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The terms forecast and prediction are often used inter-changeable,
and both confused with simple factual statements regarding past occurrence of
eruptions. Herein, the following distinctions are made:
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A factual statement describes current conditions but does not anticipate
future events. Mauna Loa erupted 35 times in the past 200 years, and the last
large debris flow from Mount Rainier was 500 years ago are two examples of
factual statements. The accumulation and analysis of such factual information
form the basis for forecasts and predictions.
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A forecast is a comparatively imprecise statement of the time,
place, and ideally, the nature and size of impending activity. For example,
Mauna Loa is likely to erupt at its summit within the next five years, as
determined from the rate of reinflation and historical eruptive patterns.
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A prediction is a relatively precise statement giving the time and
place of eruptions and measurements of ongoing monitoring results and only
secondarily on a projection of past history. For example, on the basis of
accelerated movement of thrust faults on the crater floor, the next eruption of
Mount St. Helens will occur in the summit crater between March 17 and March 20
and will be a dome-building event.
Volcanologists strive to make accurate predictions, although most often a
forecast is the most reliable statement that can be made, given the available
data and technology. In areas already developed or proposed for development,
all three types of information can be used both for land-use planning and as a
basis for developing procedures to ensure public safety in anticipation of a
volcanic eruption.
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