Alaska Volcano Observatory

From Orr and others, 2024: "Early 2021 was quiet at Mount Cleveland, so the volcano remained at UNASSIGNED—the monitoring network was insufficient to locate earthquakes, so the volcano did not warrant a GREEN/NORMAL status. On March 10, however, an earthquake large enough to be detected 100 km away on Umnak Island took place near Mount Cleveland. This earthquake, combined with the detection of weak thermal anomalies and SO2 emissions starting the following week, suggested an increased potential for an eruption. In response, AVO raised the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY on March 20. Soon afterward (March 26), an earthquake of local magnitude (ML) 4.3 took place near Mount Cleveland.
High-resolution synthetic aperture radar (SAR) data spanning March 9 to March 20 recorded slight subsidence within the crater, which seemed to correlate with the appearance of elevated surface temperatures and gas emissions there. The crater was previously floored by cold rubble, so subsidence above the conduit may have formed openings that allowed heat and gases to escape more readily. Near-infrared temperatures in the crater rose above 600 ºC [1100 ºF], indicating magma near the surface.
"Detections of thermal anomalies, SO2 emissions, a summit plume, and crater floor subsidence continued with little change over the next several months. However, by late summer, signs of unrest had declined: the volcano had quieted seismically, subsidence had ceased, gas emissions were no longer being detected, and thermal anomalies had declined in both strength and frequency. In response to this dwindling activity, AVO changed the Aviation Color Code and Volcano Alert Level to UNASSIGNED on October 20. Detections of elevated surface temperatures continued occasionally through the end of the year, but activity at the volcano overall remained low."