(1) VOLCANO OBSERVATORY NOTICE FOR AVIATION (VONA)
(2) Issued:	(20171122/1957Z)
(3) Volcano:	Great Sitkin (VNUM #311120)
(4) Current Color Code:	YELLOW
(5) Previous Color Code:
(6) Source:	Alaska Volcano Observatory
(7) Notice Number:
(8) Volcano Location:	N 52 deg 4 min W 176 deg 6 min
(9) Area:	Aleutians
(10) Summit Elevation:	5709 ft (1740 m)
(11) Volcanic Activity Summary:	Recent observations of a robust steam plume and a period of gradually increasing seismicity over several months indicate that Great Sitkin Volcano has become restless and is exhibiting behavior that is above background levels. AVO is thus raising the aviation color code and volcano alert level to YELLOW/ADVISORY. Photographs of the volcano taken by local observers on Sunday, November 19 show a light-colored vapor plume rising about 300 m (1,000 ft) above the vent area and extending about 15-20 km (9 –12 mi) to the south. Nothing unusual was observed in seismic or infrasound data around the time the photographs were taken and nothing noteworthy has been observed in satellite data since the emissions were observed. An increased number of small earthquakes was evident as early as late July 2016, and since then the level of seismic activity has fluctuated at low levels but has exhibited a gradual overall increase most notable since June 2017. Seismic activity to date has been characterized by earthquakes that are typically less than magnitude 1.0 and range in depth from near the summit of the volcano to 30 km below sea level. Most earthquakes are in one of two clusters, beneath the volcano's summit or just offshore the northwest coast of the island. The largest earthquake so far was a magnitude 2.8 on September 29, 2017. Possible explosion signals were observed in seismic data on January 10 and July 21 of this year, but no confirmed emissions were observed locally or detected in infrasound data or satellite imagery. Great Sitkin has experienced at least one other episode of unrest characterized by vigorous steaming. This last occurred in 1953 but no subsequent eruptive activity developed. Minor steaming in the vicinity of the 1974 lava dome was observed in 1986 and in 2012 but no associated eruptive activity occurred. Great Sitkin Volcano is monitored by a five-station seismic network on Great Sitkin Island and with additional seismic stations on the nearby islands of Igitkin, Adak, Kagalaska, and Kanaga. A six-element infrasound array to detect explosions (atmospheric pressure waves), was installed on Adak Island in June, 2017. AVO also uses satellite imagery to monitor Great Sitkin Volcano. The current unrest is likely the result of magma intrusion beneath the volcano. Typically, such intrusions release gas, which can increase stress in the crust and lead to an increase in the number of earthquakes. If gases build up near the surface, they may be released suddenly in small explosions or robust emissions. It is possible, but not certain, that the volcano may eventually erupt. Prior to a significant eruptive event an even greater increase in seismicity should occur as magma rises to shallower levels in the crust. An ash-producing eruption similar to the most recent historical eruption in 1974 could generate airborne ash that may pose hazards to aircraft. Ash fallout on the nearby community of Adak and the surrounding ocean may also occur.
(12) Volcanic cloud height:	not applicable
(13) Other volcanic cloud information:	Unknown
(14) Remarks:	Great Sitkin Volcano is a basaltic andesite volcano that occupies most of the northern half of Great Sitkin Island, a member of the Andreanof Islands group in the central Aleutian Islands. It is located 43 km (26 miles) east of the community of Adak. The volcano is a composite structure consisting of an older dissected volcano and a younger parasitic cone with a 3-km-diameter summit crater. A steep-sided lava dome, emplaced during an eruption in 1974, occupies the center of the crater. Great Sitkin erupted at least three times in the 20th century, most recently in 1974. That eruption produced at least one ash cloud that likely exceeded an altitude of 25,000 ft above sea level. A poorly documented eruption occurred in 1945, also producing a lava dome that was partially destroyed in the 1974 eruption. Within the past 280 years a large explosive eruption produced pyroclastic flows that partially filled the Glacier Creek valley on the southwest flank.
(15) Contacts:	Michelle Coombs, Scientist-in-Charge, USGS mcoombs@usgs.gov (907) 786-7497 Jeff Freymueller, Coordinating Scientist, UAFGI jfreymueller@alaska.edu (907) 322-4085
(16) Next Notice:

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AVO/USGS Volcanic Activity Notice

Volcano: Great Sitkin (VNUM #311120)

Current Volcano Alert Level: ADVISORY

Current Aviation Color Code: YELLOW

Issued: Wednesday, November 22, 2017, 10:57 AM AKST
Source: Alaska Volcano Observatory
Notice Number:
Location: N 52 deg 4 min W 176 deg 6 min
Elevation: 5709 ft (1740 m)
Area: Aleutians

Volcanic Activity Summary: Recent observations of a robust steam plume and a period of gradually increasing seismicity over several months indicate that Great Sitkin Volcano has become restless and is exhibiting behavior that is above background levels. AVO is thus raising the aviation color code and volcano alert level to YELLOW/ADVISORY.

Photographs of the volcano taken by local observers on Sunday, November 19 show a light-colored vapor plume rising about 300 m (1,000 ft) above the vent area and extending about 15-20 km (9 –12 mi) to the south. Nothing unusual was observed in seismic or infrasound data around the time the photographs were taken and nothing noteworthy has been observed in satellite data since the emissions were observed.

An increased number of small earthquakes was evident as early as late July 2016, and since then the level of seismic activity has fluctuated at low levels but has exhibited a gradual overall increase most notable since June 2017. Seismic activity to date has been characterized by earthquakes that are typically less than magnitude 1.0 and range in depth from near the summit of the volcano to 30 km below sea level. Most earthquakes are in one of two clusters, beneath the volcano's summit or just offshore the northwest coast of the island. The largest earthquake so far was a magnitude 2.8 on September 29, 2017.

Possible explosion signals were observed in seismic data on January 10 and July 21 of this year, but no confirmed emissions were observed locally or detected in infrasound data or satellite imagery. Great Sitkin has experienced at least one other episode of unrest characterized by vigorous steaming. This last occurred in 1953 but no subsequent eruptive activity developed. Minor steaming in the vicinity of the 1974 lava dome was observed in 1986 and in 2012 but no associated eruptive activity occurred.

Great Sitkin Volcano is monitored by a five-station seismic network on Great Sitkin Island and with additional seismic stations on the nearby islands of Igitkin, Adak, Kagalaska, and Kanaga. A six-element infrasound array to detect explosions (atmospheric pressure waves), was installed on Adak Island in June, 2017. AVO also uses satellite imagery to monitor Great Sitkin Volcano.

The current unrest is likely the result of magma intrusion beneath the volcano. Typically, such intrusions release gas, which can increase stress in the crust and lead to an increase in the number of earthquakes. If gases build up near the surface, they may be released suddenly in small explosions or robust emissions. It is possible, but not certain, that the volcano may eventually erupt. Prior to a significant eruptive event an even greater increase in seismicity should occur as magma rises to shallower levels in the crust. An ash-producing eruption similar to the most recent historical eruption in 1974 could generate airborne ash that may pose hazards to aircraft. Ash fallout on the nearby community of Adak and the surrounding ocean may also occur.

Recent Observations:
[Volcanic cloud height] not applicable
[Other volcanic cloud information] Unknown

Remarks: Great Sitkin Volcano is a basaltic andesite volcano that occupies most of the northern half of Great Sitkin Island, a member of the Andreanof Islands group in the central Aleutian Islands. It is located 43 km (26 miles) east of the community of Adak. The volcano is a composite structure consisting of an older dissected volcano and a younger parasitic cone with a 3-km-diameter summit crater. A steep-sided lava dome, emplaced during an eruption in 1974, occupies the center of the crater. Great Sitkin erupted at least three times in the 20th century, most recently in 1974. That eruption produced at least one ash cloud that likely exceeded an altitude of 25,000 ft above sea level. A poorly documented eruption occurred in 1945, also producing a lava dome that was partially destroyed in the 1974 eruption. Within the past 280 years a large explosive eruption produced pyroclastic flows that partially filled the Glacier Creek valley on the southwest flank.

Contacts: Michelle Coombs, Scientist-in-Charge, USGS
mcoombs@usgs.gov (907) 786-7497

Jeff Freymueller, Coordinating Scientist, UAFGI
jfreymueller@alaska.edu (907) 322-4085

The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.

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