|Start:||October 1, 2005 ||Observed|
|Stop:||November 2005 ± 14 Days||Observed|
|Seismicity with no confirmed eruption: ||
|Eruption Type:||Not an eruption.|
From McGimsey and others (2007): "Earthquake activity at Tanaga increased abruptly on October 1, 2005. Over the next several days, the number of located events ranged from 15 to 68 per day, in striking contrast to the typical one earthquake per month previously recorded since the seismic network was installed in 2003. The earthquakes centered a few kilometers northeast of the summit of Tanaga at a depth of 10-20 km (~6-12 mi), and the largest event had a magnitude of 1.7. AVO issued a special Information Release on October 5 to announce the activity. The activity further escalated that day and again early on October 7 with located earthquakes of magnitude 0.5-1.9 shallowing to depths of 6-12 km (~4-7.5 mi) beneath Tanaga's summit. This change in activity prompted AVO to raise the Level of Concern Color Code from Green to Yellow on October 7, 2005. By the following week, the daily earthquake count had fallen slightly, and by the next week, earthquake activity had diminished further, but remained above background levels. A several-minute-long period of unusual seismicity occurred on October 17 and may have been a landslide or small phreatic explosion, but no signs of activity were visible in satellite images. Although the daily earthquake count continued to dwindle, nearly continuous, weak volcanic tremor was recorded on seismic stations closest to nearby Takawangha Volcano on October 24 (see fig. 47 in original text). This was the first episode of tremor recorded at the Tanaga cluster since the seismic network was installed in 2003. Weak tremor continued for the next several weeks, gradually declining. Seismicity continued to decline further during November, and by the end of the month the likelihood of an eruption was considered significantly decreased. In an Information Release issued on November 25, 2005, AVO reduced the Level of Concern Color Code from Yellow to Green."