During 2-28 September, seismicity under the volcano was generally above background levels. Hypocenters were concentrated at two levels: near the summit crater and at depths of 25-30 km. Clouds often prevented observations.

On 2 September a fumarolic plume was observed during the daylight hours rising 50 m above the summit. Beginning at 2218 that day, a 33-minute series of explosive earthquakes was recorded, and at 2245 an ash explosion produced a plume that rose 4-5 km above the crater. On 3 September, scientists noticed that ash had been deposited in a 2-km-long zone on the NE slope. A plume of gas, with no ash content, rose 500 m above the volcano during 3-4 September, but had stopped by 5 September. Because of the increase in activity, the alert status was changed to Yellow, meaning more significant eruptions may occur.

No fumarolic plumes were seen during 8, 18, and 27 September, but plumes rising up to 100 m above the summit were seen during 13, 16, 17, 21, and 24 September. The alert color code returned to Green on 21 September, indicating normal activity.

Geological Summary. Klyuchevskoy is the highest and most active volcano on the Kamchatka Peninsula. Since its origin about 6,000 years ago, this symmetrical, basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during approximately the past 3,000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 and 3,600 m elevation. Eruptions recorded since the late 17th century have resulted in frequent changes to the morphology of the 700-m-wide summit crater. These eruptions over the past 400 years have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), 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.