Uncertain low-level eruptive activity occurred at Veniaminof in September 2002 (BGVN 27:10). During October 2002, seismicity was lower than when it was first noted in early September, although it was still above background levels. Visual observations were intermittent and inconclusive. The Alaska Volcano Observatory (AVO) received reports ranging from minor-steam and possible ash emissions, to no signs of activity. Satellite imagery on 2 October suggested an apparent gray, diffuse deposit extending across the caldera from the historically active intracaldera cinder cone. This could reflect a small explosion, vigorous steam emission, or redistribution of material by strong winds; no thermal anomalies were observed on satellite imagery. Footage obtained later, but recorded in early October, showed minor ash emission from the intracaldera cone rising ~100-200 m above the cone and drifting a short distance before dispersing. A faint covering of ash was visible on the caldera ice field extending from the base of the cone.

On 18 November AVO lowered the Concern Color Code from Yellow to Green. Since early October they had received no pilot reports or other observations of activity at the volcano. Also, they had not detected thermal anomalies in any clear satellite images. Though seismicity remained above levels recorded during summer of 2002, it remained roughly constant during the previous month at a level notably lower than in September.

Seismicity began to increase in mid-December, and on 6 January AVO raised the Concern Color Code from Green to Yellow. No thermal anomalies were detected on satellite imagery. Elevated seismicity continued through February 2003, with discrete seismic events occurring at a rate of 1-2 per minute during 21-28 February. Nearly constant periods of seismicity were recorded during the report week. Discrete seismic events occurred at rates up to 1-2 events per minute, along with moderate levels of volcanic tremor. Satellite imagery did not reveal increased surface temperatures, ash emission, or ash deposits. Visual observations on 22 January from the village of Perryville, located 35 km SSW of the volcano, revealed that white steam was rising from the intracaldera cone. The steaming was similar to that observed over the previous several months. The Concern Color Code remained at Yellow.

Geological Summary. Veniaminof, on the Alaska Peninsula, is truncated by a steep-walled, 8 x 11 km, glacier-filled caldera that formed around 3,700 years ago. The caldera rim is up to 520 m high on the north, is deeply notched on the west by Cone Glacier, and is covered by an ice sheet on the south. Post-caldera vents are located along a NW-SE zone bisecting the caldera that extends 55 km from near the Bering Sea coast, across the caldera, and down the Pacific flank. Historical eruptions probably all originated from the westernmost and most prominent of two intra-caldera cones, which rises about 300 m above the surrounding icefield. The other cone is larger, and has a summit crater or caldera that may reach 2.5 km in diameter, but is more subdued and barely rises above the glacier surface.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA, b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.