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USGS1983

Predicting eruptions at Mount St. Helens, June 1980 through December 1982

Thirteen eruptions of Mount St. Helens between June 1980 and December 1982 were predicted tens of minutes to, more generally, a few hours in advance. The last seven of these eruptions, starting with that of mid-April 1981, were predicted between 3 days and 3 weeks in advance. Precursory seismicity, deformation of the crater floor and the lava dome, and, to a lesser extent, gas emissions provided telltale evidence of forthcoming eruptions. The newly developed capability for prediction reduced risk to life and property and influenced land-use decisions.

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USGS1983

Seismic precursors to the Mount St. Helens eruptions in 1981 and 1982

Six categories of seismic events are recognized on the seismograms from stations in the vicinity of Mount St. Helens. Two types of high-frequency earthquakes occur near the volcano and under the volcano at depths of more than 4 kilometers. Medium- and low-frequency earthquakes occur at shallow depths (less than 3 kilometers) within the volcano and increase in number and size before eruptions. Temporal changes in the energy release of the low-frequency earthquakes have been used in predicting all the eruptions since October 1980. During and after eruptions, two types of low-frequency emergent surface events occur, including rockfalls and steam or gas bursts from the lava dome.

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USGS1983

Deformation monitoring at Mount St. Helens in 1981 and 1982

For several weeks before each eruption of Mount St. Helens in 1981 and 1982, viscous magma rising in the feeder conduit inflated the lava dome and shoved the crater floor laterally against the immobile crater walls, producing ground cracks and thrust faults. The rates of deformation accelerated before eruptions, and thus it was possible to predict eruptions 3 to 19 days in advance. Lack of deformation outside the crater showed that intrusion of magma during 1981 and 1982 was not voluminous.

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USGS1983

Eruption prediction aided by electronic tiltmeter data at Mount St. Helens

Telemetry from electronic tiltmeters in the crater at Mount St. Helens contributed to accurate predictions of all six effusive eruptions from June 1981 to August 1982. Tilting of the crater floor began several weeks before each eruption, accelerated sharply for several days, and then abruptly changed direction a few minutes to days before extrusion began. Each episode of uplift was caused by the intrusion of magma into the lava dome from a shallow source, causing the dome to inflate and eventually rupture. Release of magma pressure and increased surface loading by magma added to the dome combined to cause subsidence just prior to extrusion.

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USGS1983

Gas emissions and the eruptions of Mount St. Helens through 1982

The monitoring of gas emissions from Mount St. Helens includes daily airborne measurements of sulfur dioxide in the volcanic plume and monthly sampling of gases from crater fumaroles. The composition of the fumarolic gases has changed slightly since 1980: the water content increased from 90 to 98 percent, and the carbon dioxide concentrations decreased from about 10 to 1 percent. The emission rates of sulfur dioxide and carbon dioxide were at their peak during July and August 1980, decreased rapidly in late 1980, and have remained low and decreased slightly through 1981 and 1982. These patterns suggest steady outgassing of a single batch of magma (with a volume of not less than 0.3 cubic kilometer) to which no significant new magma has been added since mid-1980. The gas data were useful in predicting eruptions in August 1980 and June 1981.

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USGS1983

Petrologic monitoring of 1981 and 1982 eruptive products from Mount St. Helens

New material from the dacite lava dome of Mount St. Helens, collected soon after the start of each successive extrusion, is subjected to rapid chemical and petrologic analysis. The crystallinity of the dacite lava produced in 1981 and 1982 is 38 to 42 percent, about 10 percent higher than for products of the explosive 1980 eruptions. This increase in crystallinity accompanies a decrease in the ratio of hornblende to hornblende plus orthopyroxene, which suggests that the volatile-rich, crystal-poor material explosively erupted in 1980 came from the top of a zoned magma chamber and that a lower, volatile-poor and crystal-rich region is now being tapped. The major-element chemistry of the dacite lava has remained essentially constant (62 to 63 percent silica) since August 1980, ending a trend of decreasing silica seen in the products of the explosive eruptions of May through August 1980.

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USGS1983

Deep earthquakes beneath Mount St. Helens: Evidence for magmatic gas transport?

Small-magnitude earthquakes began beneath Mount St. Helens 40 days before the eruption of 20 March 1982. Unlike earlier preeruption seismicity for this volcano, which had been limited to shallow events (less than 3 kilometers), many of these earthquakes were deep (between 5 and 11 kilometers). The location of these preeruptive events at such depth indicates that a larger volume of the volcanic system was affected prior to the 20 March eruption than prior to any of the earlier dome-building eruptions. The depth-time relation between the deep earthquakes and the explosive onset of the eruption is compatible with the upward migration of magmatic gas released from a separate deep reservoir.

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USGS1983

Eruption-triggered avalanche, flood, and lahar at Mount St. Helens - Effects of winter snowpack

An explosive eruption of Mount St. Helens on 19 March 1982 had substantial impact beyond the vent because hot eruption products interacted with a thick snowpack. A blast of hot pumice, dome rocks, and gas dislodged crater-wall snow that avalanched through the crater and down the north flank. Snow in the crater swiftly melted to form a transient lake, from which a destructive flood and lahar swept down the north flank and the North Fork Toutle River.

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USGS1983

Research note: Isolation of a herpesvirus from a bald eagle nestling

Cloacal swabs collected from wild bald eagle nestlings ( Haliaeetus leucocephalus ) were tested for viruses. A virus isolated from one of these samples had a lipid coat and contained DNA. Electron microscopy confirmed that it was a herpesvirus. This appears to be the first report of a herpesvirus isolation from a wild bald eagle.

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USGS1983

Historical changes to Lake Washington and route of the Lake Washington Ship Canal, King County, Washington

Lake Washington, in the midst of the greater Seattle metropolitan area of the Puget Sound region (fig. 1), is an exceptional commercial, recreational, and esthetic resource for the region . In the past 130 years, Lake Washington has been changed from a " wild " lake in a wilderness setting to a regulated lake surrounded by a growing metropolis--a transformation that provides an unusual opportunity to study changes to a lake's shoreline and hydrologic characteristics -resulting from urbanization.

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USGS1983

Estimation of nonpoint sources of phosphorus for lakes in the Puget Sound region, Washington

Control of eutrophication of lakes in watersheds undergoing development is facilitated by estimates of the amounts of phosphorus (P) that reach the lakes from areas under various types of land use. Using a mass-balance model, the writer calculated P loadings from present-day P concentrations measured in lake water and from other easily measured physical characteristics in a total of 28 lakes in drainage basins that contain only forest and residential land. The loadings from background sources (forest-land drainage and bulk precipitation) to each of the lakes were estimated by methods developed in a previous study. Differences between estimated present-day P loadings and loadings from background sources were attributed to changes in land use. The mean increase in annual P yield resulting from conversion of forest to residential land use was 7 kilograms per square kilometer, not including septic-tank system contributions. Calculated loadings from septic systems were found to correlate best with the number of near-shore dwellings around each lake in 1940. The regression equation expressing this relationship explained 36 percent of the sample variance. There was no significant correlation between estimated septic-tank system P loadings and numbers of dwellings present in 1960 or 1970. The evidence indicates that older systems might contribute more phosphorus to lakes than newer systems, and that there may be substantial time lags between septic-system installation and significant impacts on lake-water P concentrations. For lakes in basins that contain agricultural land, the P loading attributable to agriculture can be calculated as the difference between the estimated total loading and the sum of estimated loadings from all other (nonagricultural) sources. A comprehensive system for evaluating errors in all loading estimates is presented. The empirical relationships developed allow preliminary approximations of the cumulative impact that development has had on P loading and the amounts of P loading from generalized land-use categories for Puget Sound lowland lakes. In addition, the sensitivity of a lake to increased loading can be evaluated using the mass-balance model. The methods use data that are presently available for most lakes. All the estimates are most suitable for use in developing water-quality goals, setting priorities for lake studies, and designing studies of individual lakes. The suitability of the method for management of individual lakes will often be limited by relatively high levels of uncertainty, especially if the method is used to evaluate relatively small increases in P loading.

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USGS1983

Proceedings of Workshop XIV: earthquake hazards of the Puget Sound region, Washington

This conference, the fourteenth in a series sponsored by the Earthquake Hazards Reduction Program, was held October 13-15, 1980 at Lake Wilderness, Washington. The stated purpose of the conference was to assess progress and the state of knowledge of earthquake hazards in the Puget Sound region of Washington. The small size of the group and the pleasant informal setting of the Lake Wilderness Conference Facility created conditions which encouraged abundant discussion, expansion of ideas and impromptu presentation that went well beyond the scheduled topics, most of which are covered in this volume.

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