ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, September 8, 2023, 12:41 PM AKDT (Friday, September 8, 2023, 20:41 UTC)
The most recent significant explosive event at Shishaldin Volcano, the ninth to occur during this eruptive period, happened earlier in the week on September 5. A several-hour increase in seismicity began gradually at about 3:00 am AKDT and rose more sharply around 8:30 am AKDT. A pilot in the vicinity reported an eruption cloud to about 25,000 ft above sea level and rising at 8:42 am AKDT (16:42 UTC). The initial ash cloud rose to 32,000 ft (9.7 km) above sea level and drifted to the south-southeast. The Aviation Color Code was raised to RED and the Volcano Alert level to WARNING at 9:00 am AKDT (17:00 UTC). In addition to strong tremor and sustained explosions, the eruption produced volcanic lightning detected by the World Wide Lightning Location Network (WWLLN). Seismicity decreased around 11:00 am AKDT (19:00 UTC) and satellite data confirmed that the altitude of ash emissions declined to about 25,000 ft (7.6 km) above sea level. The Aviation Color Code and Volcano Alert Level were reduced to ORANGE/WATCH at 2:14 pm AKDT (22:14 UTC), where it has remained throughout the rest of the week. Recent satellite analysis shows extensive pyroclastic debris flows on most flanks of the volcano due to the recent explosive event. The debris flow deposits extend from 0.71 miles (1.15 km) to 2 miles (3.31 km) from the crater rim.
Low-level activity has continued over the remainder of the week. Seismicity remains above background and elevated surface temperatures were observed in the summit crater throughout the week.
Collapse of accumulated lava near the summit crater can occur without warning and generate hot mass flows on the upper flanks and small volcanic ash clouds. This type of activity remains a proximal hazard as the deposits collapse occasionally without warning and generate hot flows on the upper flanks and small clouds of volcanic ash.
Shishaldin has had nine periods of explosive eruptive activity resulting in significant ash emissions since July 12. It is unknown how long this eruptive period will last. Previous eruptions of Shishaldin Volcano have lasted weeks to months with repeated cycles of activity similar to those seen since July 12. Before the current activity, the 2019–2020 eruption of Shishaldin was the first to result in lava flows outside the crater area since 1976. Minor eruptions in 2004 and 2014 erupted lava confined to the summit crater. Previous eruptions of Shishaldin have produced ash clouds like those seen during the current eruption, most recently in January 2020. Routine evaluations of satellite, seismic, and infrasound data provide warning of unrest associated with the production of ash clouds. In addition, ashfall forecast models are kept up to date on the public activity page (https://avo.alaska.edu/activity/Shishaldin.php).
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.
Slow eruption of lava is continuing at Great Sitkin Volcano. Weakly elevated surface temperatures were observed in satellite images and earthquake activity remained slightly elevated this week. Typical steaming from the vent region of the lava flow was observed in satellite images on several days. AVO geology crews spent about three hours working on the active flows on September 3 and report that the flows are warm and steaming and moving at about 1 meter every 3 to 4 days.
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 beneath Trident Volcano continued over the past week, with volcanic-tectonic earthquakes noted most days and a few deep, low-frequency earthquakes observed early in the week. No new observations of ground uplift have been reported. Views of the volcano in satellite and webcam data were partly to mostly clear and showed nothing indicative of volcanic unrest.
The current period of seismic unrest began on August 24, 2022. Earthquake depths at the beginning of the swarm were mainly deep, around 16 miles (25 km) below sea level and became progressively shallower to around 3 miles (5 km) over the following four days. Since late August 2022, most earthquakes have occurred within the shallow crust, with depths less than 4 miles (6 km) below sea level; however, an increasing number of earthquakes have been occurring deeper (greater than 9 miles or 15 km depth).
Starting in May 2023 an increase in low-frequency earthquakes and tremor has been observed—in addition to the regular earthquakes—near Trident Volcano. Such low-frequency events are often associated with the movement of magma or volcano-related fluids within the ground.
Ground uplift at Trident Volcano has also been detected in satellite radar data. Snow cover prohibits winter observations, which limits our ability to provide precise timing, but data from June 3, 2023, indicates that about 2 in (5 cm) of ground uplift has occurred since October 6, 2022. Uplift is most significant on the volcano’s south flank.
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.
Matt Haney, Scientist-in-Charge, USGS firstname.lastname@example.org (907) 786-7497
David Fee, Coordinating Scientist, UAFGI email@example.com (907) 378-5460
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