Alaska Volcano Observatory

From Cameron and others, 2023: "During 2018, AVO located more than 2,300 earthquakes at Great Sitkin Volcano, principally clustered in the shallow crust (extending from the summit to roughly 10 km [6.2 mi] below sea level). Additional earthquakes were also located between 15 and 35 km [9 and 22 mi] depth, with waveforms and frequency contents suggesting both VT and deep LP earthquakes. The magnitudes of these located events ranged from an ML of less than −1.0 to 2.48 - the largest event took place on August 31, 2018. Unfortunately, the Great Sitkin Volcano seismic network experienced several station failures in 2018, most importantly at stations GSTD and GSSP, which impaired AVO’s ability to locate earthquakes and resulted in the data gaps. The most notable of these failure periods spanned early November 2017 to mid-January 2018. Shorter-term failures of station GSSP also compromised AVO’s earthquake locating capabilities during the winter of 2018-2019. AVO identified several tremor bursts associated with this unrest during 2018, with most taking place between June and December. Interpreted as small explosion events, the tremor bursts contained a variety of waveforms with impulsive to emergent onsets, extended codas, and frequency between 1 and 15 hertz. The bursts were commonly associated with increased earthquake activity, but none produced infrasound signals identifiable by sensors in the nearby City of Adak, Alaska. The explosion of June 10, 2018, proved especially noteworthy when a Sentinel-2 satellite image acquired on June 11 at 23:00 UTC (14:00 HADT) showed a 2-kilometer [1.2 mile] long ash deposit extending southwest from the summit of Great Sitkin Volcano. Additional photographs of the summit area taken from a passing aircraft a week later showed an ash deposit on the snow. This deposit presumably came from the June 10 explosion.
To characterize the size and progression of the Great Sitkin Volcano explosions, AVO measured the durations of their signals using methodology described by Searcy and Power (2020). Tremor events lasting less than 2 minutes were excluded, though several took place during the unrest. For many of the signals, some uncertainty remains on whether they reflect explosions, short volcanic tremor episodes, or more minor steam bursts. Except for the event on June 10, which produced an identifiable ash deposit, any of these events could have been produced by any of the previously listed mechanisms.
"In response to the volcanic activity, AVO made four changes to Great Sitkin Volcano’s Aviation Color Code and Volcano Alert Level during 2018. The volcano began the year at YELLOW and ADVISORY, but on January 18, after the number of earthquakes had fallen to background levels, AVO lowered the Aviation Color Code and Volcano Alert Level to GREEN and NORMAL. It raised them again to YELLOW and ADVISORY on June 10 in response to the explosion signal that took place that day. The Aviation Color Code and Volcano Alert Level were lowered back to GREEN and NORMAL on June 27 after declining earthquake activity, then raised to YELLOW and ADVISORY again on July 1 as earthquake activity increased. Great Sitkin Volcano remained at YELLOW and ADVISORY for the rest of 2018."
From Orr and others, 2023: "On February 2 [2019], because of declining earthquake activity, the Aviation Color Code and Volcano Alert Level were lowered from YELLOW and ADVISORY, where they had been since July 1, 2018, to GREEN and NORMAL."