Description: From Neal and others (2014): "On 18 July, 2010, a magnitude 6.6 main-shock occurred 9 km (5.6 mi) northeast of the summit of Mount Cleveland. AVO field personnel on Umnak Island reported that buildings shook and fuel barrels rolled back and forth several inches (M. Kaufman, UAFGI/AVO, written commun., 2010). The earthquake also was felt in Unalaska/Dutch Harbor, although residents described the shaking as 'light' or 'weak'.
"This event triggered a subsequent earthquake cluster of more than 1,800 aftershocks greater than magnitude 2.5 and about 110 magnitude 4.0 or greater events. According to analysis by AEIC, a M 4.0 foreshock had occurred on July 17 at 14:44 UTC. Earthquakes during this time period were restricted to an approximately 25-km-wide (15-mi) area extending from eastern Chuginadak Island to the edge of the Aleutian platform about 60 km (37 mi) southeast. The largest aftershock of magnitude 6.0 occurred on July 18.
"AEIC analysis of fault plane solutions for the large events of this series determined that the larger earthquakes were located on north-northwest-trending normal faults. This was the largest event to occur in the region since the magnitude 6.5 earthquake on October 13, 2009. Similar sized earthquakes occurred in the same area on May 10, 2006, and December 26, 2007. AEIC located nearly 3,200 aftershocks through end of July, including about 60 aftershocks with magnitudes 4.0 or greater.
"The earthquake had no clear impact on Cleveland volcano; however, elevated surface temperatures reappeared at the summit throughout July and August. Based on this, AVO upgraded the Aviation Color Code and Volcano Alert Level to YELLOW/ADVISORY on August 26. On August 26, 29, and 31, pilots from Alaska Airlines flying the Anchorage to Adak route reported no volcanic ash from Mount Cleveland. Thermal anomalies continued into September but did not intensify; on September 10, the Aviation Color Code and Alert Level were downgraded to UNASSIGNED.
"On September 12, the Anchorage VAAC reported a possible weak ash signal in satellite data. If this was an eruption cloud, the plume rose only a few thousand feet above the volcano's summit and disspated quickly. A one-time VAA was released but no SIGMET was issued. AVO upgraded the Aviation Color Code and Volcano Alert Level to YELLOW/ADVISORY. Observations from a pilot in the area of Mount Cleveland reported that no ash was visible, at least below 2,900 ft. Clear satellite views continued to show elevated surface temperatures in the summit crater but no ash plumes.
"Clouds frequently prevented remote viewing of the volcano but during periods of clear conditions, thermal anomalies were noted intermittently through the end of the year. AVO maintained Cleveland at Aviation Color Code and Volcano Alert Level YELLOW/ADVISORY. There were no reports of ash or other activity except for an anomalous infrasound signal that was received on September 15 at 00:24 and 00:30 UTC (Steve McNutt, UAFGI/AVO, written commun., 2010) and may have represented a small explosion from the volcano."
Weak thermal anomalies were visible on January 1, 11, and 16 [2011], and the weather remained cloudy for the remainder of the month.
In February, a weak thermal anomaly was observed on the first. On the 9th, a pilot overflew Cleveland and reported minor, repetetive steam emissions rising hundreds of feet above the summit. The snow on the flanks was pristine, with no indication of recent ash emissions. Steam emissions are common at Cleveland and do not indicate an increased level of unrest.
In March, a weak thermal anomaly was observed on March 2, 3, and 11. A cloud-free view of the volcano on March 23 showed no unusual activity. On March 31, 2011, AVO lowered the volcano alert to UNASSIGNED and the aviation color code to UNASSIGNED, on the basis of a lack of confirmed eruptive activity over the past several months.