Redoubt update, 24 March 2009

Redoubt AVO Hut webcam image, 23 March 2009, 19:55:58 local time (image courtesy AVO/USGS). [source]

There have been six large explosions at Redoubt during the current eruptive phase. In its various updates, the AVO gives the following approximate starting times (local time) for these events:

1. 22 March, 22:38
2. 22 March, 23:02
3. 23 March, 00:14
4. 23 March, 01:39
5. 23 March, 04:31
6. 23 March, 19:41

The sixth event was captured by the AVO Hut webcam. In an update issued at 19:59 yesterday the AVO estimated from radar an eruption cloud height of ‘at least 60,000 feet above sea level’ (approximately 18200 metres). Ash emission was reported to have ended by 20:56. Pyroclastic flows were observed via the webcam, descending the north face of the volcano – one can be seen in the image reproduced at the top of this post. The picture also clearly shows the mudflows produced by the melting of the Drift Glacier and other ice and snow on the volcano, and the extensive ashfall staining the white landscape with murky grey.

In the caption to one of the images available on the AVO website (a composite of satellite images) there is a reference to seven eruption events, but with the caveat that the total of seven is ‘one preliminary interpretation and may be subject to change on further review’. So it seems best to stick with the total of six until we hear differently.

The latest from Redoubt (AVO update, 2009-03-24 06:56:32) is that seismicity is now at low levels, and has remained little changed for some hours. The subdued seismicity is clearly shown by traces from REF, RDN and NCT seismic stations, which also show clearly how rapid was the onset of explosion 5 at 19:41 yesterday, and how markedly things quietened down after it. That ‘quietened down’ is only relative: there is still a lot of low-level background tremor and occasional small quakes visible on the traces. Redoubt’s previous history would suggest that more explosions and ash emissions are to come. ‘Further explosive activity could occur with little or no warning, and could occur intermittently for weeks or months’, says the AVO.

The RSO station was knocked out yesterday, and its trace is now a rather sad and meaningless squiggle.

UPDATE: Since the above was written there’s been another AVO update (2009-03-24 09:04:19) saying that seismicity remains unchanged but noting that ‘Small discrete earthquakes are occurring’. See Akira Shirakawa’s comment below regarding a slight increase in the frequency of these quakes over the past two hours or so.

ANOTHER UPDATE: A new status report has been issued by the AVO (12:46 local time, 20:46 GMT), reporting low seismicity, no indications of ash emission, and a steam plume rising to 20000 feet (6000 metres) and extending 40 miles (64 kilometres) to the NW. Seismicity is definitely elevated and choppy: the RSAM plots (particularly the 10 minute average chart) show this clearly.

For all our Redoubt coverage: Redoubt « The Volcanism Blog.

Information
Global Volcanism Program: Redoubt – summary information for Redoubt (1103-03-)
Alaska Volcano Observatory – Redoubt – AVO information and updates for Redoubt
Alaska Volcano Observatory – main page for the AVO