23 November-29 November 2005

AVO reported on 25 November that for several weeks seismicity beneath young volcanic vents on Tanaga Island decreased significantly from levels recorded in early October. Satellite images of the island showed no anomalous temperatures or evidence of ash emissions. AVO reported that based on the decrease in earthquake counts and frequency of tremor episodes, the likelihood of an eruption had diminished. Therefore, AVO downgraded the Concern Color Code from Yellow to Green.

Sources: Alaska Volcano Observatory (AVO)

2005: October | November |

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23 November 2005              Back to Top

AVO reported on 25 November that for several weeks seismicity beneath young volcanic vents on Tanaga Island decreased significantly from levels recorded in early October. Satellite images of the island showed no anomalous temperatures or evidence of ash emissions. AVO reported that based on the decrease in earthquake counts and frequency of tremor episodes, the likelihood of an eruption had diminished. Therefore, AVO downgraded the Concern Color Code from Yellow to Green.

Sources: Alaska Volcano Observatory (AVO)

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26 October 2005              Back to Top

Elevated seismicity below young volcanic vents on Tanaga Island continued during 21-28 October. Beginning on 24 October, AVO observed weak, nearly continuous volcanic tremor in the vicinity of Takawangha volcano of the Tanaga volcano cluster. This was the first time that tremor of this sort had been observed in the volcanic cluster since the seismic network was installed in 2003. The daily number of small earthquakes continued to diminish from its peak in early October, but stayed above background levels. Tanaga remained at Concern Color Code Yellow.

Sources: Alaska Volcano Observatory (AVO)

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19 October 2005              Back to Top

Elevated seismic activity below young volcanic vents on Tanaga Island continued during 14-21 October, although the rate of small earthquakes reduced in comparison to peak values during early October. An unusual, several minute-long seismic signal on 17 October may have been a landslide or small phreatic explosion, but satellite images detected no airborne ash. The activity that began at Tanaga on 1 October was at the highest level recorded since the seismic network was installed in 2003, so the Concern Color Code remained at Yellow.

Sources: Alaska Volcano Observatory (AVO)

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12 October 2005              Back to Top

Elevated seismic activity below young volcanic vents on Tanaga Island continued during 7-14 October, although the rate of small earthquakes reduced slightly in comparison to the previous week. The activity that began at Tanaga on 1 October was at the highest level recorded since the seismic network was installed in 2003, so the Concern Color Code remained at Yellow.

Sources: Alaska Volcano Observatory (AVO)

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5 October 2005              Back to Top

AVO detected an increase in seismic activity beneath Tanaga beginning on 1 October, with 15-68 earthquakes occurring daily. Previously, less than one earthquake had occurred per month since the seismic network was installed in 2003. The earthquakes were centered roughly 2 km NE of the volcano's summit at depths of 10-20 km below sea level. The largest event was M 1.7, with most earthquakes between M 0.5-1.5. Tanaga was at Concern Color Code Green on 5 October. During 5-7 October, there was a marked increase in the rate of seismicity. The located earthquakes ranged in magnitude from 0.5 to 1.9 and ranged in depth from 6 to 12 km beneath the volcano's summit. In response to the observed changes in seismic activity, AVO raised the Concern Color Code to Yellow on 7 October. AVO reported that while the seismic activity represented a significant increase in rate, the size, depths and character of the events were not indicative of imminent eruptive activity.

Sources: Alaska Volcano Observatory (AVO)

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Summary of eruption dates and Volcanic Explosivity Indices (VEI).

Start Date	Stop Date	Eruption Certainty	VEI	Evidence	Activity Area or Unit
1914	Unknown	Confirmed	0	Historical
1829	Unknown	Confirmed		Historical
[ 1791 Jun 7 ]	[ Unknown ]	Uncertain
[ 1763 ]	[ 1770 ]	Uncertain
1550 (?)	Unknown	Confirmed		Radiocarbon (uncorrected)	Tanaga, Sajaka Two
1050 (?)	Unknown	Confirmed		Radiocarbon (uncorrected)	Tanaga
550 BCE ± 1000 years	Unknown	Confirmed		Tephrochronology	Sajaka One
1050 BCE ± 1000 years	Unknown	Confirmed	0	Ar/Ar	Sajaka One