Landslide or Debris Flow (Earthquake Trigger)

Primary reference(s)

Varnes, D.J., 1978. Slope movement types and processes. In: Schuster, R.L. and R.J. Krizek (eds), Landslides, Analysis and Control. Special report 176: Transportation research board, National Academy of Sciences, Washington, DC. pp. 11-33.

Additional scientific description

A landslide is the movement of a mass of rock, debris, or earth down a slope; a type of ‘mass wasting’, which denotes any down-slope movement of soil and rock under the direct influence of gravity. The term ‘landslide’ encompasses five modes of slope movement: falls, topples, slides, spreads, and flows. These are subdivided according to the type of geologic material (bedrock, debris, or earth). Slope movement occurs when forces acting down-slope (mainly due to gravity) exceed the strength of the earth materials that compose the slope (Varnes, 1978).

Earthquake triggered landslides typically affect steep slopes and slopes underlain by sediments that are prone to liquefaction. Rock falls are the most abundant landslides in seismic events and occur in virtually all types of rocks on slopes steeper than 40° (Keefer, 1984). The behaviour of material on hillsides is highly dependent on the amplitudes of seismic waves that reach them, and this will vary with the epicentre distance and depth, as well as the magnitude (M) of an earthquake. Keefer (1984) from a study of historic earthquakes showed that the maximum area likely to be affected by landslides in a seismic event ranges from 0 km2 at M=4 to 500,000 km2 at M=9.2. Materials most susceptible to earthquake-induced landslide were found to include weakly cemented rocks, more indurated rocks with pervasive discontinuities, residual and colluvial sand, volcanic soils with sensitive clays (e.g., Iburi–Tobu earthquake, Hokkaido; Kameda et al., 2019), loess, alluvium and deltaic deposits. First-time slides were more common than landslide reactivation. Rock falls, rockslides, soil falls and disrupted soil slides were initiated by weak shaking; coherent deeper-seated landslides required stronger shaking; lateral spreads and flows required even stronger shaking, and rock and soil avalanches required the strongest shaking (Keefer, 1984).

Within a given region, it is possible to discriminate, earthquake-triggered landslides from landslides initiated by other triggering processes. For example, Lee (2012) reported that earthquake-induced landslides in Taiwan are mostly located on steeper, longer slopes and at a higher position of the slope when compared to storm-induced shallow landslides, suggesting that topographic amplification plays an important role in earthquake-induced landslides. In hard rock terrains, earthquakes trigger a higher proportion of rock fall landslides. Zhang et al. (2014) compared earthquake-triggered landslides with rainfall-triggered landslides in the Wenchuan area of China and found that the earthquake landslides were steeper, larger landslides dominated in areas underlain by harder rocks compared with areas underlain by alluvium. In contrast, the rainfall-induced landslides were characterised by a greater volume of channelled deposits and were of a higher density but smaller area and were characterised by debris slides and debris flows. In areas that are underlain by weak rocks that are saturated, strong earthquake-induced ground shaking will result in more landslides than normal (Fan et al., 2019).

Earthquake shaking and other factors can also induce landslides underwater. These are called submarine landslides. Submarine landslides sometimes cause tsunamis that damage coastal areas (Hungr et al., 2014).

Metrics and numeric limits

Landslide movement is likely to range from moderate in velocity (1.5 metres per day) to extremely rapid. With increased velocity, the landslide mass of translational failures may disintegrate and develop into a debris flow (Varnes, 1978).

Examples of drivers, outcomes and risk management

Landslides can be extremely destructive, especially when failure is large, sudden and (or) the velocity is rapid.

Rapid soil flows, rock avalanches, and rock falls together caused more than 90% of the reported landslide deaths in the 40 historical earthquakes reported on by Keefer (1984). Rock avalanches and rapid soil flows, the two leading causes of death, are relatively uncommon, high velocity landslides that occur on slopes of a few degrees. Most deaths caused by these landslides were due to burial of cities or villages located on gently sloping ground several kilometres from the sites of landslide initiation. All but one death caused by soil slumps, block slides, or lateral spreads were due to disruption of foundations and subsequent collapse of buildings, most likely related to liquefaction. Aftershocks can be a significant trigger for further earthquake-induced landslides as reported by Liang and Zhou (2016) for the Gorkha earthquake, Nepal in 2015.

Earthquake triggered landslide impacts can cascade to dam rivers and impound lakes, which can collapse days to centuries later. They can cause extensive mountain valley flooding and leave a geomorphology that may be prone to remobilisation during heavy rainfall, potentially evolving as debris flows. Cracks and fractures can form and widen on mountain crests and flanks, conditioning the landscape for an increased frequency of landslides that lasts for decades. Increased debris load delivery to rivers can cause bank erosion and floodplain accretion as well as stream channel switching that affect flooding frequency, settlements, ecosystems, and infrastructure (Fan et al., 2019).

Instrumental monitoring to detect movement and the rate of movement can be implemented, for example, extensometers, global positioning system (GPS), seismometers, aerial photography, satellite images, LiDaR (Highland and Bobrowsky, 2008) with varying degrees of success.

While the physical damage of landslides is well documented, health impacts are complex. The risk of an increase in infectious diseases is of concern during the response and recovery phase after any major disaster. Displacement of people due to the destruction of their homes and other infrastructure can place them in unfamiliar surroundings which, if they conflict with traditional beliefs and practices with regard to water supply and hygiene, can result in unsafe behaviours. The medium- to long-term effects of changes to the environment caused by landslides, such as deforestation, and changes to river courses, can increase the risk of vector-borne diseases, and as a result, the health impacts can extend long after the initial disaster is over. Disruption of soil can also increase exposure to infectious organisms (Kennedy et al., 2015). The psychosocial and mental health impacts on survivors and rescue personnel from landslides are increasingly recorded. The prevalence of psychiatric disorders and wider support needed to reduce misuse of substances has been identified (Kennedy et al., 2015; Dell’Aringa et al., 2018). Landslides commonly occur in poor countries with steep terrain, such as the southern edge of the Himalayan arc. Increasingly, the science of landslide physics is allowing the nature of these hazards to be understood, which is leading to better techniques through which they can be managed and mitigated.