ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, February 24, 2017, 2:14 PM AKST (Friday, February 24, 2017, 23:14 UTC)

BOGOSLOF VOLCANO (VNUM #311300)
53°55'38" N 168°2'4" W, Summit Elevation 492 ft (150 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE

Volcanic unrest at Bogoslof continues. Four explosive eruptions occurred earlier this week between Feb 17and 19, producing volcanic clouds that rose as high as 25,000 - 38,000 ft asl. The most recent explosion at 17:08 AKST on 19 February (02:08 February 20 UTC) was recorded by seismic and infrasound data, and satellite data showed a cloud extending 100 miles (160 km) southeast over Unalaska Island, though no ash fall was reported in populated areas. There have been no signs of volcanic activity over the past several days, and the Aviation Color Code remains ORANGE and the Alert Level WATCH since Tuesday. Recent satellite views show no significant changes to the island, and the vent remains underwater.

Geophysical data network outages and recent changes in the eruptive pattern have made rapid characterization of volcanic activity more difficult. This week's 4 explosions were the most yet in a 2.5 day span and followed nearly 14 days without significant emissions (including strong, possibly non-eruptive seismicity on February 13), which represents a significant departure from the 1-3 day explosive intervals from earlier in the eruption sequence. Furthermore, seismic network outages from nearby Umnak Island have reduced AVO's seismic monitoring capabilities, limiting our ability to rapidly characterize Bogoslof activity.

AVO continues to use infrasound (pressure) sensors from Umnak Island and will rely on seismic data from Unalaska Island to monitor Bogoslof, which will still allow for timely detection of significant activity. In addition, we use satellite imagery to track ash clouds and information from the Worldwide Lightning Location Network to identify volcanic lightning. Although we are able to detect significant explosive activity in real-time, there is typically a lag of tens of minutes until we can characterize the magnitude of the event and the altitude of the volcanic cloud.

In addition to the significant explosive events that we are able to detect, it is likely that lower-level explosive activity is occurring that is below our ability to detect in our data sources. These low-level explosions could pose a hazard in the immediate vicinity of the volcano.

Bogoslof Island is the largest of a cluster of small, low-lying islands making up the emergent summit of a large submarine stratovolcano. The highest point above sea level prior to this eruption was about 100 m (300 ft); however, the volcano is frequently altered by both eruptions and wave erosion and has undergone dramatic changes in historical time. The two main islands currently above sea level are Fire Island and Bogoslof Island, both located about 98 km (61 mi) northwest of Unalaska/Dutch Harbor, 123 km (76 mi) northeast of Nikolski, and 149 km (93 mi) northeast of Akutan. The volcano is situated slightly north (behind) the main Aleutian volcanic front. Bogoslof volcano is within the USFWS Aleutian Maritime Wildlife Refuge and is habitat for marine mammals and seabirds.

At least 8 historical eruptions have been documented at Bogoslof. The most recent prior to 2016 occurred from July 6-24, 1992, and produced episodic steam and ash emissions including an ash cloud up to 26,000 ft (8 km) asl on July 20, followed the next day by extrusion of a new 150 m (500 ft) by 275 m (900 ft) lava dome on the north end of the island. Previous eruptions of the volcano have lasted weeks to months, and have on occasion produced ash fall on Unalaska. Eruptions of the volcano are often characterized by multiple explosive, ash-producing events such as we have seen in 2016, as well as the growth of lava domes.

CLEVELAND VOLCANO (VNUM #311240)
52°49'20" N 169°56'42" W, Summit Elevation 5676 ft (1730 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE

No significant activity was observed in satellite or web camera views this week, including mostly clear views this morning. No significant volcanic activity has been detected in seismic or infrasound data over the past week.

We have had no recent observations of the lava dome that began extruding in the summit crater in late January, and it is unclear whether it is still slowly growing. However, the dome's presence may act as a plug on the vent that restricts gas emissions, which could pressurize the system resulting in explosive activity with little to no warning.

Cleveland volcano is monitored with a limited real-time seismic network, which inhibits AVO's ability to detect precursory unrest that may lead to an explosive eruption. Rapid detection of an ash-producing eruption may be possible using a combination of seismic, infrasound, lightning and satellite data.

Cleveland volcano forms the western portion of Chuginadak Island, a remote and uninhabited island in the east central Aleutians. The volcano is located about 75 km (45 mi) west of the community of Nikolski, and 1500 km (940 mi) southwest of Anchorage. The most recent significant period of eruption began in February, 2001 and produced 3 explosive events that generated ash clouds as high as 39,000 ft above sea level. The 2001 eruption also produced a lava flow and hot avalanche that reached the sea. Since then, Cleveland has been intermittently active producing small lava flows, often followed by explosions that generate small ash clouds generally below 20,000 ft above sea level. These explosions also launch debris onto the slopes of the cone producing hot pyroclastic avalanches and lahars that sometimes reach the coastline.

OTHER ALASKA VOLCANOES

Information on all Alaska volcanoes is available at : http://www.avo.alaska.edu.

AVO scientists conduct daily checks of earthquake activity at all seismically-monitored volcanoes, examine web camera and satellite images for evidence of airborne ash and elevated surface temperatures, and consult other monitoring data as needed.

For definitions of Aviation Color Codes and Volcano Alert Levels, see: http://www.avo.alaska.edu/color_codes.php

SUBSCRIBE TO VOLCANO ALERT MESSAGES by email: http://volcanoes.usgs.gov/vns/

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CONTACT INFORMATION:

Dave Schneider, Acting Scientist-in-Charge, USGS
djschneider@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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