Report on Great Sitkin (United States) — June 2002
Bulletin of the Global Volcanism Network, vol. 27, no. 6 (June 2002)
Managing Editor: Richard Wunderman.
Great Sitkin (United States) Abnormal tremor and earthquake swarms in May 2002
Please cite this report as:
Global Volcanism Program, 2002. Report on Great Sitkin (United States). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 27:6. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200206-311120.
52.076°N, 176.13°W; summit elev. 1740 m
All times are local (unless otherwise noted)
On 27 and 28 May the Alaska Volcano Observatory (AVO) detected anomalous seismicity at Great Sitkin, a volcano located 1,895 km SW of Anchorage, Alaska. On 27 May two periods of seismic tremor lasted for 20 and 55 minutes and on 28 May earthquake swarms began at 0306 and 1228. The earthquake swarms each began with a relatively large event (ML 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 earthquake swarms represent significant changes from background seismicity at Great Sitkin. However, aftershocks declined significantly overnight, and seismicity returned to background levels with a lack of recorded tremor since 27 May. Satellite imagery showed no signs of surface volcanic activity, and no reports of anomalous activity were received by AVO.
Geologic Background. The Great Sitkin volcano forms much of the northern side of Great Sitkin Island. A younger parasitic volcano capped by a small, 0.8 x 1.2 km 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. 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 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.
Information Contacts: Tom Murray and John Eichelberger, Alaska Volcano Observatory (AVO) (URL: http://www.avo.alaska.edu/).