Increased earthquake activity at Great Sitkin volcano, alerts raised, Alaska

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Earthquake activity at Great Sitkin volcano has increased over the past month and is now above background levels, forcing the Alaska Volcano Observatory (AVO) to raise the Aviation Color Code to Yellow and the Alert Level to Advisory at 20:58 UTC on February 26, 2020. The last known eruption of this volcano took place in June 2019.

There has been no evidence of eruptive activity in geophysical data streams or in satellite observations, AVO said.

The volcano is monitored with a local real-time seismic network, which will typically allow AVO to detect changes in unrest that may lead to an explosive eruption. Rapid detection of an ash-producing eruption would be accomplished using a combination of seismic, infrasound, lightning, and satellite data.

A small steam explosion occurred at the volcano at 05:40 UTC on June 2, 2019.

Great Sitkin on February 16, 2020. Credit: Copernicus EU/Sentinel-2, TW

Geological summary

Great Sitkin forms much of the northern side of Great Sitkin Island. A younger parasitic volcano capped by a small, 0.8 x 1.2 km (0.5 – 0.7 miles) ice-filled summit caldera was constructed within a large late-Pleistocene or early Holocene scarp formed by massive edifice failure that truncated an ancestral volcano and produced a submarine debris avalanche.

Deposits from this and an older debris avalanche from a source to the south cover a broad area of the ocean floor north of the volcano. The summit lies along the eastern rim of the younger collapse scarp.

Deposits from an earlier caldera-forming eruption of unknown age cover the flanks of the island to a depth up to 6 m (20 feet). The small younger caldera was partially filled by lava domes emplaced in 1945 and 1974, and five small older flank lava domes, two of which lie on the coastline, were constructed along the northwest- and NNW-trending lines.

Hot springs, mud pots, and fumaroles occur near the head of Big Fox Creek, south of the volcano.

Historical eruptions have been recorded since the late-19th century.

Featured image: Great Sitkin on February 16, 2020. Credit: Copernicus EU/Sentinel-2, TW

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