DESCRIPTION:
Mount St. Helens, Washington,
Eruption Advisories and Statements, Monitorings and Warnings

When Mount St. Helens awoke in March 1980, there was an immediate need for the rapid dissemination of information about volcanic events and hazards. An emergency coordination center (ECC) was established at the U. S. Forest Service (USFS) facility in Vancouver, Washington. The facility was staffed 24 hours a day by USFS personnel experienced with emergency response. During critical times, the ECC was also staffed by representatives of the U. S. Geological Survey Cascades Volcano Observatory, emergency management agencies, and private companies. A communications network and telephone call-down procedures were developed to facilitate rapid dissemination of information about the activity of the volcano. Information was also disseminated through public meetings, press conferences, and briefings with governmental agencies and private businesses (Miller and others, 1981). Written "volcanic and seismic activity reports" were issued daily. These "daily updates" provided information on the status of the volcano and any significant changes or observations during the previous 24 hours.

Since 1980, this notification system has been modified in response to changes in volcanic activity, funding, and the concerns of government, business, and the public. Key changes include the capability of issuing written predictions weeks in advance of most eruptions; eliminating the need for 24-hour duty for both USFS/ECC and CVO staff except when eruptions are imminent; entering all predictions and updates into a computer "news" system for easy review by those on the call-down list; updating volcanic activity reports when the volcano is quiet; and, most recently, developing a seismic alarm to alert scientists to small events that occur without precursors.

Volcanic activity at Mount St. Helens is carefully monitored by the U. S. Geological Survey and the University of Washington. Some kinds of events, such as crater-wall avalanches or steam- driven explosions from the dome have occurred without warning in the past and may do so again. However, our experience since early 1980 at Mount St. Helens and elsewhere indicates that the monitoring is sufficient for us to detect the ascent of fresh magma that must take place before another large eruption. As in the past, interpretation of phenomena related to magma ascent will enable us to provide warnings and updated assessments of hazards.

Lahar and flood hazards are monitored by the U. S. Geological Survey and the National Weather Service; the latter agency has responsibility for providing warnings of floods, including lahars. Currently monitoring includes: (1) lake-level gages on Castle, Coldwater, and Spirit Lakes, (3) sensors to measure movement on the Castle Lake debris dam, (3) flow-vibration sensors in the North Fork Toutle River valley to detect passage of lahars or floods, and (4) streamflow gages in the North Fork, South Fork, and main channel of the Toutle River and in the Muddy River.

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:

• 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.

• 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.

• 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.

The goal of the monitoring and research activities of CVO is to give timely warnings of eruptive and related hydrologic hazards at Mount St. Helens. Three types of written public statements about volcanic activity are issued by CVO to provide hazard information to the public and to government agencies:

• A factual statement describes current conditions of the volcano but does not anticipate future events; such statements are revised when warranted by new developments.

• A forecast is a comparatively nonspecific statement about volcanic activity to occur weeks to decades in advance. A forecast is based on projections of past eruptive activity or is used when monitoring data are not well understood. This kind of statement is particularly useful for land use planning and development of emergency response plans.

• A prediction is a comparatively specific statement giving place, time, nature, and, ideally, size of an impending eruption. Correct predictions were made of all 14 eruptions at Mount St. Helens from June 1980 to the end of 1982.

Water-related hazard information is provided to the National Weather Service and other Federal, State, and local agencies involved in flood mitigation planning.

A primary goal of the Volcano Hazards Program is forecasting and predicting eruptions. Several increasingly specific and useful steps lead toward prediction Initially, when little is known about the past history and preeruption behavior of a volcano, we may only be able to give factual information about current unrest; for example, that swarms of small earthquakes are occurring beneath the volcano, similar to those which have preceded eruptions elsewhere.

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.

The eruptive activity of Mount St. Helens has provided a good test for scientists who faced the challenge of obtaining, relaying, and explaining in easily understandable terms the information needed by the Federal, State, and local officials charged with land management and public safety. It should be reemphasized, however, that a quick response at Mount St. Helens was possible only because decades of systematic research before 1980 had contributed to a good understanding of the volcano's eruptive behavior and potential hazards. ...

Throughout the 1980s, the ability of scientists at CVO and the University of Washington to provide warnings for dome-building eruptive episodes has been exceptional. Indeed, for all episodes (except for one small event) since May 1980, scientists- using data from seismic, ground deformation, and volcanic gas monitoring-have provided reliable forecasts from several hours to several days, even weeks, in advance of these events. The table gives a typical example of the timely information for one 1982 eruption given to government officials charged with emergency management and to the general public via news releases.

August 18-23, 1982, Eruption of Mount St. Helens
Type of Notice and When Issued	Excerpt
Extended Outlook Advisory: 1 p.m. July 30	"an eruption will probably begin within the next 3 weeks." "...the eruption will consist primarily of dome growth."
Advisory Update: 11:30 a.m., August 16	"eruption will begin within the next 4 days, possibly within 2 days ... the eruption will consist primarily of dome growth, but as with all dome growth, minor explosive activity is also possible."
Eruption Alert: 6:55 a.m., August 17	"Seismicity and rates of deformation in the crater have accelerated sharply ... the expected eruption will probably begin within the next 24 hours."
Updated Eruption Alert: 7:45 a.m. August 18	"The dome is already growing internally, but we have not seen any discrete event yet, for example, an explosion, a change in the character of seismicity or deformation ... or gas emissions, that in other eruptions has signaled the onset of ... eruptions. We still expect lava to eventually work its way through the dome and to be extruded as a new lobe on the surface of the dome."
Eruption Update: 7:15 p.m. August 18	"Lava finally broke through to the top of the dome this morning, and a new lobe is flowing slowly onto the western and southern sides of the dome."
End-of-eruption Advisory: 8:45 p.m., August 23	"Deformation and gas emissions have returned to their background levels, so this eruption is essentially over. Minor sagging and spreading of the new lobe may continue for a few days, accompanied by occasional rockfalls and dust plumes."

At Mount St. Helens, the track record for predicting eruptions, especially dome-building ones, is better than any previously accomplished for any volcano in the world. Our improving predictive ability, however, has not been tested by any large explosive eruptions.

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