mechanisms are strong vibrations from earthquakes and excessive amount of water from a winter snow melt of a heavy rainstorm. Earthquakes are the most common type of strong vibrations and thus trigger many mass movements. In many cases, the resulting landslide causes far more damage and poses a greater threat to people than the earthquake itself.