Sunday, January 25, 2009

Alaska Volcano Observatory Issues A Potential Imminent Eruption Alert For Mount Redoubt Due To A Sudden Increase In Seismicity

Update March 22nd: Mount Redoubt erupts to 50,000 feet, ashfall expected in Susitna Valley with slight chance in Anchorage. Details HERE.

On January 25th, 2009, the Anchorage Daily News reported that the Alaska Volcano Observatory has upgraded the aviation color code for Mt. Redoubt from yellow to orange, indicating that they believe an eruption is imminent, perhaps "within hours or days". A sudden increase in seismic activity at Mount Redoubt triggered the alert, since sudden seismicity at a given volcano is usually a prelude to an eruption. The alert was issued at 2:09 A.M. Sunday.

The volcano, which lies about 50 miles west of Kenai and 100 miles southwest of Anchorage, last erupted over a four-month period, from 1989 to 1990. You can read a detailed 47-page report about that eruption HERE. By the way, people have been known to climb this mountain (when it's quiet). Read about one such journey documented in the Redoubt Reporter.

Late Sunday afternoon, scientists from the observatory flew over the volcano and determined that it hadn't erupted yet. In addition, seismic activity has now tapered off. Nevertheless, monitoring of Redoubt will now proceed around the clock. Here's the latest statement, in part, from the AVO:

Redoubt Volcano Activity Notifications
2009-01-25 16:28:22 - Information Statement

Unrest at Redoubt Volcano continues. AVO conducted an overflight of the volcano this afternoon, and observations confirm that an eruption has not occurred. Increased steaming through previously observed sources in the snow and ice cover were seen and sulfur gas emissions were noted. There was no significant disruption of the glacial ice, nor any apparent increased water discharge down the Drift River.

Seismicity remains well above background levels. Beginning at 00:58 AST (9:58 UTC) this morning, nearly continuous volcanic tremor was recorded at stations near Redoubt's summit, prompting AVO to raise the Aviation Color Code to ORANGE and the Alert Level to WATCH at 02:09 AST (11:09 UTC). Seismicity began to decline at about 05:30 AST (14:30 UTC) this morning, but remains elevated as of this writing.The current activity at Redoubt could be precursory to an eruption, perhaps within hours to days. A further increase in seismicity is expected to accompany an eruption. Staff are currently monitoring the volcano 24 hours a day.

It is because once a volcano reaches an ORANGE state that it can erupt without warning that the AVO issued their alert. The late afternoon decrease in seismicity does NOT mean the threat is over. This same pattern was observed before the eruption of Mount Spurr, just west of Anchorage, in 1992. On June 3rd, 1992, seismicity became unstable and began to oscillate; seventeen days later, Spurr blew its cork, and in later eruptions, spewed an average of 3mm of ash all over Anchorage. Read a report about the 1992 Mt. Spurr eruption HERE.

Wind patterns will determine where the ash cloud goes and whether or not there is ashfall. If the winds aloft are strong, the ash cloud will fan out, but little of it will fall on adjacent areas. But if winds are weak, the ash cloud will begin to fall shortly after it forms aloft. The most immediate threat of any ashfall would be from Nikiski to Kenai and to Clam Gulch; secondary threat areas would be southward to Seldovia and northward to Palmer-Wasilla.

The AVO has an experimental Puff Ash Cloud Prediction Model which you can access HERE. It's updated every three hours and will give you an idea of where the ash could go if the volcano was to erupt immediately. The information derived is for personal use only and is not considered "official".

Visit the USGS Volcano website to find out what you should do in the event of an ashfall in your area.

1 comment: