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U.S. Geological Survey
Thursday, March 12, 2009, 11:23 AM AKDT (Thursday, March 12, 2009, 19:23 UTC)

60°29'7" N 152°44'38" W, Summit Elevation 10197 ft (3108 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

Current Status
On the basis of declining seismicity, a possible decrease in heat flux, and no apparent change in gas emission, the likelihood of an eruption of Mount Redoubt within days to weeks has diminished. Accordingly, AVO lowered the alert level to YELLOW/ADVISORY on Tuesday, March 10, 2009, and ceased round the clock staffing of the AVO operations center. The volcano remains on a heightened monitoring schedule, and AVO scientists will continue to evaluate conditions at the volcano regularly. The volcano remains restless and it is still possible, though far from certain, that the current episode of volcanic unrest at Mount Redoubt could result in an eruption.

Over the past two weeks the overall level of seismic activity has decreased, and the periods of sustained volcanic tremor, common during late January and most of February, have been largely absent. Discrete earthquakes are still occurring, but they are typically small and their observed rate is similar to that detected prior to this period of unrest. Volcanic gas emissions are still well above background levels, and melting of the upper Drift glacier in the vicinity of the 1989-90 eruption vent is continuing. These conditions could persist for many months and do not indicate that an eruption is imminent. It remains possible for conditions at the volcano to change rapidly, advancing from relatively low levels of unrest to eruption in time periods as short as 24 hours. If this happens, seismic activity should increase markedly providing some advance warning. At the present time, the overall trend is one of declining unrest and a much lower probability of an eruption in the near term.

Chronology of Unrest
November 5, 2008: Volcano alert level raised to YELLOW /ADVISORY in response to elevated gas emissions and visual evidence of ice melt in the volcano’s summit crater. Reports of sulfur smell in the vicinity of the volcano as early as August, 2008.

January 25, 2009: Volcano alert level raised to ORANGE/WATCH in response to a sudden and distinctive increase in seismicity beneath the volcano.

January 25 - February 26, 2009: Observations and measurements of snow and ice melt, minor flooding, debris flows, and steam emission in the upper Drift glacier area. During this period, elevated levels of volcanic gas (CO2, SO2, and H2S) were detected by airborne measurements. The nature of the gas emissions indicated a magmatic source. Seismic activity during this period was characterized by frequent discrete earthquakes and episodes of strong, continuous, volcanic tremor that also was detected at seismic stations as far as Mounts Spurr and Iliamna (45 to 90 km distant). No ash emissions were observed. Steam plumes were occasionally observed rising to about the summit of the volcano.

February 26 - March 10, 2009: During this period, the overall level of seismic activity decreased and episodes of sustained volcanic tremor were largely absent. Discrete earthquakes are still occurring; however, most are small and similar to those detected during normal background activity. Volcanic emissions and snow/ice melt continue, occasional steam plumes observed by web camera.

March 10, 2009: AVO lowers the volcano alert level to YELLOW /ADVISORY in response to declining seismic activity. The AVO operations center in Anchorage has ceased 24 hour staffing. However, AVO staff continues regular surveillance of the volcano by the web camera, overflights, airborne measurements of gas output, seismic analysis, and examination of satellite data.

Analysis of Unrest
The current episode of unrest is most likely a result of intrusion of new magma beneath the volcano. The main evidence for an influx of new magma is: (1) measurement of significant amounts of magmatic gas, including CO2, SO2, and H2S from fumaroles in the vicinity of the 1989-90 vent; (2) increased heat flux causing ice/snow melt of the upper Drift glacier (about 4-5 million cubic meters through March 10, 2009), and fluctuating water discharge from streams draining the lower Drift glacier, (3) elevated seismicity since January 23, 2009, including hour-long periods of continuous volcanic tremor that is consistent with the movement of fluids (including heated ground water) and gases within the volcano. The influx of magma and rise of hot magmatic gases resulted in a reinvigoration of the volcano’s hydrothermal system and this activity was likely the cause of some of the shallow seismicity. The increased heat output caused melting and disruption of snow and ice on Drift glacier and this led to greater than normal water outflow and at least one sediment-water outburst flood on Drift glacier.

Although we do not know how much new magma has intruded beneath Mount Redoubt, we estimate that most of the new magma is probably at depths greater than about 5 km (about 3 miles). It is possible that a small amount of the magma may have risen to shallower depths in late January-February when seismicity, degassing, and melting intensified. There is no evidence indicating a large volume of magma is present at shallow depths (within 2 km, or about a mile, of the surface).

On the basis of reduced seismic activity, it appears that the new magma intruded beneath the volcano is no longer moving toward the surface, or is doing so at a greatly reduced rate. Thus the probability of an eruption of Mount Redoubt within days to weeks is low. The volcano remains in a restless condition and it is still plausible that the unrest observed thus far will lead to an eruption on a longer time scale. We expect that elevated levels of volcanic gas emission and additional melting of the upper Drift glacier will continue for some time, perhaps many months.

Potential Future Activity
Redoubt is an active volcano and future eruptions are a certainty. The 1989-90 eruption was seismically monitored, but little was known about the seismic behavior of the volcano prior to that eruption. It is uncertain if future activity will be more or less like that of 1989-90. In 1989, seismic activity escalated rapidly, and explosive events occurred after only about 24 hours of precursory seismicity. During the current unrest, the sudden onset of strong volcanic tremor on January 25, 2009 was preceded by about two days of elevated seismicity, and the tremor indicated an eruption of Redoubt appeared likely. The activity observed thus far is distinctly unlike the activity that preceded the 1989-90 eruption, and has implications for future unrest and possible eruptive behavior.

Based on our observations and understanding of Redoubt Volcano to date, AVO considers the following scenarios as possible outcomes of the current period of unrest.

1. Gradual decline in earthquake activity, gas emission, and heat output, and return to normal background conditions. Occasional periods of slightly elevated activity that abates. Period of unrest ends, no eruption occurs.

2. After a period of relative quiescence, lasting some weeks to months, seismic activity increases, possibly rapidly, and the possibility of an eruption becomes more likely. The increase in seismicity likely would be accompanied by increasing volcanic gas emission, snow and ice melt, and increased melt-water runoff. If this situation arises, the following outcomes are plausible:

a. Hydrothermal system becomes invigorated, and this increases the possibility of phreatic activity (water related steam emissions or steam explosions), or explosive phreatic eruption. This could lead to magmatic involvement and an explosive magmatic or phreatomagmatic eruption.

b. Explosive eruption occurs, associated with rapid assent of magma to shallow levels. Activity could be similar to that of 1989-90 and may occur after a brief period of precursory seismicity, possibly as short as 24 hours.

c. Hydrothermal system becomes invigorated as it did in January-February, 2009, but again does not culminate in eruptive activity.

At present, AVO regards both scenarios 1 and 2 as about equally likely, and gives equal relative probability to scenarios 2a, 2b, and 2c should conditions evolve toward scenario 2. There are several examples worldwide of an explosive eruption occurring many months to more than a year following the onset of increased heat output. In fact, the 1966 eruptions of Redoubt occurred a year following the onset of increased steaming observed at the summit.

Ongoing Hazards at Current Level of Unrest
Heat and volcanic gas output could gradually decline over the coming months. However, parts of upper Drift glacier were disturbed by melting, and readjustment of the glacier will occur over time. Rapid settling of the ice could initiate sudden onset, fast moving and potentially hazardous outflows of water and sediment in the drainages adjacent to and downstream of Drift glacier. It is possible that settling of the ice may also initiate ice and rock avalanches, or unusual water flow on the surface of Drift glacier. These flows of water and or ice would be hazardous to anyone on Drift glacier or along its outflow streams.

If the hydrothermal system remains active for some time, additional melting, ice collapse, and minor downstream flooding should be expected. It is also possible that stream-driven (phreatic) explosions may occur without warning and it is possible for such explosions to produce plumes (possibly containing fine rock fragments) that could rise well above the summit of the volcano. Volcanic gas emissions are likely to continue for some months, and sulfur odors could be noted by people in the region. The output of volcanic gas will likely decline, but emission rates could be variable. During low wind conditions, potentially hazardous levels of gas could accumulate in low lying areas in the Drift glacier drainage.

Heavily ice-mantled Redoubt volcano is located on the western side of Cook Inlet, 170 km (106 mi) southwest of Anchorage and 82 km (51 mi) west of Kenai, within Lake Clark National Park. Redoubt is a stratovolcano which rises to 10,197 feet above sea level. Recent eruptions occurred in 1902, 1966-68, and 1989-90. The 1989-90 eruption produced mudflows, or lahars, that traveled down the Drift River and partially flooded the Drift River Oil Terminal facility. The ash plumes produced by the 1989-90 eruption affected international air traffic and resulted in minor or trace amounts of ash in the city of Anchorage and other nearby communities.


Tom Murray, Scientist-in-Charge, USGS
tlmurray@usgs.gov (907) 786-7497

Steve McNutt, Coordinating Scientist, UAF
steve@giseis.alaska.edu (907) 474-7131

The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.
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