Science

Felt earthquakes on Mount Hood (Oregon) occur every two years on average. Seismic monitoring , in effect since 1977, indicates a generalized concentration of earthquakes just south of the summit area and 2-7 kilometers below sea level. A seismic swarm in July 1980, during which nearly 60 earthquakes (mostly 5-6 kilometers deep with a maximum bodywave magnitude of 2.8) were recorded in a 5-day...

Debris flows ( lahars ) pose the greatest hazard to people near Mount Rainier . A debris flow is a mixture of mud and rock debris that looks and behaves like flowing concrete. Giant debris flows sometimes develop when large masses of weak, water-saturated rock slide from the volcano's flanks. Many of these debris flows cannot be predicted and may even occur independently of a volcanic eruption...

Eruptions of Mount Rainier usually produce much less volcanic ash than do eruptions at Mount St. Helens . However, owing to the volcano's great height and widespread cover of snow and glacier ice, eruption triggered debris flows ( lahars ) at Mount Rainier are likely to be much larger--and will travel a greater distance--than those at Mount St. Helens in 1980. Furthermore, areas at risk from...