Earthquake Hazards: Landslides and rockfalls

The obvious hazard of an earthquake is the shaking of the ground. I'm not aware of anyone being killed directly by the ground shaking. The exception might be when the ground opens up. There have probably been some indirect deaths of the earth shaking if someone died from a heart attack.

Most of the injuries and deaths are the result how the ground shaking affects other things. These are the earthquake-related hazards. Shaking of the earth shakes buildings. This is the obvious earthquake hazard.

Related to the collapse of man-made structures is the threat of the destabilization of the ground. One of these hazards is liquefaction. Other earthquake-related hazard caused by ground destabilization, which many people don't consider but which becomes obvious when mentioned, are landslides and rockfalls.

While landslides and rockfalls are similar, rockfalls are primarily when only rocks (which can small or large) are what come down. Landslides are the downward sliding of a mass of earth, debris, and even rocks. Generally landslides are fairly dry, meaning they aren't mud which would be a mudslide.

The shaking of the earth can easily destabilize cliffs and steep slopes. The susceptibility to landslides and rockslides is dependent on a variety of factors. Proximity to the epicenter certainly has an effect. How water-saturated the ground is, such as from rain or snowmelt, can further weaken slopes. Additionally, lots of loose, unconsolidated, or fractured rocks can make the problem worse.

Here's something to be aware of. Landslides don't just happen close to the earthquake's epicenter.

On 23 August 2011, the largest earthquake to strike the eastern U.S. since 1897 struck Virginia. At a magnitude 5.8 it wasn't really a "large" earthquake, just the biggest one to shake the area in over 100 years. However, a study determined that earthquake triggered landslides "at distances four times greater and over an area 20 times larger than previously documented for M-5.8 earthquakes worldwide" (2011 Virginia quake triggered landslides at extraordinary distances).

Previous studies put the maximum distance that a 5.8 quake could cause at about 37 miles (60 km). With the 2011 earthquake that maximum distance was about 152 miles (245 km). Area-wise the previous "maximum" was 579 square miles (1,500 square km). The new maximum (for the same size of earthquake) is 12,895 square miles (33,400 square km). Extraordinary distance limits of landslides triggered by the 2011 Mineral, Virginia, earthquake

Besides the possibility of rocks falling, or the side of hill sliding down, and causing damage, injury, or death, tsunami and seiches can be generated by landslides falling into a body of water.

Landslides and rockfalls can also cause a river to dam up, flooding the area upstream of the obstruction and then creating more flooding hazards if the unstable dam fails.

Earthquake Lake (also known as Quake Lake) in Yellowstone was created from the 1959 Hebgen Lake earthquake. This magnitude 7.5 rupture caused a landslide that blocked most of the flow of the Madison River. Within about a month, the river had filled an area six miles long and up to 190 feet (57 m) deep. Because there wasn’t a natural outlet for the water, the U.S. Army Corp of Engineers constructed a spillway to help minimize the threat of the water breaching the top of the landslide and potentially causing the failure of the natural dam.

State Highway 287 slumped into Hebgen Lake from earthquake in 1959. USGS photo


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