On 29 July, a fumarolic plume rose 100-600 m above the volcano. At the end of July, clouds largely obscured Shiveluch. Low seismic activity occurred during the following week, so the Level of Concern Color Code was Yellow. Seismicity under Shiveluch increased sharply at 0344 on 6 August. Consequently, the hazard status changed to Orange, indicating an eruption or imminent eruption. The volcano itself continued to be concealed by heavy clouds preventing visual observations. Satellite imagery detected no ash or thermal anomaly.

Beginning at 1300 on 6 August, the level of high-frequency volcanic tremor gradually decreased, reaching background levels by 1600. At 1740 seismic data indicated a possible short-lived gas-ash explosion. Based on seismicity, the cloud was estimated to have risen to ~6,000-8,000 m, although this could not be confirmed because the volcano remained obscured by clouds. An increase in tremor level occurred from 2010 to 2120. Seismicity decreased to near background levels, although shallow events continued to occur, and the hazard status was decreased to Yellow. Seismic activity gradually built in the following days as small earthquakes trembled beneath the volcano, but became quiet on 16-17 August.

An increase in seismic activity took place on the morning of 23 August. At 1343, gas-ash explosions rose from Shiveluch to altitudes of 8,000 m and moved to the SE. The following day, gas-ash explosions reached 3,000-4,000 m in altitude, and were accompanied by shallow seismic events. By 25 August, seismicity was close to background levels, and the hazard status was downgraded to Yellow.

A short-lived explosive eruption was observed at 1135 on 29 August sending an ash-rich plume to an estimated altitude of 10 km. The ash cloud drifted SE, and was recorded by geostationary weather-satellite imagery moving E across the Bering Sea. Increased seismicity ensued at 2231-2237 followed by volcanic tremor. Seismicity decreased significantly after this, and denoted the end of this explosion. Since Shiveluch had several short-lived explosive eruptions in the past weeks during partial dome collapses, the hazard status was upgraded to Orange. The status was again downgraded to Yellow at the end of the month, although future unrest seems likely as seismicity continues above background levels.

Geological Summary. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1,300 km3 andesitic volcano is one of Kamchatka's largest and most active volcanic structures, with at least 60 large eruptions during the Holocene. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes occur on its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large open caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

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