1. If landslides occur at one place, the failed mass does not move again and therefore can be assumed safe.
False: The failed mass or an old landslide may be re-activated or triggered in the future. For example, at Shiveluch Volcano (Kamchatka, Russia) large-scale failures have occurred at least eight times in its history: approximately 10,000, 5700, 3700, 2600, 1600, 1000, 600 C-14 BP and 1964 AD. (Belousov, Belousova and Voight, 1999)
2. Soil and rock tests that alone can show us whether the slope is safe.
False: These tests only give us the properties of these materials, such as cohesion and friction. Additional information such as environment, seismicity, and internal magma pressure also need to be assessed.
3. Engineering solutions can stop volcanic landslides from occurring.
False: Because volcanoes erupt and produce extreme force, landslides cannot be stopped by normal engineering solutions such as those used in shallow, non-volcanic landslides. See more about engineering volcanoes here.
4. Scientists can accurately predict volcanic collapses.
False: Volcanic collapses are very unpredictable due to unknowns associated with magmatic systems and seismicity.
5. Erosion will not cause slope failure.
False: Erosion can cause situations such as the undercutting of volcanic slopes. In other words, erosion at the base of a slope will cause the foundation that the volcano is on to be unstable, hence increasing the chances of collapse.
6. Long-standing slopes should be considered safe since they haven’t failed yet.
False: Although the slope may be safe in normal conditions (under gravity alone), external triggering mechanisms such as earthquakes or magma injection in conduits within the volcanic edifice can cause slopes to fail.
7. There is no evidence associated with slope failure that scientists can monitor.
False: Although slope failures are difficult to predict, there are hints that can be used to alert people in high-risk areas. These include (from http://www.tulane.edu/~sanelson/geol204/slopestability.htm):
- Springs, seeps, or saturated ground in areas that have not typically been wet before.
- New cracks or unusual bulges in the ground, street pavements or sidewalks.
- Soil moving away from foundations.
- Broken water lines and other underground utilities.
- Leaning telephone poles, trees, retaining walls or fences
- Offset fence lines.
- Sunken or down-dropped road beds.
- Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content).
- Sudden decrease in creek water levels though rain is still falling or just recently stopped.
- A faint rumbling sound that increases in volume is noticeable as the landslide nears.
- Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris. (from USGS Landslide Hazards – http://landslides.usgs.gov/learning/prepare/)
- Volcanic activity or nearby seismicity.