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Elevated earthquake activity, possible steam explosion at Great Sitkin volcano, Alaska

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Seismicity at Alaska's Great Sitkin volcano has been at elevated levels over the past couple of days, followed by a signal that may represent a short-lived steam explosion on June 11. The last known eruption of this stratovolcano took place in 1974.

Seismicity at Great Sitkin volcano was at elevated levels over the past 5 days, the Alaska Volcano Observatory (AVO) reported 21:26 UTC on June 11, 2018.

This activity was followed by a signal at 19:39 UTC that may represent a short-lived steam explosion detected by seismic data.

AVO has thus raised the Aviation Color Code to Yellow and Alert Level to Advisory.

The last time the observatory raised Aviation Color Code to Yellow and Volcano Alert to Advisory was on November 22, 2017. The status was unchanged until January 18 of this year.

On November 19, 2017, local observers photographed a light-colored vapor plume rising about 300 m (984 feet) above the vent and drifting 15 – 20 km (9 – 12 miles)  S. A satellite image acquired on November 21 showed steam continuously jetting from a small fumarole on the W side of the 1974 lava flow within the summit crater, and at least one area where snow and ice had been melted. 

Seismicity at this volcano had fluctuated but increased overall since July 2016, most notably in June 2017. The seismic activity was characterized by earthquakes less than M 1, and occurred either just below the summit or just offshore the NW coast of the island, 30 km (18.6 miles) below sea level. Possible explosion signals were recorded in seismic data on January 10 and July 21, 2017, but there were no confirmed emissions.

The last known eruptive episode of this volcano took place from February 19, 1974 to September 16, 1974 (+/- 15 days) (VEI2).

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.

The 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, although it is currently (June 2018) only partly operational. AVO also uses satellite imagery to monitor Great Sitkin Volcano.

Featured image: Steaming from Great Sitkin Volcano as visible from the town of Adak (40 km or 25 miles to the SW). Credit: Alan Beauparlant

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