Unrest at Shiveluch continued from 1 January through 9 April 2004, including above-background seismicity and lava-dome growth with associated pyroclastic flows. Gas-and-steam plumes rising as high as 4.5 km altitude and ash plumes rising to 4-6 km altitude were frequent. Plumes were noted as far as 175 km from the volcano. During the period, US and Russian satellites repeatedly detected thermal anomalies. For viewers on the ground the volcano was obscured by clouds throughout much of the period.

Earthquakes occurred at depths of 0-5 km with local magnitudes (Ml) of 1.25-2.6. About 70 shallow earthquakes with Ml over 1.75 occurred during the week ending 16 January. These were exceeded the following week by 206 earthquakes with Ml of 1.75-2.6 and about 40 ash explosions. Intermittent spasmodic volcanic tremors of 0.5-1.0 µm/s were also recorded that week. These events caused the level of concern to raise from Yellow to Orange, where it remained throughout the remainder of the report period.

Accompanying these events were pyroclastic flows with run-out distances of 1-2 km. Ash plumes rose as high as 6 km, extending in various directions for several kilometers. Gas-and-steam plumes rose to 3.5-4.5 km. One extended 50 km to the SE on 22 January while another, on 26 January, extended over 75 km to the SW.

Events and activities similar to those described above were noted throughout the report period. Shallow earthquakes were recorded almost daily through February, >10/week was typical except for the period in late January noted earlier. However, during late February and through March and April, strong earthquakes occurred, numbering 14-24 per week. Spasmodic volcanic tremor was registered throughout this latter period, attaining a maximum velocity of 0.8 µm/s during 4-6 March.

Gas-and-steam plumes, some containing ash and extending as far as 175 km, were noted throughout the period. During the beginning of April, one ash-gas explosion delivered ash up to 9.0 km while 13 other explosions sent plumes up to 4.0-7.2 km and spasmodic tremor with velocities of 0.2-0.7 µm/s was recorded.

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 Girina, Kamchatka Volcanic Eruptions Response Team (KVERT), a cooperative program of the Institute of Volcanic Geology and Geochemistry, Far East Division, Russian Academy of Sciences, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia, the Kamchatka Experimental and Methodical Seismological Department (KEMSD), GS RAS (Russia), and the Alaska Volcano Observatory (USA); Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), the Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and the Alaska Division of Geological and Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.