The deaths of at least eight people in a massive landslide in Washington State have focussed attention upon this natural hazard.
Several people are still missing, and there’s little hope of survivors. The death toll from the slide, which destroyed around 30 homes near the town of Oso, is likely to rise.
So what causes landslides and how big a hazard are they?
What is a Landslide?
The British Geological Survey defines a landslide as “a mass movement of material, such as rock, earth or debris, down the slope of a hill or cliff. They can happen suddenly or move slowly over long periods of time”.
This is a broad definition; there are many factors influencing the type and nature of landslides and different authorities classify them in slightly different ways, although they essentially cover the same thing. The United States Geological Survey identifies five categories, defined according to the type of movement.
These are: falls; topples (both self-explanatory); slides (distinguished by the movement of one unit of rock over another); flows (which, as the name suggests, have a high water content) and lateral spreads (broadly similar to flows but more prevalent on gentler slopes).
This is a simplification as the USGS also recognises different categories of slide and flow; and the situation is further complicated as more than one type of movement may be present in the same event.
What Causes a Landslide?
The key factor for a landslip to occur is that a slope, whether steep or shallow, is unstable. Many geological factors (such as the type of rock, grain size and steepness of the slope) influence the susceptibility of a particular location to a landslide. When the gravitational force reaches a certain threshold (which varies by location, rock type and so on) the slope fails and a landslide occurs. This may be sudden or it may be slow.
Although there are very many possible causes which may act singly or together upon a slope, there are key events which may trigger landslides. Again, these are many and varied but the major causes of landslides include: volcanic or earthquake activity; heavy rain; isostatic rebound (melting of glacial ice which causes land to rise); and human activity such as mining.
Landslides can occur underwater as well as on land, and can even cause tsunamis. A notable example is the M7.2 Grand Banks earthquake of 1929, which triggered a landslide. This, in its turn, generated a tsunami which cost 28 lives in Newfoundland.
Landslides, with their many causes and their geographical spread, are a worldwide hazard, in contrast to such events as earthquakes or volcanoes which are largely restricted to particular locations. Landslides are, however, more likely to occur in some regions than in others.
According to the USGS, in the United States these areas are “the coastal and mountainous areas of California, Oregon, and Washington, the States comprising the intermountain west, and the mountainous
and hilly regions of the Eastern United States.”
Landslides can move vast quantities of rock. There’s no definitive list of the largest to occur, but the BBC suggests that the landslide which signalled the beginning of the Mount St Helen’s eruption in 1980 may have been the largest in recorded history, moving an estimated 3 cubic km of rock in just a few seconds.
Geological excavations show that volumes of rock many times larger can move downslope. The BGS reports that an estimated 50 cubic km of material was moved in an earthquake-triggered landslide in Iran over 10,000 years ago but even this is dwarfed by the submarine Storegga slide off Norway’s continental shelf around 1,500 years later. This may have moved 3,300 cubic km over an area the size of Scotland.
Washington Landslide: a Recurrent Hazard
The evidence points to recent heavy rain in an area rendered vulnerable by steep slopes as the main trigger for Washington State’s deadly landslide. Events such as this occur worldwide and cost many lives (a study reported in Nature suggests that “between 2004 and 2010, 2,620 fatal landslides killed a total of 32,322 people.”
Bearing this in mind, landslides must be considered a significant hazard and a major threat to human life.