Alaska Volcano Observatory

From McGimsey and others (2014): "Shishaldin Volcano is one of the most active volcanoes of the Aleutian arc. Minor unrest and eruptive activity have been documented for all but a couple of the past 20 years. In 2008, Shishaldin maintained the pattern of producing steam plumes, occasionally with minor phreatic ash emissions (Neal and others, 2011). Thermal anomalies (TA) were observed on September 5 and December 18, 2008. This activity increased in early 2009 with the occurrence of a significant 2-pixel thermal anomaly and a slight increase in seismicity on January 5. AVO issued a VAN the following day upgrading the Aviation Color Code/Volcano Alert Level to YELLOW/ADVISORY. That day, pilots and ground observers reported a constant steam plume rising about 1,000 ft (about 300 m) above the summit and trailing of 16-25 km (10-15 mi) to the southeast. Satellite views on January 8 showed a steam-filled crater and no ash on the snow-covered flanks, and a pilot's photograph taken January 11 shows a pulsing steam plume. A few days later, on January 13, AVO seismologists identified low-amplitude, minor tremor in the seismicity at Shishaldin, which continued for several weeks and then apparently faded out.
"On January 17, the on-island Shishaldin Web camera, which had been out of service since October 22, 2008, was made operational again, although images were not recorded until February 20. Over the next couple of weeks, thermal anomalies were few, seismicity remained low, and although the reports of steaming continued, steam emissions from Shishaldin are considered normal. The return to background conditions prompted AVO to downgrade the Color Code/Alert Level to GREEN/NORMAL on February 11, 2009. A February 28 Web camera image shows a modest steam plume rising from the summit. Image quality problems with the Web camera in March and April led to the camera being taken off-line on April 30.
"Over the next 7 weeks, occasional thermal anomalies were observed along with continuous low-level tremor, which was not considered unusual activity for Shishaldin. Then, on April 7, a PenAir pilot reported Shishaldin streaming more vigorously than in the previous 16 months of observing during his weekly flights by the volcano. A thermal anomaly was reported in satellite imagery that day as well. Activity continued during the next couple of weeks, and on April 20, thermal activity at the summit spiked with the recording of multiple thermal anomalies having saturated pixels, indicative of high ground temperatures (more than 300C; about 600F). This level of thermal activity was last observed at Shishaldin during the run-up to the 1999 eruption (J. Dehn, AVO UAFGI, 2009 internal log entry 31659). A pilot reported steaming on May 5, and an observer on a different flight that day reported also seeing dark colored, linear features on the northern side of the summit. These would later be interpreted to be minor streams of 'dirty' water; no significant deposits were produced.
"Throughout June, thermal anomalies were detected on about one-third of the days, with a particular strong anomaly noted on June 9; no unusual seismicity was detected. A clustering of thermal anomalies appeared to coincide with favorable vertical view angles. On the night of June 25, an ASTER thermal infrared satellite image showed a thermal anomaly and a 22-km-long (14 mi) steam plume extending east-northeast from Shishaldin. Clouds blanketed most of Unimak Island, but the top of Shishaldin was visible above the cloud deck. An observer in Cold Bay, Alaska, called on June 29 to report increased steaming at Shishaldin during the previous couple of days. During the first full week of July, thermal anomalies increased in strength, with a return of saturated pixels (high ground temperatures). Based on this trend, as well as the persistence of the thermal anomaly, AVO elevated the Color Code/Alert Level to
YELLOW/ADVISORY on July 10, 2009. Neither seismicity nor deformation had changed appreciably, and satellite data showed no significant sulfur dioxide gas emissions.
"Airborne emissions were detected in the daily analysis of satellite imagery on July 13, which was a day of rare, cloud free conditions, and a pilot also reported a steam plume rising 2,000 ft (600 m) above Shishaldin and moving to the northwest. Then, on July 15, the Ozone Monitoring Instrument (OMI) satellite imagery appeared to show a small plume-like cloud rich in SO2 originating at Shishaldin; a PUFF simulation using current winds supported the emission source as being Shishaldin.
"During the remainder of July and the first half of August, weather permitting, views of Shishaldin showed steaming from the summit. Thermal anomalies were observed in satellite images, particularly in August. A Plate Boundary Observatory (PBO) crew working on Unimak Island replaced the AVO Web camera for Shishaldin, and it began recording images on August 10; the Web camera would go off-line again on October 11, 2009.
"Following the report of a thermal anomaly on August 16, no more anomalies were detected through the remainder of 2009 except for a weak thermal anomaly on November 2. In mid-September, seemingly anomalous air waves were detected on pressure-sensors located on Shishaldin (station SSLN_BDF), which could be indicative of minor explosions. Retrospective analysis of pressure-sensor data for the previous 2 months revealed that these air waves are common phenomenon and correlated to episodic gas bursts, as documented in 2003-04 (Petersen and McNutt, 2007).
"The persistent absence of thermal anomalies, decrease in steam emissions, and seismicity considered to be within background levels, prompted AVO to downgrade the Aviation Color Code/Volcano Alert Level for Shishaldin to GREEN/NORMAL on October 19, 2009. The volcano remained quiet for the remainder of 2009."