During late January to early April 2002, seismic activity at Shiveluch remained above background levels and the lava dome in the active crater continued to grow. Explosions, avalanches, pyroclastic flows, and plumes from combinations of steam, gas, and ash, all occurred without warning and rose as high as 7-10 km altitude. Many shallow earthquakes (ML less than or equal to 2.5) occurred within the volcano's edifice along with other shallow seismic events. The Alert Level was raised from Yellow to Orange during a period of increased activity, 12 February-mid-March. Then the Level was returned to Yellow.

Typical activities from the end of January through the first half of February 2002 included weak seismic events and short-lived explosions, which sent ash-gas plumes to heights of 0.8-1.0 km above the ~2.5 km-high dome (reaching 3.3-3.5 km altitude). Occasional explosions sent plumes to higher altitudes; on 25 January a plume rose to ~4.5 km altitude, while an explosive eruption on 1 February sent ash-gas plumes to heights over 5.0 km.

For the latter eruption, Advanced Very High Resolution Radiometer (AVHRR) satellite images show thermal anomalies up to 10 pixels. Many of these "hot" pixels were at the detector saturation level of 48°C, and appeared against typical background temperatures as low as -25°C. A satellite image on 2 February revealed a 40-pixel thermal anomaly; however, only 5-7 of the pixels had temperatures above 40°C, indicating that only those pixels were influenced by hot material on the ground. The other pixels with elevated temperatures were associated with a cloud caused by hot avalanches.

On the evening of 12 February, the character of the seismicity changed suggesting the occurrence of more intense gas-ash explosions. During the next 30 days, explosions occurred frequently, producing ash-and-gas plumes that rose over 1 km above the dome and occasionally over 3 km above it. On 22 February, a series of shallow seismic events registered for ~1 hour, possibly related to ash-gas explosions or a hot rock avalanche. A similar series was recorded on the night of 27 February, and the next morning observers from Klyuchi town (46 km S) reported a 2-km-long pyroclastic flow to the SE of the dome.

AVHRR images of the resulting plumes showed that some extended to distances of 100 km in various directions, depending upon local wind conditions. A satellite image on 21 February showed a circular ash cloud, 20 km in diameter, at an altitude of ~7.1 km. By 15 March seismic activity declined but remained above background levels. Short-lived explosions continued to send plumes as high as 7.5 km altitude during the last week of March.

Thermal anomalies and pixel size on AVHRR imagery. Dave Schneider (USGS, AVO) provided an explanation of relevant aspects of pixel size and thermal anomalies. The size of a "raw" AVHRR pixel varies across and along the scan of the sensor. The instrument scans from side to side as the satellite moves in orbit. Along the nadir (the line directly beneath the satellite sensor), the pixel size is ~1.1 km on a side. At the far extreme of the scan (55° from nadir) the pixel size increases to 2.4 x 6.5 km. The raw image looks very distorted due to this change in pixel size, so the raw data are resampled to a resolution of 1 x 1 km when processing the data for display. This resampling can generate artifacts, including duplicate "hot" pixels. Analysts usually recognize and account for this problem, and then accurately report the appropriate number of hot pixels.

Another complication is the cause and significance of hot pixels. An AVHRR pixel will begin to appear anomalously warm, compared to its neighbors, when very hot material (hundred's of degrees) occupy a very small percentage of the total pixel area (much less that 1%). So, when a report mentions four hot pixels, and the pixel is 1 km square, one might be tempted to interpret this as 4 km² of hot material. However, this is not correct. Typically, only a very small portion of a pixel area is hot, but sufficiently hot to reach the pixel's saturation value of ~50°C. The numbers of hot pixels are not all that relevant in an absolute sense. In a relative sense, however, the number of hot pixels can be important, for example, during episodes where we have seen anomalies grow from 1-4 hot pixels and then reach 10-20 hot pixels after an eruption.

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; Tom Miller and Dave Schneider, 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.