Event Name : Little Sitkin 2012/8
|Start:||August 22, 2012 ||Observed|
|Stop:||November 26, 2012 ||Observed|
|Seismicity with no confirmed eruption: ||
|Eruption Type:||Not an eruption.|
From Herrick and others (2014): "Seismic activity at Little Sitkin began to increase on August 22. Although volcano-tectonic earthquakes accounted for most of the seismicity, some unusual signals also were observed, most notably monochromatic earthquakes. A series of 5 monochromatic earthquakes that had unusually low resonant frequencies (0.6 Hz) that occurred during August 24-28. These earthquakes occurred at depths depths between 10 and 20 km (6-12 mi) and were recorded on seismic stations as far away as 80 km (50 mi) on Amchitka and Semisopochnoi Islands. These signals at Little Sitkin appear similar to that seen at Izu-Oshima Volcano in Japan (Ukawa and Ohtake, 1987). Two brief episodes of tremor also were recorded during this time.
"On August 30, earthquake activity escalated dramatically, prompting AVO to upgrade the Aviation Color Code and Volcano Alert Level to YELLOW/ADVISORY. AVO instituted a heightened seismic watch schedule to examine seismic data every 2 hours. An RSAM alarm was established for Little Sitkin to alert seismologists in the event of a sudden escalation in seismicity. Frequent earthquakes continued through August 31, then decreased during the first few days of September. Of the 110 earthquakes located during the first week of September, epicenters averaged 3.5 km (2.2 mi) from the summit. After a short flurry of earthquakes on September 13, seismicity decreased, but remained well above background. On September 24, AVO reduced the frequency of seismic data checks to once every 6 hours.
"Over the next 3 months, seismicity consisted of low-frequency tremor bursts with occasional brief swarms of VT events. Three notable flurries of seismicity occurred on October 13, 15, and 29. Another pulse of VT activity began on November 15 and continued through November 26. Six?hour seismic checks for Little Sitkin were discontinued on October 8.
"On January 9, 2013, after more than a month of relative quiet and continued decrease in overall seismicity, AVO downgraded the Aviation Color Code and Volcano Alert Level to GREEN/NORMAL. (Note that prior to the onset of seismicity and upgrade to YELLOW/ADVISORY in August, the volcano had been designated UNASSIGNED. Technically, it should have returned to UNASSIGNED, but this did not happen until March 29, 2013).
"Satellite images of the largest hot spring area west of the modern Little Sitkin cone showed that no significant change had occurred in the area since the last observations in late-October and early-September 2012. AVO received no reports from mariners or air crews of any changes at the surface; however, this part of the Aleutian Arc receives few visitors throughout the year and minor changes in activity could well go unnoticed.
"Subsequent analysis of InSAR results from images that span the time period of increased seismicity indicates 1-2 cm (0.4-0.8 in.) of inflation beneath the modern Little Sitkin cone. This result, along with the evolution of the seismic sequence in 2012, strongly suggests a magmatic intrusion as the source of observed seismicity and geodetic change (Haney and others, 2014)."