The Challenge of Predicting Eruptions

When the average repose period and other information regarding a particular volcano's eruptions, for example, when the amount of inflation preceding the previous eruption is matched by current conditions at that volcano, a general forecast can be made that the volcano is "ready" to erupt. The start of microearthquakes or other common eruption precursors would lead to an updated forecast -- that the volcano may erupt soon.

In the past, forecasts of eruptions were based solely on recurring patterns of unrest before eruptions. The occurrence of one particularly diagnostic type of unrest, for example, volcanic tremor, might be the basis for a prediction that the volcano would erupt within a specified number of hours or days. The appearance of other known eruption precursors helped narrow the time window and lent certainty to the prediction.

We now recognize the need to understand why particular patterns and events occur before some eruptions, and this need requires a thorough physical understanding of the volcano's internal plumbing and the processes associated with the generation, transport, storage, and ultimately, eruption of magma. For example, a combination of seismic and geodetic data demonstrates the existence of a complex magma reservoir 2 to 6 kilometers beneath Kilauea's summit from which all eruptions on the volcano ultimately originate. Earthquake foci outline the area of magma storage, whereas horizontal, vertical, and tilt changes above the reservoir define the depth to "centers" of inflation (swelling) or deflation. Understanding of this storage system has greatly improved the ability to determine when Kilauea is fully inflated and ready to erupt. Accurate short-term (within days to weeks) prediction of Hawaiian eruptions remains elusive, as both Kilauea and Mauna Loa may reach a highly inflated state, and wait with no further ground deformation or increase in seismicity until eruptions occurs. some Kilauea rift eruptions are preceded within hours by a strong earthquake swarm whose foci migrate toward the point of outbreak, giving a short but accurate prediction of this type of activity. Volcano monitoring, combined with study of Kilauea's volcanic history, yields the information necessary for long-term eruptions forecasts.

As with Hawaiian eruptions, the dome-building eruptions of Mount St. Helens are not predictable many months ahead. Prediction of dome-building eruptions were made, however, within days or weeks, using very simple methods, with relatively little prior knowledge or understanding of the volcano's plumbing system.

Accurate predictions are still rare in volcanology, and probabilities associated with eruption from a given volcanic system may change after an eruption takes place. Often volcanic systems are in delicate balance and may be considered "ready" to erupt; this determination of readiness allows a medium-range forecast of increased likelihood of eruption. For many currently dormant but potentially active volcanoes, we may only be able to give factual information regarding past activity without specifying what the future holds. For well-studied, historically active volcanoes we can make more specific forecasts of future activity. The most accurate predictions are in the short-term where either rapid ground movements or an earthquake swarm directly precedes eruption at the surface.