Increased seismic activity at Semisopochnoi volcano, Aleutian Islands

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Earthquake activity at Semisopochnoi volcano, Aleutian Islands that began in January continues, and has increased in intensity over the past few days, Alaska Volcano Observatory (AVO) reports.

In addition, AVO detected brief periods of seismic tremor, which can indicate movement of magma or magmatic gases, and increased the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory on March 25, 2015.

No other signs of volcanic activity in remote sensing or other data were observed. 

The last time this volcano appeared in GVP's weekly volcanic report was during the week of June 18 – 24, 2014:

"AVO reported that the earthquake swarm that had started at Semisopochnoi on June 9 continued until June 23. No eruptive activity was indicated."

The volcano last erupted in 1987 (VEI 2).

Geologic summary

Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is 1221-m-high Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked 774-m-high Mount Cerberus volcano was constructed during the Holocene within the caldera. Each of the peaks contains a summit crater; lava flows on the northern flank of Cerberus appear younger than those on the southern side.

Other post-caldera volcanoes include the symmetrical 855-m-high Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented historical eruptions have originated from Cerberus, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone within the caldera could have been active during historical time. (GVP)

Featured image: ​Semisopochnoi volcano. Photo by Steve Ebbert, 1997 (U.S. Fish and Wildlife Service).

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