Cascade Range Volcanoes -
Ash Accumulation of 10cm or More
-- Excerpt from:
W.E. Scott, R.M. Iverson, J.W. Vallance, and W. Hildreth, 1995,
Volcano Hazards in the Mount Adams Region, Washington:
USGS Open-File Report 95-492
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The most serious tephra hazards in the region
are due to Mount St. Helens,
the most prolific producer of tephra in the Cascades
during the past few thousand years. This map
provides estimates of the annual probability of
tephra fall affecting the region,
based on the combined likelihood of
tephra-producing eruptions occurring at
Cascade volcanoes, the relationship between
thickness of a tephra-fall deposit and distance
from its source vent, and regional wind patterns.
Probability zones extend farther east of the
range because winds blow from westerly directions
most of the time. The map shows probabilities for a fall of
10 centimeters (about 4 inches) or greater.
Northern Skamania County has an annual
probability of a tephra fall of 10 centimeters or more of about 1 in
100 (1%) to 1 in 500 (0.2%).
Even though Mount Adams is a meager tephra producer, the region
around Mount Adams has the highest probability of
tephra fall of anywhere in the western conterminous United States,
owing to its location just downwind of Mount St. Helens.
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