ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, July 28, 2023, 12:56 PM AKDT (Friday, July 28, 2023, 20:56 UTC)
Two significant episodes of elevated eruptive activity occurred at Shishaldin Volcano over the past week on July 22–23, and July 26. The Aviation Color Code and Volcano Alert Level was raised to RED/WARNING for the first of these. Low-level eruption of lava confined to the summit crater continued between these events.
On July 22, a sustained ash plume with a height of 37,000 ft (11 km) above sea level erupted between 11:20 pm AKDT (07:20 UTC July 23) and 12:30 am on July 23 (08:30 UTC) and drifted NE from the volcano (https://www.avo.alaska.edu/images/dbimages/1690181861.mp4). The explosive eruption was preceded by a hours-long increase in seismic tremor, infrasound detection of explosions, and increased surface temperatures visible in satellite data. Satellite data showed a sulfur dioxide plume drifting after this event with an estimated mass of 10 kt (http://www.avo.alaska.edu/admin/imagedb/image.php?imageid=194580). The Alaska Volcano Observatory issued a Volcanic Activity Notice announcing of the lead up to explosive activity at 4:53 pm AKDT (00:53 UTC July 23). The Aviation Color Code and Volcano Alert Level was raised to Red/Warning at 11:43 pm AKDT (07:43 UTC, July 23) when the ash cloud was detected in satellite data and reported by pilots and then lowered back to Orange/Watch on July 23, 4:18 am AKDT (12:18 UTC) when the ash plume became detached from the volcano and was drifting downwind.
Following the eruptive sequence on July 22–23, satellite data continued to indicate strongly elevated surface temperatures consistent with low-level eruption of lava in the summit crater. Seismicity dropped to low levels and infrasound data did not detect significant explosive activity. Minor ash emissions to an altitude of 10,000 ft (3 km) above sea level were observed by pilots flying by the volcano on July 24.
On July 26, a sustained low-level ash plume with a height of 15,000 ft (6 km) above sea level erupted starting at 5:00 am AKDT (13:00 UTC) and drifted ENE from the volcano. Ash emissions, while diffuse, were visible in satellite data extending about 78 mi (125 km) from the volcano until clouds obscured views starting around 11:30 AKDT (17:30 UTC) (https://www.avo.alaska.edu/images/dbimages/1690565951.mp4). This eruptive event was preceded by about 7 hours of seismic tremor, infrasound detections of explosions, and 5 hours of increased surface temperatures visible in satellite data. Satellite data showed a sulfur dioxide plume drifting after this event with an estimated mass of 22 kt, the most significant sulfur dioxide plume of the current eruption (http://www.avo.alaska.edu/admin/imagedb/image.php?imageid=194581). The Aviation Color Code and Volcano Alert Level remained at ORANGE/WATCH for this event. AVO issued three Volcanic Activity Notices announcing the lead up to explosive activity at 2:11 am AKDT (10:11 UTC); for the onset of confirmed eruption of ash at 3:51 am AKDT (11:51 UTC); and for the decline in eruptive activity at 1:27 pm AKDT (21:27 UTC).
Satellite observations acquired after the eruptive sequences on July 22–23 and 26 show pyroclastic flow and mud flow deposits extending as far as 1.9 (3 km) down the N, NW and NE flanks and as far as about 1 mi (1.5 km) down the S and SE flanks. Ash deposits now cover the SW to NE flanks. No lava flows outside the crater were observed.
Following the July 26 episode, the volcano has been mostly obscured by clouds. A clear web camera view from today shows no significant activity (https://www.avo.alaska.edu/images/image.php?id=194583). Strongly elevated surface temperatures continue to be observed in satellite data which is consistent with low-level eruptive activity in the summit crater. Seismicity has remained at low levels and no infrasound signals of explosive activity have been detected.
Shishaldin has had five periods of elevated eruptive activity resulting in significant ash emissions during the current eruption which started on July 12. It is unknown how long this eruption will last, but previous eruptions of Shishaldin Volcano have lasted weeks to months with repeated cycles of activity similar to those seen in the last three weeks. Before the current activity, the 2019–2020 eruption of Shishaldin was the first to result in lava flows outside of the crater area since 1976. Minor eruptions in 2004 and 2014 erupted lava confined to the summit crater. Eruptions from Shishaldin have produced ash clouds in the past like those seen during the current eruption, most recently in January 2020. Satellite, seismic, and infrasound data is routinely monitored for signs of explosions that might produce ash clouds. In addition, ashfall forecast models are kept up to date on the public activity page (https://avo.alaska.edu/activity/Shishaldin.php).
Shishaldin Volcano is monitored by local seismic and infrasound sensors, web cameras, and a telemetered geodetic network. In addition to the local monitoring network, AVO uses nearby geophysical networks, regional infrasound and lighting data, and satellite images 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 continued at Great Sitkin over the past week, adding to a thick lava flow in the summit crater. Satellite radar data through July 19 show the eastern lobe of the lava flow moving into glacial ice surrounding the crater and causing it to deform and crack. Earthquake activity remained slightly elevated this week. Steaming from the surface of the active lava flow was observed in clear satellite views on July 24 and in web camera views on July 25. Weakly elevated surface temperatures were observed in satellite data on July 27.
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 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 continued at Cleveland volcano this week although at reduced levels from the previous week, and only one earthquake has been located since July 26. Many of the most recent earthquakes are located less than 4 miles (6 km) below the surface. While these earthquakes are small (less than magnitude 2), they are unusual for Cleveland.
Clouds obscured views of the volcano by satellite much of the week but normal steam and gas emissions from the summit crater were observed in web camera images on July 24 and 27 and 28.
Episodes of lava effusion and explosions can occur without advance warning. Explosions from Mount Cleveland are normally short-duration and only present a hazard to aviation in the immediate vicinity of the volcano. Larger explosions that present a more widespread hazard to aviation are possible but are less likely and occur less frequently.
Cleveland volcano is currently monitored with a five-station real-time seismic network and three nearby web cameras. Based on past events, explosive eruptions of Cleveland may occur with little or no warning. Rapid detection of an ash-producing eruption may be possible using a combination of seismic, infrasound, web camera, lightning, and satellite data.
Cleveland volcano forms the western portion of Chuginadak Island, a remote and uninhabited island in the east central Aleutians. The volcano is located about 45 miles (75 km) west of the community of Nikolski, and 940 miles (1500 km) southwest of Anchorage. The most recent significant period of eruption began in February 2001 and produced 3 explosive events that generated ash clouds as high as 39,000 ft. (11.8 km) above sea level. The 2001 eruption also produced a lava flow and hot avalanche that reached the sea. Since then, Cleveland has been intermittently active producing small lava flows, often followed by explosions that generate small ash clouds generally below 20,000 ft. (6 km) above sea level. These explosions also launch debris onto the slopes of the cone producing hot pyroclastic avalanches and lahars that sometimes reach the coastline.
Earthquake activity beneath Trident Volcano remained elevated over the past week. Most of the earthquakes were small with magnitudes below 1.0, except one with a magnitude of 2.5. Sequences of 14–24 mile-deep (23–38 km) low-frequency earthquakes and tremor were noted each day. No eruptive activity or other signs of unrest were observed in clear satellite and web camera images.
AVO issued an Information Statement on July 25 providing a more detailed update on the volcanic unrest at Trident Volcano and the broader Katmai volcanic cluster (https://www.avo.alaska.edu/news.php?id=1595).
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.
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.
Earthquake activity beneath Aniakchak volcano was low over the past week but slightly elevated from the previous week. Two to six small earthquakes were detected daily and one larger earthquake with a magnitude 2.7 was detected on July 27. No unusual activity was observed in mostly clear views of the volcano this week. Satellite radar data from July 21 showed no detectable suface changes at the volcano.
The current period of seismic unrest began in October 2022. Increases in seismic activity have been detected previously at other similar volcanoes, with no subsequent eruptions. We expect additional shallow seismicity and other signs of unrest, such as gas emissions, elevated surface temperatures, and additional surface deformation to precede any future eruption, if one were to occur.
Aniakchak volcano is monitored by local seismic and infrasound sensors, web cameras, regional infrasound and lightning networks, and satellite data.
Aniakchak volcano, located in the central portion of the Alaska Peninsula, consists of a stratovolcano edifice with a 6 mile (10 km)-diameter summit caldera. The caldera-forming eruption occurred around 3,500 years ago. Postcaldera eruptions have produced lava domes, tuff cones, and larger spatter and scoria cone structures including Half-Cone and Vent Mountain all within the caldera. The most recent eruption occurred in 1931 and created a new vent and lava flows on the western caldera floor while spreading ash over much of southwestern Alaska. Aniakchak volcano is 15 miles (25 km) southeast of the nearest community, Port Heiden, and 416 miles (670 km) southwest of Anchorage, Alaska.
Earthquake activity beneath Semisopochnoi volcano was low over the past week with only a handful of earthquakes detected all week. Satellite and web camera views of the volcano throughout the week were often obscured by clouds, but a small steam plume from the north crater of Mount Young was observed in web camera views on July 27 and 28.
The last evidence of ash emission from the volcano was on May 5, 2023 and consisted of a minor dusting of ash on the northwest flank of the north crater of Mount Young.
Small eruptions producing minor ash deposits within the vicinity of the active north crater of Mount Young and ash clouds usually under 10,000 ft. (3 km) above sea level have characterized recent periods of eruption since 2018. Additional ash-producing events could occur again with little or no warning.
Semisopochnoi volcano is monitored by local seismic and infrasound sensors, web cameras, regional infrasound and lightning networks, and satellite data.
Semisopochnoi volcano occupies the largest, young volcanic island in the western Aleutians. The volcano is dominated by a 5-mile (8 km) diameter caldera that contains a small lake and several post-caldera cones and craters. The age of the caldera is not known with certainty but is likely early Holocene. Prior to 2018, the previous known historical eruption of Semisopochnoi volcano occurred in 1987, probably from Sugarloaf Peak on the south coast of the island, but details are lacking. Another prominent, young post-caldera landform is Mount Young, a three-peaked cone cluster in the southwest part of the caldera. Mount Young has been intermittently active since 2018. The island is uninhabited and part of the Alaska Maritime National Wildlife Refuge. It is located 40 miles (65 km) northeast of Amchitka Island and 130 miles (200 km) west of Adak.
Matt Loewen, Acting Scientist-in-Charge, USGS, email@example.com, (907) 786-7497
David Fee, Coordinating Scientist, UAFGI, firstname.lastname@example.org, (907) 378-5460
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