USGS Volcano Notice - DOI-USGS-AVO-2023-09-22T11:03:55-08:00

A significant explosive event at Shishaldin occurred on September 15, 2023, generating an ash cloud that reached an altitude of approximately 30,000 ft (9.1 km) above sea level. Ash emissions began at 5:10 PM AKDT on September 15, 2023 (01:10 UTC on Sept 16) after a period of rapidly increasing seismicity. The Alaska Volcano Observatory issued a notice at 2:52 PM AKDT (22:52 UTC) to report the increased seismicity and likelihood of an eruption. Once the explosive event began, AVO increased the Aviation Color Code and Volcano Alert Level to RED/WARNING. The National Weather Service issued SIGMETs for airborne ash and Special Weather Statements for ashfall in response to the event. The ash emissions were accompanied by infrasound and volcanic lighting and resulted in reported ashfall in False Pass, King Cove, and nearby marine waters. Activity declined beginning at 6:30 PM AKDT (02:30 UTC September 16), although elevated seismicity, infrasound, ash emissions and ashfall continued until around 9:00 PM AKDT (05:00 UTC September 16). AVO lowered the Aviation Color Code and Volcano Alert Level to ORANGE/WATCH at 12:44 AM AKDT (08:44 UTC) on September 16. Significant sulfur dioxide emissions were observed in satellite imagery associated with this event. 

Since this event, Shishaldin seismicity declined but has remained elevated over the course of the week with tremor and earthquakes up to magnitude 1.6 detected. Elevated surface temperatures, steaming from the vent, and new small lahars on the upper flanks of the volcano have been observed in satellite and web camera views this week. These minor deposits are likely due to collapse of previously accumulated lava near the summit crater and can occur at any time without associated with explosive activity. No explosions have been detected in geophysical data. 

There have been ten periods of elevated eruptive activity resulting in significant ash emissions and mass flows of volcanic debris on the volcano's flanks since the onset of the current eruption. The last four periods of elevated eruptive activity have occurred approximately every 10.5 days and all ten have been preceded by increases in seismicity in the hours before they occur. If this inter-eruptive period regularity holds, the next event may occur around September 25 or 26. 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.  

The ongoing eruptive period started on July 12, and it is unknown how long this eruptive episode will last. However, previous eruptions of Shishaldin Volcano have lasted weeks to months with repeated cycles of activity like those seen over the last month. 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.    

Slow eruption of lava is continuing at Great Sitkin Volcano. Weakly elevated surface temperatures and steaming were observed from the eruptive vent region in satellite and web camera images and earthquake activity remained slightly elevated this week. Satellite imagery and radar observations 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. 

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 throughout the week. Minor ground deformation is possibly ongoing but difficult to resolve in current data. Views of the volcano in satellite and web camera data were mostly partly cloudy and showed nothing indicative of volcanic unrest.      

The current period of unrest began with a swarm of earthquakes 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.