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ALASKA VOLCANO OBSERVATORY
Wednesday, May 29, 2002 10:30 AM ADT (1830 UT)
GREAT SITKIN VOLCANO
52°05'N 176°08'W, Summit Elevation: 5,710 ft (1,740 m)
Current Level of Concern Color Code: GREEN
On May 27 and May 28 AVO detected anomalous seismicity at Great Sitkin Volcano, located in the central Aleutian Islands 1175 miles southwest of Anchorage. The seismicity consisted of two periods of seismic tremor on May 27 lasting for 20 and 55 minutes and two earthquake swarms on May 28 beginning at 1306 and 2228 UTC. The earthquake swarms each began with a relatively large earthquake (ML 2.2 and ML 4.7) followed by tens to hundreds of smaller aftershocks, most located 5-6 km southeast 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 seismicity is approaching background levels again. Satellite imagery shows no signs of volcanic activity at the surface, and no reports of anomalous activity have been received by AVO. Great Sitkin remains at color code GREEN.
Great Sitkin Volcano is a basaltic andesite volcano that occupies most of the northern half of Great Sitkin Island, a member of the Andreanof Islands group in the central Aleutian Islands. It is located 43 km (26 miles) east of the community of Adak. The volcano is a composite structure consisting of an older decapitated volcano and a younger parasitic cone with a 2-3 km diameter summit crater. A steep-sided dome occupies the center of the crater. Great Sitkin erupted at least three times in the 20th century, most recently in 1974 when a lava dome formed in the crater accompanied by at least one ash cloud that reached ~10,000 ft. above sea level. A poorly documented eruption occurred in 1945, also producing a lava dome that was partially destroyed in the 1974 eruption. Within the past 280 years a large explosive eruption produced pyroclastic flows that partially filled the Glacier Creek valley on the southwest flank.
AVO will continue to monitor the situation closely and will issue further updates as information becomes available.