| LANDSLIDES triggered by earthquakes | ||||||||||||||||||||
| Definition: What are landslides? | Causes of landslides | |||||||||||||||||||
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A landslide is a general term given to describe the various forms of mass movement. Many definitions have been applied to the term and they all vary depending on the objective of the author. For example, Cruden (1991)defined a landslide as, "a movement of a mass of rock, earth or debris down a slope" However, this rather vague interpretation was confronted by a more detailed approach from Hutchinson (1988). He suggested that slope movements were categorised into individual groups based on the mechanism of failure. Hutchinson classed them as: fall, topple, i)rotational and ii)translational slides, lateral spreading, flow and complex. The common features of landslides according to the above criteria, are seen as slope failures, mudflows, rock falls and rock slides to name a few. |
Landslides are not individual events. they often occur in conjunction with at least one other contributary factor. There are a countless number of factors involved in the landslide process and in respect, landslides can be the cause of other secondary effects. Some triggering factors of landslides are due to climatic, tectonic and human reasons. In the event of a rainstorm in troical regions, heavy rainfall can induce slope failure and snowmelt in cold climates can also cause landslides. Where human activities are concerned in the nature of mining and quarrying, the surface material and underlying bedrock can get disturbed by sudden shaking and vibrations. This impact on the crustal level can be further explained by the soil type. Obviously, different soil or rock types respond in distinct ways to the amount of movement it is subjected to. The particle bonding dictates the overall strength or weakness of the rock and its ability to withstand external impacts. In a similar context the water content of a rock or soil is important in identifying the cause of a landslide hazard. This is true for clay in particular, which according to Dikau et al (1996), has a 30-40% water content that builds on the stress loading and oversteepening of the slope. The raised water table after a rainstorm would saturate the soil rendering it weak and therefore the slope failure would naturally occur due to gravitational forces. Subsequently, a process known as liquifaction is liable to take place such as in the case of the Alaskan earthquake-induced landslide of 1964. The process of liquefaction triggered by the ground shaking effects of an earthquake causes the soil particles to break apart and sink. The water contained is displaced and rises to the surface. The result is that buildings and infrastructure could be swallowed into the ground by several meteres in depth. The Alaskan earthquake illustrates the fact that seismicity is a major landslide trigger. It has been historically noted that many landslides occurred as the result of a major earthquake. Earthquakes destabilise slopes by shear stress thereby weakening the structure of slope material. This kind of seismic hazard can lead to not only immediate destruction but also the long-term effects have been studied by geologists and earth scientists alike. | |||||||||||||||||||
| Past landslides - Case studies and findings | ||||||||||||||||||||
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The main point in Keefer's work was that major slope erosion (especially in Hawaii) was primarily due to earthquake induced landslides. This was evident in the large volumes of landslide material dislodged by past earthquakes. Adams (1980) produced studies along the same basis, focusing on New Zealand and New Guinea. His interests in the fluvial sediment discharge proved conclusively that 40-67% of material eroded from the location had derived from earthquake-induced landslides. In one of Crozier's (1986,1991) vast range of studies, his team conducted an extensive research into the prehistoric landslide records for south Taranaki, New Zealand. It was specifically aimed at investigating the triggering mechanisms of landslides. Using modern mapping and dating techniques, Crozier et al discovered the great significance of a seismic event in light of the deep-seated landslides. Justified by the existence of faults at the site, the role of earthquake originated landslides outplayed proposal of rainstorms being the dominant factor. With attention to the final sample of published articles, it draws consideration to the notion of a massive landslide on an intracontinental scale. In comparison to the previous works mentioned, this most recent analysis comes from Herve & Ritz(1999). Their observations of the Bogd fault patterns in the mountainous region of Mongolia established a huge soil block slide in 1957. An earthquake of M8.3 was responsible for the ruptured strike-slip fault which showed signs of surface fracturing. Together with the simultaneous input of the wet climate the 50km3 paleolandslide was agreed to be a strong indicator that active tectonics are the most influential processes in the representation of landslides. REFERENCES CITED; Crozier, Michael J (ed) 1986 Landslides: causes, consequences & environment Croom Helm, London Crozier,MJ, Deimel,MS and Simon,JS Investigation of earthquake triggering for deep-seated landslides, Taranaki, New zealand Quaternary International, vol 25 pp65-73 1995 Dikau, Richard et al (eds) 1997 Landslide recognition: identification, movement and causes John Wiley & Sons, Chichester Keefer, David The importance of earthquake-induced landslides to long-term slope erosion and slope-failure hazards in seismically active regions Geomorphology 10 (1994) pp265-284 Philip, Herve & Ritz, Jean-Francois Gigantic paleolandslide associated with active faulting along the Bogd fault (Gobi-Altay, Mongolia) Geology, March 1999 v27 no3 211-214 Strecker, Manfred & Marrett, Randall Kinematic evolution of fault ramps and its role in development of landslides and lakes in the northwestern Argentine Andes Geology, April 1999 v27 no4 pp307-310 Web page created by: Jennifer Cheng Last updated November 1999 | ||||||||||||||||||||
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