ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, November 24, 2023, 12:00 PM AKST (Friday, November 24, 2023, 21:00 UTC)
Low level eruptive activity continued at Shishaldin Volcano over the past week. Although decreasing since November 21, seismicity remains elevated with seismic tremor and low frequency earthquakes observed daily. Weak explosions were observed in infrasound data on November 19 and 21. Elevated surface temperatures were observed in satellite images on November 20–22 with robust steam emissions observed in the occasional clear web camera views of the summit early in the week. Elevated sulfur dioxide emissions were observed in satellite data on November 19 and 20. Satellite radar images showed no obvious changed in the summit crater over the past week.
Thirteen significant explosive events have occurred at Shishaldin since July 12, 2023. These events have been preceded by increases in seismicity in the hours to days before they occur. It is unknown how long this period of ongoing activity will last. However, previous eruptions of Shishaldin Volcano have lasted weeks to months with repeated cycles of eruptive activity like those seen since July.
Local seismic and infrasound sensors, web cameras, and a geodetic network monitor Shishaldin Volcano. In addition to the local monitoring network, AVO uses nearby geophysical networks, regional infrasound and lightning data, and satellite observations to detect eruptions.
Shishaldin volcano, located near the center of Unimak Island in the eastern Aleutian Islands, is a spectacular symmetric cone with a base diameter of approximately 10 miles (16 km). A 660 ft wide (200 m) funnel-shaped summit crater typically emits a steam plume and occasional small amounts of ash. Shishaldin is one of the most active volcanoes in the Aleutian volcanic arc, with at least 54 episodes of unrest including over 28 confirmed eruptions since 1824. Most eruptions are relatively small, although the April-May 1999 event generated an ash column that reached 45,000 ft. (14 km) above sea level.
Slow eruption of lava is continuing at Great Sitkin Volcano. Seismicity has been low during the past week. The volcano was obscured by clouds in satellite data for most of the week but satellite radar observations through November 20 showed continued growth of the summit lava flow.
The current lava flow at Great Sitkin Volcano began erupting in July 2021. No explosive events have occurred since a single event in May 2021.
The volcano is monitored by local seismic, geodetic, and infrasound sensors, web cameras, regional infrasound and lightning networks, and satellite data.
Great Sitkin Volcano is a basaltic andesite volcano that occupies most of the northern half of Great Sitkin Island, a member of the Andreanof Islands group in the central Aleutian Islands. It is located 26 miles (43 km) east of the community of Adak. The volcano is a composite structure consisting of an older dissected volcano and a younger parasitic cone with a ~1 mile (1.5 km)-diameter summit crater. A steep-sided lava dome, emplaced during the 1974 eruption, occupies the center of the crater. That eruption produced at least one ash cloud that likely exceeded an altitude of 25,000 ft (7.6 km) above sea level. A poorly documented eruption occurred in 1945, also producing a lava dome that was partially destroyed in the 1974 eruption. Within the past 280 years a large explosive eruption produced pyroclastic flows that partially filled the Glacier Creek valley on the southwest flank.
Earthquake activity in the vicinity of Trident Volcano remains slightly elevated over the past week with small earthquakes observed each day within the Katmai group of volcanoes. The number of deeper volcanic earthquakes within the Katmai area continues to be low. No volcanic activity was observed in mostly clear satellite and web camera images.
The current period of unrest began in August 2022. Since then, most earthquakes have occurred within the shallow crust, with depths less than 4 miles (6 km) below sea level; however, deeper quakes (greater than 9 miles or 15 km depth) were notable during much of the unrest period. Surface uplift, low-frequency earthquakes, and seismic tremor – all consistent with magmatic activity – have been observed over this extended period as well.
Increases in seismic activity have been detected previously at Trident Volcano and other similar volcanoes and did not result in eruptions. We expect additional shallow seismicity and other signs of unrest, such as gas emissions, elevated surface temperatures, and ground movement, to precede any future eruption if one were to occur.
Trident Volcano is monitored by local seismic sensors, web cameras, regional infrasound and lightning networks, and satellite data.
Trident is one of the Katmai group of volcanoes located within Katmai National Park and Preserve on the Alaska Peninsula. Trident consists of a complex of four cones and numerous lava domes, all andesite and dacite in composition, that reach as high as 6,115 ft (1,864 m) above sea level. An eruption beginning in 1953 constructed the newest cone, Southwest Trident, and four lava flows on the flank of the older complex. This eruption continued through 1974 and produced ash (an initial plume rose to 30,000 ft or 9 km above sea level), bombs, and lava at various times. Fumaroles remain active on the summit of Southwest Trident and on the southeast flank of the oldest, central cone. Trident is located 92 miles (148 km) southeast of King Salmon and 273 miles (440 km) southwest of Anchorage.
Following the seismic swarm that began near Bogoslof around October 22, 2023, the frequency of earthquakes has now subsided to background levels. During the swarm 5 to 10 events per hour and up to a total of ~1,100 earthquakes were recorded in one week. The decline in seismicity has been observed over the past 3 weeks, with the last moderate earthquake (M2.7) recorded on November 9. No other signs of volcanic unrest have been detected. In response, AVO lowered the Aviation Color Code and Volcano Alert Level to UNASSIGNED earlier this morning.
Bogoslof volcano is monitored using a single local seismic station, distant seismic and infrasound instruments, satellite data, and lightning networks.
At least nine historical eruptions have been documented at Bogoslof volcano. The most recent occurred from December 2016 to August 2017 and produced seventy main explosive events that generated volcanic ash clouds that rose as high as 42,500 ft (13 km) above sea level, and greatly modified the topography of Bogoslof Island. Previous eruptions of the volcano have lasted weeks to months and have on occasion produced ash fall on the community of Unalaska. Eruptions of the volcano are often characterized by multiple explosive ash-producing events as well as the growth of lava domes.
Bogoslof Island is the largest of a cluster of small, low-lying islands making up the summit of a large submarine stratovolcano. The highest point above sea level is about 490 ft (150 m); however, the volcano is frequently altered by both eruptions and wave erosion and has undergone dramatic changes in historical time. The two main islands currently above sea level are Fire Island and Bogoslof Island, both located about 61 miles (98 km) northwest of Unalaska/Dutch Harbor, 76 miles (123 km) northeast of Nikolski, and 93 miles (149 km) northeast of Akutan. The volcano is situated slightly north (behind) the main Aleutian volcanic front. Bogoslof volcano is within the U.S. Fish and Wildlife Service Alaska Maritime National Wildlife Refuge and is habitat for marine mammals and seabirds (https://www.fws.gov/refuge/alaska_maritime/).
Matt Loewen, Acting Scientist-in-Charge, USGS mloewen@usgs.gov (907) 786-7497
David Fee, Coordinating Scientist, UAFGI dfee1@alaska.edu (907) 378-5460
The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.