Alaska Volcano Observatory

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."