From Cameron and others, 2017: "Low-frequency events at Semisopochnoi Island were first noted on June 1, 2014. Semisopochnoi's seismic network had been returned to service on May 22, 2014, only 2 weeks prior to these events. The swarm began on June 9 at about 18:00 UTC (10:00 a.m. AKDT), and the AVO network recorded about 40 volcanic-tectonic earthquakes over the next 2 days. These earthquakes were small (less than M2) and clustered around Mount Cerberus in the center of the caldera. Depths ranged from 2 to 10 km (1.2 to 6.2 mi).
"Tremor was first noted on June 12, coincident with an increase in the rate of earthquakes recorded. Over the next day, the number of earthquakes doubled over the total located in the previous 3 days. The number of located earthquakes, all volcano-tectonic, doubled again on June 14, marking the peak daily earthquake count of the sequence (223 earthquakes). Throughout the increase in activity, the depth range of the events and epicentral area did not change. On June 13, AVO issued a VAN upgrading the Aviation Color Code and Volcano Alert Level to YELLOW/ADVISORY based on the ongoing swarm . Following the peak in earthquake activity on June 14, the number of located events decreased to 20-30earthquakes located in the last week of June, and further decreases in seismicity continued into July and August.
"On June 23, a M7.9 earthquake occurred in the region, 73 km (46 mi) west of Semisopochnoi Island at a depth of 118 km (73 mi) in the subducting slab. It was accompanied by more than 2,500 aftershocks, 60 with magnitudes of 4.0 or greater. There was no direct causality between the M7.9 earthquake and the Semisopochnoi volcanic-tectonic earthquake (VT) swarm; AVO did not note any change in the rate of VT seismicity at Semisopochnoi following the M7.9 earthquake.
"Zhong Lu of Southern Methodist University evaluated the deformation related to the volcanic unrest. More than 10 cm (4 in.) of inflation occurred between 2003 and June 26, 2014, about one-half of which occurred between June 15 and June 26, 2014. Maximum uplift was approximately in the center of the caldera. Modeled source depth ranges from 5 to 10 km (3 to 6 mi). (Zhong Lu, written commun., 2015).
"The consensus interpretation of the 2014 swarm is that it represents an intrusion of magma. This would be consistent with the style and character of the seismic swarm as well as the deformation signal. Although the seismograph network was not operational before May 22, the absence of significant activity between May 22 and June 9 suggests that the intrusion occurred in mid-June. An earlier pulse of magma possibly was emplaced before May 22, but without an operational seismograph network, this cannot be verified.
"In response to the swarm, AVO instituted heightened seismic watch schedules, requested that NOAA initiate HYSPLIT model ash cloud trajectory runs, and solicited pilots and the U.S. Coast Guard to report any unusual volcanic activity (such as steaming or ground cracks). Daily satellite analysis by AVO staff included careful examination of the volcano using all available imagery. Throughout the sequence, no surface activity was noted in the few clear-weather views of the island. The volcano is remote, and aerial reconnaissance by AVO staff was determined to be too costly to obtain.
"The 2014 unrest was the first significant departure from background at Semisopochnoi since installation of the seismic network in 2005. Background seismicity at Semisopochnoi had been relatively low (fewer than 50 earthquakes located per year until 2014), with short periods of weak seismic tremor. The last confirmed eruption at Semisopochnoi was from a satellitic vent in 1987 (Miller and others, 1998). The volcano, however, was deforming. Lu and Dzurisin (2014) discovered an area about 2 km (1.2 mi) in diameter that subsided during 2004-2010 at a rate of about 10 mm/yr (0.4in/yr). They suggested compaction of young volcanic deposits in the caldera as a mechanism to explain this subsidence; alternative hypotheses include localized subsidence due to depressurization of a shallow hydrothermal system or localized changes in the groundwater table."

From Cameron and others, 2023: "Activity at Semisopochnoi Island, in the Rat Islands, Alaska, began in September 2018 and continued through the fall. It was characterized by sporadic, weak eruptive activity from the north cone of Mount Young. Though seismic tremor and explosion signals captured most of this activity, AVO also made infrequent infrasound detections and satellite observations of steam emissions and small ash deposits. Retrospective analysis showed that activity continued through the end of the year, but real-time observations were limited by a prolonged data outage beginning November 1, 2018.
"The first activity detected at Mount Young in 2018 included two minor seismic bursts on September 8 at 08:10 and 09:44 UTC (September 7 at 22:10 and 23:44 HADT). This seismicity was preceded by weak tremor beginning on August 25, but AVO only recognized the tremor during retrospective analyses. Satellite imagery taken during the summer showed a seasonal lake within the crater of the north cone on Mount Young, but satellite imagery from September 4 showed the crater steaming, and satellite imagery from September 5 showed the lake to be three times larger than normal. On September 10, ash deposits extending about 1 km west of the north cone were visible in satellite imagery, and within the crater, AVO noted a new tephra cone about 75 m [250 ft] in diameter.
"Seismicity increased on September 16; strong tremor beginning at 16:31 UTC (07:31 HADT) caused AVO to raise the Aviation Color Code and Volcano Alert Level from GREEN and NORMAL to YELLOW and ADVISORY. The lake in the crater dried up around the same time, and on September 15 and 17, AVO observed fumaroles and steam in satellite imagery. On September 17, an increase in seismicity and tremor strength, in addition to the recognition of the ash deposit in the September 10 satellite image, led AVO to increase the Aviation Color Code and Volcano Alert Level again to ORANGE and WATCH.
"From September 16 to 29, seismicity alternated between continuous tremor and tremor bursts. At the same time, atmospheric propagation conditions improved between Semisopochnoi Island and the infrasound array on Adak Island (a 13-minute delay), leading to the first infrasound explosion detection on September 21. Satellite imagery indicated weakly elevated surface temperatures on September 19 and the presence of steam on September 20. Tephra deposits were seen east and southeast of the north cone of Mount Young in satellite imagery the following week. A small pit formed in the cone’s crater by September 27, and between September 29 and October 1, a new tephra cone grew around the pit and thick tephra deposits accumulated on the adjacent crater floor.
"Weak tremor was reported on October 2–4, 8, and 10, and a small SO2 plume was detected in OMPS satellite data on October 6, but no infrasound was detected in early October. AVO saw no evidence of eruptive activity in satellite images during this period and the crater lake returned by October 11. This lull in activity led AVO to lower the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY on October 12.
"Strong seismic tremor began again on October 26 at 04:47 UTC (October 25 at 19:47 HADT), accompanied by a small ash plume reaching an altitude of 12,000 ft (3,700 m) ASL and infrasonic tremor detections at the array on Adak Island. This activity triggered an increase in the Aviation Color Code and Volcano Alert Level again to ORANGE and WATCH. Small explosions, accompanied by ground-coupled airwaves and some infrasound detections, continued until November 1. Clouds obscured all satellite views during this time, but a possible SO2 signal was detected in OMPS satellite data on October 30.
"Unfortunately, the satellite connection to the real-time seismic data receive facility (on Amchitka Island) failed on November 1 and was not recovered until June 2019. During the outage, AVO observed no changes in satellite imagery and detected no explosions from the nearby Adak Island infrasound array. Owing to a lack of evidence of ongoing eruptive activity, AVO lowered the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY on November 21. With continued apparent quiescence, AVO changed the Aviation Color Code and Volcano Alert Level to UNASSIGNED on December 19. After the missing seismic data were recovered in 2019, a retrospective analysis found that strong tremor and likely eruptive activity continued through at least November 11, 2018. Satellite imagery showed steam in the crater on December 1, 10, and 19, 2018.
"Although the unrest in 2014-2015 was associated with considerable deformation, InSAR measurements have shown little to no deformation since then, and even the 2018 activity lacked associated surface displacement. In a preliminary analysis of Sentinel-1 interferograms, a small signal occasionally appeared on the west flank of Mount Young’s north cone, but this signal could simply reflect surface deformation, atmospheric noise, or a change in spectral properties due to freshly deposited ash. A more spatially extensive signal might exist in the center of the caldera near the modeled deformation source of the 2014-2015 inflationary episodes (DeGrandpre and others, 2019). This signal could represent deformation from a deeper source, but the magnitude of surface displacement is so small (less than 1 millimeter [0.04 inches]) that the mechanisms producing the signal would be more likely related to processes like gas exsolution or crystallization rather than magma volume flux. Additional analysis is required to eliminate atmospheric noise as a possible source of the apparent displacement."

From Orr and others, 2023: "Activity at the north cone of Mount Young, which began in September 2018, continued in 2019 with sporadic eruptive activity. Seismic tremor and explosion signals captured most of this activity, along with infrequent infrasound detections and occasional satellite observations of steam and small ash deposits. Although the eruptive style and geophysical characteristics of the 2019 unrest were similar to those of 2018, AVO’s ability to observe them in real time was limited for the first half of the year owing to a prolonged data outage. This outage, caused by a communication failure at the regional data network telemetry hub in Amchitka, lasted from November 2018 to June 2019, at which point communications were restored and the missing data were recovered.
"Owing to the data outage, the first activity observed at the volcano in 2019 comprised satellite observations of steam in January and a small ash deposit on June 2. Although the north crater of Mount Young has persistently steamed since 2018, the ash deposits observed on June 2 appear to be a more recent change, having followed tremor bursts in mid-May that probably correspond to when they erupted. AVO began receiving seismic data again on June 11 but kept the Aviation Color Code and Volcano Alert Level at UNASSIGNED until early July, when increased tremor prompted a change to YELLOW and ADVISORY. On July 18, the detection of SO2 emissions in TROPOMI satellite data and the recording of ground-coupled airwaves triggered AVO to raise the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH. SO2, steam plumes, ground-coupled airwaves, and infrasound were detected throughout the summer, but no ash deposits were observed. After a period of quiescence, AVO lowered the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY on September 18.
"Beginning on December 7, the detection of many explosions by regional infrasound sensors indicated a renewal of activity at Semisopochnoi Island, triggering AVO to raise the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH. In addition to producing infrasound, the explosions were accompanied by ash emissions visible in satellite imagery, ground-coupled airwaves, and one SO2 detection (on December 12). No ash plumes reaching altitudes greater than 10,000 ft (3,000 m) ASL and no large ash deposits were observed during this eruptive period. The last activity recorded at the volcano in 2019 was an infrasound detection on December 18. On January 9, 2020, AVO lowered the Aviation Color Code and Volcano Alert Level back to YELLOW and ADVISORY."
From Orr and others, 2024: "The overall seismicity remained elevated, however, and on February 15, a series of small explosions and tremor bursts were detected. No ash emissions or deposits were identified in satellite imagery, which was frequently cloudy, but the seismic activity led AVO to return the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH. Seismicity did not increase further and clear imagery later confirmed the volcano was not erupting, although steam emissions were visible. On February 26, AVO lowered the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY.
"Activity in mid-March was characterized by nearly continuous tremor and frequent, small explosion signals, but no observations of ash plumes. Regional infrasound arrays detected an explosion at the volcano on March 11, followed by more on March 15-16, all of which were accompanied by ground-coupled airwaves recorded on the local seismic network. During this March explosive period, a dark ash deposit appeared around the north cone of Mount Young and TROPOMI imagery showed a possible SO2 plume. In response to the infrasound and remote sensing observations, AVO raised the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH on March 16. This activity was short-lived; by March 22, a partly cloudy WorldView-1 image showed that a robust steam plume emanated from the north cone of Mount Young and a water lake had appeared deep within its crater. After two weeks with no sign of eruption activity, AVO lowered the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY on April 1, 2020.
"The volcano emitted steam and SO2 regularly throughout the spring and summer, and by June 15, the crater lake had mostly dried up. Planet Labs and Sentinel-2 satellite imagery from June 21 captured an ash deposit extending nearly 4 km [2.5 mi] southward from the north cone crater, accompanied by robust steam emissions and a high-temperature short-wave infrared anomaly within the crater. The appearance of this deposit was associated with ground-coupled airwaves detected on June 16 and 17, but no infrasound. Seismicity at the volcano initially remained elevated, appearing as low-level tremor and small earthquakes, but AVO detected no further explosions in 2020 and its activity waned in the fall. An outage of local real-time monitoring data began on November 11, 2020. Considering the absence of data, the lack of eruptive activity detected since June, and the decaying trend in seismicity, AVO changed the Aviation Color Code and Volcano Alert Level of Semisopochnoi Island to UNASSIGINED on November 20, 2020."

From Orr and others, 2024: "Eruptive activity at Semisopochnoi Island, which began in September 2018, was characterized in 2021 by frequent, low-level ash emissions and explosions from the north cone of Mount Young. An equipment failure on Amchitka Island meant that activity observations during the first half of the year were limited to those from satellite imagery and regional infrasound and seismic stations. In early June 2021, the equipment on Amchitka Island was repaired and the local network on Semisopochnoi Island was completely upgraded. These upgrades involved a change from analog to digital telemetry; the replacement of short-period vertical-component seismometers with three-component broadband seismometers; the addition of broadband infrasound sensors at stations CERB, CESW, and CEPE; and the addition of webcams at stations CEPE and CETU. A six-element infrasound array was also installed on Amchitka Island in late May 2021 to provide improved regional monitoring.
"Seismic activity was low prior to the network outage; thus, the first activity noted in 2021 was a satellite observation of a small ash deposit from the north cone of Mount Young on February 6. In response, AVO raised the Aviation Color Code and Volcano Alert Level from UNASSIGNED to YELLOW and ADVISORY the next day. Ash emissions appeared in satellite imagery on February 8, which led AVO to further elevate the volcano to ORANGE and WATCH.
In the following months, satellites recorded intermittent ash emissions and regional infrasound sensors detected small explosions. Ash emissions increased considerably on April 16, when an ash cloud as high as ~20,000 ft (~6,100 m) ASL extended more than 350 km [220 mi] from the north cone of Mount Young. This change prompted AVO to raise the Aviation Color Code and Volcano Alert Level from ORANGE and WATCH to RED and WARNING. The next day, ash emissions declined and the volcano was lowered back to ORANGE and WATCH. Satellite imagery and regional infrasound sensors recorded intermittent, small explosions over the next few months.
"Seismicity was initially low after the local geophysical sensors were restored in June, but both seismicity and the number of discrete explosions increased into July and August. Seismic activity remained elevated, although variably so, for the rest of the year, during which time tremor and explosion signals dominated the data. On September 20, increased ash emissions observed in satellite imagery again prompted AVO to raise the Aviation Color Code and Volcano Alert Level from ORANGE and WATCH to RED and WARNING. A large SO2 cloud associated with this increase in activity appeared in TROPOMI satellite data. The following day, ash emissions declined, and Semisopochnoi Island was lowered to ORANGE and WATCH. The activity at the volcano for the rest of the year consisted of low-level ash emissions, seismic tremor, phreatomagmatic explosions, and many satellite SO2 detections.
"In combination with carrying out geophysical maintenance, AVO geologists sampled ash deposits on Semisopochnoi Island on May 30 and June 1. Tephra samples were collected across all parts of the island, but the samples most proximal to Mount Young, taken from a 4-cm-thick [1.6-in-thick] deposit, were from 1.6 km [1 mi] southwest of the center of the north cone. The tephra was fine-grained, easily remobilized, and present over the entire island except along the north coast. The coarsest grains in the samples were less than 4 mm [0.16 in] in diameter. Approximately 50 percent of the material was less than 50 micrometers [0.002 inches] in diameter. No larger ballistic particles were observed, including on the flanks of the north cone. The internal structures of deposits were not preserved; thus, the samples represent a bulk aggregate of deposits from throughout the spring of 2021.
"A microscopic analysis of the samples showed that they are mostly crystal-lithic grains or loose crystals. A minor proportion of the grains are scoriaceous juvenile material characterized by low vesicularity and microlitic glass. Microlite phases comprise plagioclase, clinopyroxene (augite and pigeonite), orthopyroxene, and iron-titanium oxides."
Frequent low-level ash emissions and small explosions continued through the first half of 2022. Starting June 12, there was a pause in activity that led AVO to reduce the alert level and color code to YELLOW/ADVISORY on July 8, but they were increased again to ORANGE/WATCH on August 21, after a renewal of activity. On that day ash emissions reached almost 20,000 ft asl.
Minor eruptive activity continued through mid-September. On September 29, AVO again lowered the color code and alert levels to YELLOW/ADVISORY due to an eruptive pause and a decline in seismic activity.
Starting on October 31, a few small explosions occurred. This, along with tremor starting on November 5, led AVO to raise the alert levels to ORANGE/WATCH once more on November 7. A partial network outage starting that day reduced AVO's ability to track the eruption, but frequent steam plumes were observed in this period.
Due to a lack of ash emissions or explosions, AVO reduced the alert levels to YELLOW/ADVISORY on November 23.
On December 27, webcam images showed fresh ash on the snow and a steam plume. The alert levels were raised to ORANGE/WATCH on December 28, 2022.
Ash deposits and explosion signals continued into mid January, 2023. Steam plumes and seismic activity continued after that, but the lack of explosive activity caused AVO to lower the alert levels to YELLOW/ADVISORY on February 22, 2023.
On March 19, fresh ash was once again seen in webcam images, and explosions were detected. The alert levels were returned to ORANGE/WATCH. Minor eruptive activity continued through early May.
On May 17, the alert levels were lowered to YELLOW/ADVISORY.
No further eruptive activity was observed after this. Seismic activity declined to low levels as well. The alert levels were lowered to GREEN/NORMAL on August 3, 2023.