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U.S. Geological Survey
Wednesday, January 28, 2009, 4:19 PM AKST (Thursday, January 29, 2009, 01:19 UTC)

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

Summary of Current Unrest

Since last fall, the Alaska Volcano Observatory (AVO) has detected increasing volcanic unrest at Redoubt Volcano. Starting on Friday, January 23, the level of seismic activity increased markedly, and on Sunday AVO raised the Aviation Color Code to ORANGE and the Volcano Alert Level to WATCH. On the basis of all available monitoring data AVO regards that an eruption similar to or smaller than the one that occurred in 1989-90 is the most probable outcome. We expect such an eruption to occur within days to weeks.

In response to the current increase in activity, AVO has deployed a web camera approximately 7.5 miles north-northwest of the summit and will install additional seismic equipment at the volcano as weather permits. A second web camera also is pointed at Redoubt from a platform within Cook Inlet. The observatory in Anchorage is currently staffed 24 hours a day. We plan continued visual surveillance of the volcano's summit region, frequent airborne measurements of gas output, and frequent analysis of satellite and weather-radar data.

Observations and Background

Beginning in September 2008, AVO received reports of a strong hydrogen sulfide (H2S) odor downwind of Redoubt. During an observation flight on September 26, a melt hole was observed in the upper Drift glacier, down slope from the location of historical eruptive vents. Gas-measurements flights in October and November detected elevated levels of sulfur dioxide (SO2), carbon dioxide (CO2), and H2S. Through the fall, melt holes increased in size, and several points of steaming and gas emissions were observed in the area of the lava domes extruded during the 1989-90 and 1966-68 eruptions.

Concurrent with visual observations and gas data, a subtle increase in seismic activity was also detected beneath the volcano. Seismicity took the form of episodic, very weak volcanic tremor, consistent with the movement of fluids and gases within the volcano. AVO also located deep (30 km, or 19 miles, below sea level) long-period earthquakes, which were few in number but possibly indicative of magma recharge deep within Redoubt’s plumbing system.

Beginning on the evening of Friday, January 23, 2009, seismic activity increased at stations closest to Redoubt’s summit. The seismicity consists of a combination of discrete, relatively small earthquakes and periods of more continuous volcanic tremor. This activity intensified early Sunday morning, January 25, at which time AVO raised the Aviation Color Code to ORANGE and the Volcano Alert Level to WATCH. Since Sunday, seismicity has waxed and waned but has remained well above background levels. The seismic unrest observed over the past few days is unusual for Redoubt and has not been seen since just prior to the 1989-90 eruption. Overflights on January 26 and 27 documented continued production of water vapor and volcanic gas plumes from the summit crater; increased melting has produced small flows of debris at the north base of the volcano.

The most recent eruptions of Redoubt, in 1966-68 and 1989-90, were characterized by large explosions that produced ash clouds reaching as high as 40,000 ft asl. During the 1989-90 eruption, the largest ash fall from a single event in Kenai was 5 millimeters (0.2 inches), but ash fall was widespread and noted as far away as Fairbanks, along the Richardson Highway, and the Yukon Territory border. Other ash-producing events during the 1989-90 eruption deposited several millimeters of ash on the Kenai Peninsula and trace amounts of ash in Anchorage and more distant locations over several months. The 1989-90 eruption also several disrupted air traffic operations in and out of Anchorage. Other hazards from past and likely future eruptions include hot pyroclastic flows that may travel several miles from the volcano, especially to the north, and volcanic mudflows, or lahars, that travel many miles from their source. Lahars form as hot pyroclastic flows, often caused by collapse of a growing lava dome, swiftly melt large volumes of snow and ice downslope of the summit crater. In 1989-90, lahars flowed east down the Drift River, at least three of which reached Cook Inlet and one of which partially flooded the Drift River Oil Terminal facility. Lahars produced by eruptions of Redoubt Volcano may range from dense, gravel-rich flows to watery, sediment-laden floods. All such flows can transport boulder-size particles, and water within these flows temporarily may be at boiling temperatures. It could take a typical lahar 1-2 hours to travel from the base of the volcano to the mouth of the Drift River.

Interpretation and Hazards

Based on our current understanding of Redoubt's past eruptions, both historical and from the geologic record, and our analysis of the current episode of unrest, AVO considers the following future scenarios as possible:

1) Failed Eruption: No eruption occurs because magma does not reach the surface. Earthquake activity, gas output, and steaming slowly decrease over several weeks or months. Continued heat flux may cause continued, modest melting of snow and ice on the edifice and subsequent increased, but not hazardous outflow into the Drift River.

2) Eruption similar to or smaller than 1989-90: Unrest continues to escalate culminating in an eruption that is similar to or smaller than the one that occurred in 1989-90. An eruption such as this would likely spread volcanic ash throughout Cook Inlet and other parts of south-central Alaska depending upon the prevailing winds. Communities around the volcano, especially to the east, northeast, and southeast, would likely experience trace to several millimeters (less than 0.4 inches) of ash fall as a result of discrete explosive events. Such events could also generate pyroclastic flows that swiftly melt snow and ice to form mudflows, or lahars, that would likely travel east down Drift River, possibly reaching and flowing into Cook Inlet. If summit lava domes form, as they did in 1989-90, they may repeatedly collapse and generate pyroclastic flows that would likely travel north from the summit crater and form lahars. Smaller lahars could also form in other drainages if hot debris accumulated on other flanks of the volcano. An eruption consisting of multiple explosive events, episodic lava-dome growth and collapse, and lahars may last weeks to months.

3) Larger Explosive Eruption: A significantly larger eruption could occur, perhaps similar to eruptions that are thought to have taken place prehistorically. Such an eruption might involve the production of larger ash clouds, pyroclastic flows on several flanks of the volcano, and larger lahars more frequently reaching Cook Inlet down Drift River and affecting other drainages around the volcano as well.

4) Flank Collapse: The intruding magma or other processes could destabilize a portion of the Redoubt edifice that could result in a large volcanic landslide. At least twice in the last 10,000 years, debris flows generated by such landslides have reached Cook Inlet. It is also likely that a landslide of this type would be accompanied by an eruption. Because of the scarcity of these events in the geologic record, a flank collapse and eruption is considered very unlikely. A flank collapse may be accompanied by visible deformation of the edifice and AVO will be looking for such signs.

Based on all available monitoring data and AVOs knowledge of the volcano, scenario number two, an eruption similar to or smaller than that of 1989-90, appears to be the most probable outcome at this time. We consider one and three to be somewhat less likely, and scenario four to be much less likely.

Comparing the time frame of pre-eruptive activity in 1989-90 (the only other eruption for which seismic data were available) with the current unrest, we would expect such an eruption to begin within the next few days or weeks. It is likely that the onset of an explosive eruption would be preceded by a further increase in seismicity. An explosive eruption would be accompanied by a sharp increase in seismicity. Should earthquake activity or other monitoring data suggest that an eruption is expected within hours, or is underway, AVO would move Redoubt from its current Aviation Color Code ORANGE to RED, and Alert Level WATCH to WARNING.

Please see http://volcanoes.usgs.gov/activity/alertsystem/index.php for more information about our aviation color codes and volcano alert levels.

More information about Redoubt Volcano can be found at http://www.avo.alaska.edu/activity/Redoubt.php


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

Chris Nye, Acting Coordinating Scientist, ADGGS
cnye@giseis.alaska.edu (907) 474-7430

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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