Alaska Volcano Observatory

From Orr and others, 2024: "In late July 2016, Great Sitkin Volcano entered a period of increased unrest characterized by an elevated frequency of earthquakes, anomalous steaming from its summit crater, and small explosive events (Dixon and others, 2020). Similar small earthquakes and steaming from the summit crater continued throughout 2020.
"AVO located 3,393 earthquakes at Great Sitkin Volcano during 2020, a large increase from earlier years. This increase may, in part, reflect network upgrades made during the 2019 field season that replaced older analog sensors with broadband sensors capable of digital telemetry. This new instrumentation has proven more reliable at Great Sitkin Volcano than the older equipment, improving AVO’s ability to detect and locate earthquakes in the area.
"In late January 2020, Great Sitkin Volcano began to experience an increase in earthquake activity that was interpreted as unrelated to the recently improved seismic detection capabilities, prompting AVO to raise the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY on February 26. This activity peaked in late March and then slowly declined throughout the remainder of 2020. In response to the declining rates of seismicity, the Aviation Color Code and Volcano Alert Level were lowered back to GREEN and NORMAL on October 21. No seismic or infrasound signals indicative of explosive events were detected at the volcano during 2020.
"The hypocenters of earthquakes at Great Sitkin Volcano in 2020 were principally clustered within the shallow crust, extending from the summit to roughly 10 km [6 mi] depth below sea level. Earthquakes were also recorded at depths between 10 and 35 km [6 and 22 mi]; these had waveforms and frequency contents suggesting both volcano-tectonic (VT) and deep LP events. Located events ranged in magnitude from ML -1.2 to ML 3.5. The ML 3.5 event occurred on March 6 (at 15:31 UTC; 05:31 HAST) under the east rim of the summit crater at a depth of 0.2 km [0.1 mi]. This earthquake was the strongest event recorded beneath Great Sitkin Volcano since its period of unrest began in 2016. For comparison, the largest earthquake recorded beneath the volcano’s edifice since monitoring began in 1999 was ML 4.3. That event took place on May 28, 2002, and was located beneath the southeast flank of the volcano (Pesicek and others, 2008).
"The other notable earthquake activity at Great Sitkin Volcano in 2020 consisted of a cluster of seven earthquakes that occurred between January 31 and February 1. These seven events ranged from ML -0.6 to 0.27 and had depths ranging from 22.6 to 27.0 km [14.0 to 16.8 mi].
"AVO noted no major changes or unusual activity at the summit crater and dome during 2020, although the degree of visible steaming decreased compared to the 2016-2019 period (Dixon and others, 2020; Cameron and others, 2023; Orr and others, 2023). Photographs taken by passing airplanes in March, shortly after the ML 3.47 earthquake of March 6, show small snow-free areas and minor steaming at the summit of the volcano. Satellite imagery of Great Sitkin Volcano acquired in June and July indicated weakly elevated surface temperatures."