Event Name : Iliamna 2011/12
|Start:||December 22, 2011 ||Observed|
|Debris-avalanche, volcanic avalanche, or landslide: ||
|Fumarolic or hydrothermal activity: ||
|Seismicity with no confirmed eruption: ||
|Eruption Type:||Not an eruption.|
From Herrick and others (2012): "From January 15 through 17, AVO detected a swarm of volcanic-tectonic (VT) events with the largest event in the sequence being an M2.7 earthquake on January 17; subsequent analysis (H. Buurman, UAFGI, written commun., 2014) places the beginning of the swarm as early as December 22, 2011. On January 27, a pilot called AVO to inquire about webicorder signals displayed on AEC (Alaska Earthquake Center, formerly AEIC, the Alaska Earthquake Information Center) and AVO Web sites. Anticipating additional questions about elevated seismicity at Iliamna, AVO posted explanatory information on the public-access webicorder plot.
"On March 7, AVO received a telephone call from Dennis Anderson, a photographer from Diamond Ridge above Homer, Alaska. Anderson reported observations of avalanche activity and appearance of new crevasses on the Red Glacier of Iliamna. Public attention resulted in local media calls to AVO and several news stories on the avalanche and increase in seismicity. On March 8, the largest events of the unrest sequence were recorded (M2.96 and M3.01). On March 9, based on the sustained increase in seismicity, AVO upgraded the Aviation Color Code and Volcano Alert Level for Iliamna to YELLOW/ADVISORY. Following the upgrade of Color Code and Alert Level, AVO increased the frequency of seismic checks to once every 6 hours.
"Examination of photographs from Anderson and satellite images from early March showed clear evidence of avalanching of debris on the upper Red Glacier. Photographs from March 12 showed a vapor plume, 2-4 km (1-2.5 mi) above the crater, drifting north from the summit area. AVO received no reports of increased or anomalous sulfur smell.
"On March 14, a fumarolic plume was visible on satellite images that drifted north-northwest from the summit; the length and prominence of this cloud-feature was unusual for Iliamna. Based on the appearance of this plume in mid-infrared images, it is possible that the feature was a predominantly water-rich meteorological cloud influenced by a strong Iliamna fumarolic plume.
"In mid-March, a Web camera (AnnaCam) managed by Hilcorp on a platform in the middle of Cook Inlet was repositioned to allow for visibility of both Iliamna and Redoubt within the same field of view. At the same time, AVO installed a new Web camera pointed at Iliamna (station NNL) on the Kenai Peninsula.
"AVO seismologists determined that the 2012 earthquake sequence was occurring just south of the summit at 0-4 km (1-2.5 mi) depth. The swarm appeared similar to activity in 1996 (Roman and others, 2004), although the 2012 swarm also had low frequency earthquakes, which were not present in 1996 (H. Buurman, UAFGI, written commun., 2014).
"A gas measurement flight to Iliamna on March 17 noted levels of carbon-dioxide (CO2) and SO2 at similar values as in 1996 (C. Werner, USGS, written commun., 2012). AVO observers on this flight noted visibly disturbed ice in the area of the upper Red Glacier. Close-up views of the main fumarolic field revealed robust steam- and gas-plumes with possible jetting from some sources. The high southern flank fumarolic field also was more active than usual. Several possible new vents were noted high on the eastern flank that later in the year appeared less active. Minor, recent icefalls also were noted on the western flank.
"Observations from the gas flight verified that the upper Red Glacier descending Iliamna's eastern flank had undergone a surge in early 2012. This fast creep (or slow slide) event was not a true avalanche, but it was sourced in the same area as large avalanches of the recent past (Huggel and others, 2007). At the time of the observations, it was unclear if the surge was related to changes in the volcano, such as the increase in seismicity, associated increases in heat and gas flux, or just a result of heavy snowfall during the winter of 2011-2012.
"Into late March, distinctive plumes continued to appear in mid-infrared satellite images. On March 21, sulfur odors were reported by a pilot about 24 km (15 mi) west of Iliamna. Web camera images showed occasional plumes from the summit fumarole fields and above background seismicity persisted through the year's end. Possible thermal anomalies in satellite images were detected on a number of occasions during the year; it remains uncertain if these were a clear departure from background thermal conditions.
"A second gas flight to Iliamna on June 20 measured SO2, hydrogen sulfide (H2S), and CO2 emissions similar to the March 17 flight when measurements were considered elevated over background and were comparable to the highest measurements from the episode of unrest in 1996. A third flight on August 13 determined that volcanic gas emission continued at elevated levels. Less robust visible vapor plumes suggested a possible decrease in activity, but relatively dry and warm atmospheric conditions may have played a role in the apparent change.
"On August 21, a citizen called AVO to report a plume rising from Iliamna. Conditions were clear that day and the mountain was backlit. The observer called it an 'uncommon plume' that billowed from the summit with more vigor than typically noted. Later that month (August 27), another observer noted a sulfur odor at Anchor Point on the Kenai Peninsula. Back trajectories from a HYSPLIT model indicated that a gaseous plume from Iliamna would have drifted over Anchor Point at that time, suggesting Iliamna was a likely source. Problems with the seismic network led to data drops and challenges in tracking seismicity in the late summer. When data flow resumed in late September and AVO determined that elevated seismicity at Iliamna had continued.
"A photograph was taken by a resident on October 13 that showed a flow feature on the southeastern flank of Iliamna. A review of seismic records showed that a possible landslide signal had been recorded at 8:16 p.m. AKDT on October 12 (04:16 UTC October 1), consistent with the apparent flowage deposit captured in the photograph. The dimensions of the avalanche were determined on October 22 with the help of satellite images. At the widest part, the feature was 2,800 x 200 m (1.7 x 0.1 mi). The landslide had originated near the existing fumarolic area high on the southeastern flank just below the summit. Some small melt slides had been visible in that area several days earlier.
"Elevated seismicity continued through October and November with hypocenters roughly coincident with the 1996 activity. Two bursts of low-frequency seismicity occurred on November 24. Based on the continued levels of seismicity, AVO maintained Aviation Color Code and Volcano Alert Level YELLOW/ADVISORY into 2013. Seismicity and gas measurements during the 2012 unrest are likely explained by a magmatic intrusion to shallow levels below the volcano (Prejean and others, 2012.)"