On 27 and 28 May, AVO recorded anomalous seismicity at Great Sitkin. The seismicity consisted of two periods of seismic tremor on 27 May (lasting for 20 and 55 minutes) and two earthquake swarms on 28 May (beginning at 0406 and 1328). The earthquake swarms each began with a relatively large event (ML (local magnitude) 2.2 and ML 4.3) followed by tens to hundreds of smaller aftershocks, most located 5-6 km SE of the crater at depths of 0-5 km. Both the tremor and the earthquake swarms represent significant changes from what is considered to be normal, "background" seismicity at Great Sitkin. However, aftershocks declined significantly overnight, and no tremor episodes were detected after the 27th. There were neither signs of surface volcanic activity on satellite imagery nor ground-level reports of anomalous activity. Great Sitkin remained at Concern Color Code Green.

29 May 2002            Back to Top

On 27 and 28 May, AVO recorded anomalous seismicity at Great Sitkin. The seismicity consisted of two periods of seismic tremor on 27 May (lasting for 20 and 55 minutes) and two earthquake swarms on 28 May (beginning at 0406 and 1328). The earthquake swarms each began with a relatively large event (ML (local magnitude) 2.2 and ML 4.3) followed by tens to hundreds of smaller aftershocks, most located 5-6 km SE of the crater at depths of 0-5 km. Both the tremor and the earthquake swarms represent significant changes from what is considered to be normal, "background" seismicity at Great Sitkin. However, aftershocks declined significantly overnight, and no tremor episodes were detected after the 27th. There were neither signs of surface volcanic activity on satellite imagery nor ground-level reports of anomalous activity. Great Sitkin remained at Concern Color Code Green.

Sources: Alaska Volcano Observatory (AVO)

Below is a summary of eruption dates and Volcanic Explosivity Indices (VEI).

The following references are the sources used for data regarding this volcano. References are linked directly to our volcano data file. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title. Additional discussion of data sources can be found under Volcano Data Criteria.

Coats R R, 1950. Volcanic activity in the Aleutian Arc. {U S Geol Surv Bull}, 974-B: 35-47

Coombs M L, White S M, Scholl D W, 2007b. Massive edifice failure at Aleutian arc volcanoes. {Earth Planet Sci Lett}, 256: 403-418

Henning R A, Rosenthal C H, Olds B, Reading E (eds), 1976. Alaska's volcanoes, northern link in the ring of fire. {Alaska Geog}, 4: 1-88

IAVCEI, 1973-80. Post-Miocene Volcanoes of the World. {IAVCEI Data Sheets, Rome: Internatl Assoc Volc Chemistry Earth's Interior}.

Keller F, Meuschke J L, Alldredge L R, 1954. Aeromagnetic surveys in the Aleutian, Marshall, and Bermuda Islands. {Eos, Trans Amer Geophys Union}, 35: 558-572

Miller T P, McGimsey R G, Richter D H, Riehle J R, Nye C J, Yount M E, Dumoulin J A, 1998. Catalogue of the historically active volcanoes of Alaska. {U S Geol Surv Open-File Rpt}, 98-582: 1-104

Motyka R J, Liss S A, Nye C J, Moorman M A, 1993. Geothermal resources of the Aleutian arc. {Alaska Div Geol Geophys Surv, Prof Rpt}, no 114, 17 p and 4 map sheets

Myers J D, 1994. {The Geology, Geochemistry and Petrology of the recent Magmatic Phase of the Central and Western Aleutian Arc}. {Unpublished manuscript}, unpaginated

Romick J D, Kay S M, Kay R W, 1992. The influence of amphibole fractionation on the evolution of calc-alkaline andesite and dacite tephra from the central Aleutians, Alaska. {Contr Mineral Petr} 112: 101-118

Simons F S, Mathewson D E, 1955. Geology of Great Sitkin Island, Alaska. {U S Geol Surv Bull}, 1028-B: 29-32

Smith R L, Shaw H R, Luedke R G, Russell S L, 1978. Comprehensive tables giving physical data and thermal energy estimates for young igneous systems of the United States. {U S Geol Surv Open-File Rpt}, 78-925: 1-25

Constructed within the caldera of an older shield volcano forming the northern half of Great Sitkin Island, 1740-m-high Great Sitkin volcano contains a small, 0.8 x 1.2 km ice-filled summit caldera. Deep glacial valleys radiate from the summit, which lies at the eastern rim of the caldera, which was formed by massive edifice failure that produced a submarine debris avalanche that traveled more than 40 km to the north. The entire island is blanketed with a light-brown to black pumice layer up to 6 m thick. This deposit is overlain over much of the NW side of the island by ash deposits from a subsidiary vent NW of the caldera. 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. In 1945, a 400-600 m wide, flat-topped lava dome was emplaced through a glacier filling the steep-sided, 180-m-deep summit caldera.