|Start:||February 18, 2012 ||Observed|
|Stop:||February 18, 2012 ||Observed|Description:
From Herrick and others (2014): "An increase in seismic activity at Kanaga was first noted on February 18 during a routine seismic check by AVO seismologist S. Stihler, who reported a tremor-like event (interpreted as a possible explosion) followed by smaller events over the next hour. A possible correlative airwave occurred on the Adak seismic station 40 km (25 mi) northeast of Kanaga about 2 minutes after the tremor event (M. Haney, USGS/AVO, written commun., 2012). Ten minutes after the tremor event, a faint ash signal in satellite images may have been a small ash cloud from Kanaga, drifting to the east at an altitude of about 6 km (19,700 ft) ASL. Based on these observations, AVO upgraded the Aviation Color Code and Volcano Alert Level to YELLOW/ADVISORY. Additional analysis of the seismic signal, 'reduced displacements,' showed relatively small values, consistent with hydrothermal or possibly phreatic activity (S. McNutt, UAFGI, written commun., 2012).
"Adak resident M. Tillion contacted AVO by telephone on February 19 to share photographs and observations of Kanaga on the day after the possible explosion at about 12:26 Adak time or 10:26 UTC. From the White Alice site, west of the community of Adak, a resident noted an acrid odor that caused throat irritation. Kanaga volcano, 37 km (23 mi) northwest of Adak, was steaming strongly from the summit; M. Tillion thought that the plume may have contained some ash but further analysis suggests these were just shadowed clouds. There was, however, evidence of tephra or flowage deposits on the eastern flank extending down from the summit area.
"Cloudy conditions prevented any direct observations throughout much of February. Slightly elevated surface temperatures at the summit were noted in satellite images on February 21. On February 23, four bursts of tremor-like events were recorded on the Kanaga network. Satellite images from February 26 showed that the summit crater was clear with no evidence of ongoing eruptive activity. Seismicity remained low and on March 2, AVO downgraded the Aviation Color Code and Volcano Alert Level to GREEN/NORMAL, where it remained for the rest of the year.
"Retrospectively, the combination of satellite images, aerial photographs, and field inspection demonstrated that a brief explosive event from Kanaga’s summit had indeed occurred, modifying the summit crater and producing a very small tephra fall deposit. A March 5 satellite radar image clearly showed a new open fissure along the southern rim of the summit crater. The feature was about 600 m (1,970 ft) in length and continued a short distance down the upper western flank of the volcano. In places, the fissure was 15 m (50 ft) wide and white clouds of vapor issued from several points along the fissure. An additional fumarolic cloud issued from the 15-m-diameter (50-ft), circular hole in the bottom of the crater. On March 9, nearly 3 weeks after the explosion signal, no ejecta or ash fall deposits were noted in satellite images, however, additional snowfall and reworking on the steep flanks likely would have obscured any primary deposits. Given the transient nature of the event, the limited tephra fall, and residual steaming fissure, AVO concluded that this event was a sudden phreatic explosion originating in the shallow, hydrothermally active summit region of Kanaga. The orientation and location of the feature has no clear relationship to the 1993-1995 eruptive vent complex that involved effusion of lava and ash from both the summit crater and a fissure system on the upper eastern flank of the volcano (Neal and others, 1995; Waythomas and others, 2002).
"Notable plumes of white vapor from Kanaga’s summit were seen throughout the spring by ship and airborne observers as well as Adak residents. Ship-based observers on April 1 suggested that three distinct locations along the fissure were producing the most intense clouds. At least one satellite image on April 1 showed a possible plume from the summit. Bursts of seismicity were noted in mid- and late April. The seismic record was hampered by periods of noise and scattered data outages in late spring and early summer.
"An AVO crew working via helicopter on the Kanaga seismic network in mid-June was able to describe the fissure and apparently new ash on the northern and eastern flanks; they were unable to land and inspect the volcano’s summit area, however, due to high winds. Steam was steadily rising from the summit fissure. On June 20, a faint sulfur odor was detected at the KINC seismic station located 2 km (1 mi) to the east of the volcano summit; our limited experience on the volcano makes it difficult to know how atypical this observation is from normal conditions. The team noted traces of what appeared to them to be recent ash on the northern flank down to an elevation of about 250 m (820 ft) ASL.
"Aerial photographs by Roger Clifford in both summer and fall provided excellent views of the summit fissure. Figure 29 shows a summer image of the nearly snow-free Kanaga cone; the fissure can be seen wrapping around and just outboard of the summit crater rim. The maximum opening across the fissure is about 15 m (50 ft) and white vapor issues from several point sources within the fissure. In November, Roger Clifford captured the western extent of the fissure where it crosses the summit and extends about 100 m (330 ft) down the western flank.
"Brief episodes of elevated seismicity occurred during the rest of the year at Kanaga. On June 27, unusual, emergent seismic events were detected on records from the northern seismic stations of the Kanaga network. Periods of tremor also were noted. The significance of this seismicity with respect to the summit fissure or ongoing activity at Kanaga is unknown.
"Satellite and seismic data from before February 18 was further analyzed to look for any changes prior to the explosion and opening of the summit fissure. A satellite image from January14, 2012, showed no structure in the area later cut by the fissure, although the southern rim was bare and lightly steaming. In October 2011, an unusual series of low frequency earthquakes had been noted, but any relationship of this to the 2012 activity remains unclear.
"In response to this event, AVO established automated PUFF runs to simulate ash cloud trajectories in the event of a magmatic eruption, increased the frequency of daily satellite checks, and attempted limited field verification of events in conjunction with seismic network maintenance."