From Neal and McGimsey (1997): "Two seismic swarms occurred beneath Iliamna Volcano in southern Cook Inlet in 1996. The first occurred between May 10 to May 28. The second swarm began abruptly at the end of July and peaked in late August or early September. Earthquakes as large as M 3.2 and at rates of up to 82 per day were recorded. The swarm began to decay by late 1996 and appeared to be over by early February, 1997. Most of the earthquakes during both swarms were shallower than 5 km and nearly all were unequivocal volcano-tectonic (VT) events."
Waythomas and Miller (1999) summarize the activity as follows: "Elevated levels of seismic activity beneath Iliamna Volcano were recorded by the Alaska Volcano Observatory (AVO) in 1996 (McGimsey and Neal, 1997). Earthquakes as large as magnitude 3.2 and as many as 82 earthquakes per day were recorded between May and September 1996. During this period, an increased flux of carbon dioxide and sulfur dioxide gas was detected over the volcano."
Roman (2000) suggests that "[t]he coincidence of the swarms with increased volcanic gas emissions suggests hot new magma intruded beneath the volcano in 1996."

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.)"

From Dixon and others (2015): "Activity at Iliamna Volcano in 2013 was highlighted by the observation of three significant rock/snow/ice avalanches, which often are seen on Iliamna. Monitoring highlights included a flight to measure gas emissions in April and substantial upgrades to AVO's geophysical monitoring network in August and September. Iliamna began 2013 at Aviation Color Code and Volcano Alert Level YELLOW/ADVISORY, based on seismicity interpreted to be caused by a magmatic intrusion (Neal and others, 2012). On January 9, 2013, citing decreasing seismicity, AVO downgraded the Aviation Color Code and Volcano Alert Level to GREEN/NORMAL, and Iliamna remained at this level throughout the rest of 2013.
"On February 2, 2013, seismic signals indicated that a substantial avalanche had occurred on the eastern flank of Iliamna on the Red Glacier (fig. 3 in original text) at 13:58 UTC (4:58 a.m. AKST). This slide was preceded by 3 days of elevated seismicity with more than 40 earthquakes, several as large as ML=3. An hour before the avalanche, small, repeating earthquakes occurred, gradually increasing in rate to merge into a continuous signal. After about 5 minutes of sustained broad-frequency signal, the sequence abruptly ended, signaling the end of the avalanche. This avalanche also produced a signal seen on the infrasound array at Dillingham, Alaska, approximately 320 km (200 mi) southwest of Iliamna. Poor weather obscured views of the volcano, and prevented immediate visual confirmation of the avalanche. AVO mentioned the avalanche in the February 8, 2013 weekly update, noting that avalanches are common at Iliamna, and are not indicative of volcanic unrest. Later in February, AVO received visual confirmation of the avalanches when a local resident sent photographs from early February, showing the avalanche deposits on the Red Glacier (fig. 4 in original text).
"On April 18, AVO scientists conducted a gas-measurement flight to Iliamna. During this flight, they detected emissions slightly greater than Iliamna's long-term background values (C. Werner, U.S. Geological Survey, written commun., 2013). Iliamna's typical fumaroles were visible and documented by AVO observers on the gas-measurement flight.
"On July 24, a citizen observer used the 'Is Ash Falling?' notification system on AVO's Web site to report 'increased' steaming without ash on Iliamna's eastern flank during the evenings of the week of July 15. AVO noted no anomalous seismicity or evidence of increased steaming in satellite data. When Iliamna is backlit in the evening hours, as viewed from the Kenai Peninsula, its normal fumarolic plume often appears more prominent.
"During August and September, AVO improved and upgraded geophysical equipment at Iliamna. Two new broadband seismometers were installed at pre-existing station sites ILW and IVE with a Web camera at site IVE.
"Satellite data from August 25 showed two modest avalanches down the northern-northeastern flank of Iliamna. On September 20, AVO received photographs of a new, significant, eastern flank avalanche at Iliamna (fig. 5 in original text). AVO had incomplete seismic data through September and AVO was not able to find evidence of the event in the existing seismic data. As determined in satellite imagery, the avalanche had a headwall scarp measuring 300 m (980 ft) wide by 40 m (130 ft) tall; source-to-terminus, the feature was about 1,200 m (3,940 ft) long.
"A photograph taken by a local resident on November 5 clearly depicts a flow feature on the eastern flank of Iliamna (fig. 6 in original text). A review of seismic records revealed a landslide at 16:32 UTC (7:32 AKST) on November 5. Careful analysis of seismic data on November 5 and 6 suggests two additional smaller avalanches after the larger one, although we do not have further photographic confirmation."

From Cameron and others, 2017: "Iliamna Volcano during 2014 experienced several significant rock/snow/ice avalanches on its eastern flank, and numerous smaller avalanches. Seismic data indicated a likely avalanche at 12:11 UTC (3:11 a.m. AKST) on January 28. This event was identified primarily by its characteristic seismicity - precursory seismicity of small, discrete events transitioning into a continuous signal consistent with a slide (Scott Stihler, UAFGI/AVO, written commun., 2014). In mid-May, satellite imagery of Iliamna showed recent avalanche deposits extending about 1.5 km (0.93 mi) eastward from a near-summit source area (Christina Neal, USGS/AVO, written commun., 2014) that may have been the source of the seismic signal 4 months earlier.
"AVO conducted its annual overflight of Iliamna to measure gas concentrations on April 10, recording normal background levels of about 32 tons/d of SO2. Because of an instrument malfunction, CO2 was not measured on this flight (Cynthia Werner, USGS/AVO, written commun., 2015). Iliamna remained at Aviation Color Code and Volcano Alert Level GREEN/NORMAL throughout 2014.
"On July 20, AVO received a phone call from a citizen on the Kenai Peninsula reporting a new avalanche at Iliamna, identified by the presence of new dark streaks within the snowfield. A review of the seismic data showed that the avalanche likely occurred at about 16:00 UTC (8:00 a.m. AKDT) on July 20. The deposit extended about 2.8 km (1.7 mi) east from the summit (Rick Wessels, USGS/AVO, written commun., 2014).
"Satellite imagery from August 19 documented new fresh avalanches at Iliamna, slightly larger than those in July. In early October, AVO observed evidence for a small, recent landslide or avalanche on the upper headwall of the Red Glacier on the eastern side of Iliamna. A retrospective analysis of the seismic data showed that the avalanche likely occurred in the early morning of October 3 (Jacqueline Caplan-Auerbach, Western Washington University, written commun., 2014). This new avalanche extended about 2 km (1.2 mi) from its source area."

From Cameron and others, 2020: "Members of the public noted typical amounts of steaming from the near-summit fumaroles at Iliamna Volcano, particularly in March 2016, and a large landslide occurred May 22. Iliamna remained at Aviation Color Code and Volcano Alert Level GREEN and NORMAL, respectively, throughout 2016.
"Radio station KSRM in Soldotna, Alaska, contacted AVO on Saturday, March 12, stating that people on the Kenai Peninsula could see 'quite a plume' coming from Iliamna Volcano. AVO noted no anomalous seismicity or evidence of increased steaming in satellite data. Web camera imagery showed white vapor and gas plumes rising from the volcano’s prominent and long-lived fumaroles. When Iliamna Volcano is backlit in the evening hours, as viewed from the Kenai Peninsula in spring and fall, its normal fumarolic plume often appears more prominent.
"At 07:58 UTC May 22 (11:58 pm May 21 AKDT), AVO detected seismic and pressure sensor signals associated with a large avalanche on the flank of Iliamna Volcano. Seismic signals from the avalanche were detected on volcano monitoring networks throughout Cook Inlet and as far away as the Mount Katmai area, 250 km (155 mi) southwest. The event was also recorded on distant infrasound stations in Dillingham, Alaska, 326 km (202 mi) and Fairbanks, Alaska, 610 km (379 mi) southwest and northeast of Iliamna Volcano, respectively. AVO issued an Information Statement about the landslide on Sunday, May 22. The sizeable avalanche covered about 7.2 km2 and extended down the east flank of the volcano (fig. 7; Dave Schneider, USGS, written commun., 2016)"

From Cameron and others, 2023: "Like in previous years, Iliamna Volcano experienced at least one large avalanche in 2018, as well as many smaller ones. The Aviation Color Code and Volcano Alert Level at the volcano remained GREEN and NORMAL throughout the year. Late in the morning of July 11, a resident from the Kenai Peninsula observed a fresh slide deposit on the east face of Iliamna Volcano. A retrospective analysis of seismic data indicated a probable avalanche signal at 00:24 UTC on July 11, 2018 (July 10 at 14:24 AKDT). The same resident observed a deposit from a slightly larger avalanche on August 1, 2018, but retrospective seismic analysis did not yield a possible signal for this later event. These deposits are very similar to those of Iliamna Volcano avalanches observed in prior years (Cameron and others, 2020; Dixon and others, 2020).
"On September 6, 2018, AVO measured the gas emissions at Iliamna Volcano during its annual overflight. These values proved largely unchanged from those of the previous year: instruments recorded an H2 S/SO2 value of about 2, a CO2 / (SO2 +H2 S) value of about 6, and an SO2 emission rate of 45±30 metric tons per day (t/d). The large uncertainty of the SO2 rate is due to a local wind field around the volcano that produces swirling air currents."

From Orr and others, 2023: "Although Iliamna Volcano maintained an Aviation Color Code and Volcano Alert Level of GREEN and NORMAL throughout 2019, AVO observed seismicity episodes suggestive of ice and rock avalanches, and two large avalanches were confirmed in satellite images. Avalanches like these are common on Iliamna Volcano, where they are composed mostly of ice and snow (as much as 80 percent by volume) (Schneider and others, 2010), and they are highly mobile, traveling at mean speeds of approximately 50 meters per second (m/s) [110 miles per hour] and reaching peak speeds, estimated via numerical modeling, of more than 70 m/s [160 miles per hour] (Caplan-Auerbach and Huggel, 2007; Schneider and others, 2010).
The first of the two large ice and rock avalanches took place on the south flank of the volcano in late March. This first avalanche started at an elevation of 2,800 m [9,200 ft] and ran for about 3 km [1.9 mi], reaching a final elevation of 1,450 m [4,760 ft]. A seismic signal inferred to be from the avalanche was detected on stations as far as approximately (~) 100 km [60 mi] away at 18:00 UTC (10:00 AKDT) on March 26, with a recorded duration of 3-4 minutes. This timing for the ice and rock avalanche was roughly confirmed by Landsat satellite images taken before and after the seismic signal, although the signal of the avalanche did not appear in infrasound data.
"The second, much larger ice and rock avalanche took place on the east flank of Iliamna Volcano at 00:03 UTC on June 21 (June 20 at 16:03 AKDT) (Toney and others, 2021). An oblique aerial photograph taken the same day and Copyright 2019 DigitalGlobe, Google Earth Copyright 2019 DigitalGlobe, Google Earth satellite imagery acquired the next day showed the flow deposit on Iliamna Volcano’s east-facing Red Glacier. Red Glacier has hosted many debris avalanches in prehistoric and historical time (Waythomas and others, 2000); its most recent avalanche of comparable size took place in May 2016. The June 21 rock and ice avalanche initiated less than 1 km [0.6 mi] from the volcano’s summit and traveled east for about 8 km [5 mi]. Measurements taken from satellite imagery estimate that the deposit covered an area spanning approximately 7.1×10^6 m^2 [76,000,000 ft^2]. Assuming an average deposit thickness of 0.5 m, this yields a volume of about 3.6×10^6 m^3 [39,000,000 ft^2].
"The second ice and rock avalanche on Iliamna Volcano, like the July 15 ice and rock avalanche on Mount Spurr, generated energetic seismic and acoustic signals that were recorded both locally and regionally (Toney and others, 2021). One local seismic station recorded at least 100 minutes of precursory seismicity (J. Caplan-Auerbach, Western Washington University, written commun., 2019). High-frequency signals associated with the event itself were recorded on local and regional (greater than 100 km [60 mi] away) seismic networks, and LP seismic signals were recorded more than 600 km [370 mi] away. Pre-avalanche seismicity was also documented during Red Glacier avalanches in 1994, 1997, 2003, and 2016 (Caplan-Auerbach and Huggel, 2007; J. Caplan-Auerbach, Western Washington University, written commun., 2016). The infrasound array in Dillingham detected acoustic waves from the event, as did the infrasound arrays in the Alaskan cities of Sand Point and Fairbanks."

From Orr and others, 2024: "A large ice and rock avalanche took place on the east flank of Iliamna Volcano on August 5, 2021, at 07:21 AKDT (15:21 UTC). Afterward, satellite imagery and oblique aerial photographs showed a dark-colored flow deposit on Iliamna Volcano’s east-facing Red Glacier. This avalanche initiated less than 1 km [0.6 mi] from the volcano’s summit and traveled at least 4 km almost due east, generating seismic and acoustic signals recorded locally and regionally. High-frequency signals were clearly recorded on Iliamna Volcano’s seismic network. Farther away, the KENI infrasound array at Kenai, and the HOM infrasound station in the City of Homer, Alaska, detected acoustic waves from the event.
"Red Glacier has hosted many avalanches historically and prehistorically (Waythomas and others, 2000); the most recent ice and rock avalanche of comparable size to this event occurred in June 2019 (Toney and others, 2021; Orr and others, 2023). These mass flows are generally composed of mostly ice and snow with smaller amounts of rock (involved through entrainment or as part of the original failure region). Avalanches on Red Glacier are highly mobile, traveling at mean speeds of ~50 meters per second (m/s) [~110 miles per hour] (CaplanAuerbach and Huggel, 2007) and peak speeds, estimated via numerical modeling and seismic force inversion, of greater than (>) 70 m/s [160 miles per hour] (Schneider and others, 2010; Toney and others, 2021). Judging by the size of the deposit and the amplitudes of the seismic and infrasound signals, the August 2021 event appears to be smaller than the June 2019 ice and rock avalanche."