USGS Volcano Notice - DOI-USGS-AVO-2023-10-20T11:04:01-08:00

ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, October 20, 2023, 12:53 PM AKDT (Friday, October 20, 2023, 20:53 UTC)

SHISHALDIN (VNUM #311360)
54°45'19" N 163°58'16" W, Summit Elevation 9373 ft (2857 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE

No explosive activity occurred at Shishaldin this past week, though observations suggest degassing of magma at shallow levels beneath the volcano's summit.

Seismic unrest consisted primarily of tremor and low-frequency earthquakes and has remained fairly consistent since the last explosive event on October 3. Elevated sulfur dioxide gas emissions were detected in satellite data frequently over the past week. Elevated surface temperatures were observed in satellite views nearly every day, though temperatures are inconsistent with lava at the surface. Clear webcam views this week showed degassing from the summit and a hot, linear feature that extends over 1000 feet down the northeast flank from the summit.

For a full update on the current eruption of Shishaldin, please see the recent Information Statement: https://avo.alaska.edu/activity/report.php?type=1&id=404251&mode=hans

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 24 confirmed eruptions since 1775. Most eruptions are relatively small, although the April-May 1999 event generated an ash column that reached 45,000 ft. above sea level.

GREAT SITKIN (VNUM #311120)
52°4'35" N 176°6'39" W, Summit Elevation 5709 ft (1740 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE

Slow eruption of lava is continuing at Great Sitkin Volcano. Weakly elevated surface temperatures were observed from the eruptive vent region in satellite images and there were a few small earthquakes each day. Satellite radar observations through October 17 showed continued growth of the eastern branch 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.

TRIDENT (VNUM #312160)
58°14'3" N 155°6'9" W, Summit Elevation 3599 ft (1097 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

Earthquake activity continued this past week but at lower levels than previous weeks, with 23 earthquakes located in the vicinity of Trident, and 66 located within the broader Katmai group of volcanoes. The largest event in the area was a M2.3 quake located about 0.6 miles (1 km) SW of nearby Mageik on Thursday, October 19. During the past two weeks, the number of deeper earthquakes under Trident has tapered off, with only one event on October 13 located at 18 miles (29 km) below sea level. This may suggest that deeper movement of fluids and/or magma is diminishing.

This week, satellite and web camera data showed no significant volcanic activity.

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.

CONTACT INFORMATION:

Matt Haney, Scientist-in-Charge, USGS mhaney@usgs.gov (907) 786-7497

Jessica Larsen, Acting Coordinating Scientist, UAFGI jflarsen@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.