Koryaksky, which had lacked fumarolic activity during the last ~ 51 years, began steaming after seven months of local seismicity. The increased seismicity became prominent in March 2008 (figure 1) and the first steaming was recorded on 6 October 2008. The first fumarole vent to appear was located at ~ 3 km elevation on the upper NW slope. Koryaksky, a large snow-laden stratovolcano, forms the most prominent feature of the Avachinskaya volcano group. Episodes of elevated seismicity have previously occurred, as reported in December 2003 (BGVN 28:12).

Figure 1. Seismicity of Koryaksky (and Avachinsky, to the SE) recorded during February 2008-February 2009. a) A map of the region showing location and depths of earthquakes. The white line is trace of cross section AB; a 35-km-diameter circle encloses epicenters of earthquakes plotted in c). b) Histogram showing daily earthquakes with respect to time; ascending curve is the cumulative number of earthquakes (reaching a total of 970 for the interval). c) Hypocenters projected onto the vertical plane of cross section AB. Courtesy of KB GS RAS.

Only two earthquakes struck on 23 December 2008, both at ~ 5 km depth. Observers also heard a booming sound from the volcano at night. NOAA 17 satellite data collected at 2357 UTC on 23 December revealed that a dense ash plume extended over 60 km laterally, and an ash-poor ash plume continued beyond that for another 140 km NE. During 24-25 December observers in the Nalychevo valley saw a dark ash column rise about 200-300 m from the upper NW-flank vent.

On 28 December 2008 moderately explosive vulcanian-type eruptions occurred. Ash plumes rose to ~ 4 km altitude and extend NW. Observers also saw significant fumarolic activity at two vents. During a break in cloud cover on 30-31 December observers saw gas-and-steam plumes, which were thought to contain small amounts of ash. They drifted along the surface of the NW flank, some reaching ~ 4 km altitude. A 2 January KVERT report noted background seismicity during 31 December-6 January, with 1-7 volcanic earthquakes per day and possible episodes of tremor during 30 December to 1 January.

During 6-8 January 2009 strong fumarolic activity continued. According to visual data, gas-steam plumes extended SW from three vents. Gray deposits were visible at the area near the summit. Figures 2 and 3 illustrate the scene on 8 and 10 January 2009. Strong fumarolic activity also prevailed on 14 and 18-19 February 2009.

Figure 2. Gas and steam escaping from Koryaksky on 8 January 2009. Three vents on the NW slope trace an older fissure. Photo by Alexandr Socorenko.

Figure 3. View of Koryaksky looking W from Petropavlovsk on 10 January 2009. The photo was taken from the roof of Institute of Volcanology and Seismology. Photo by Alexandr Socorenko.

KVERT issued reports on 4 March 2009 noting increased activity, with some ash-bearing plumes extending over 200 km to the NE-ENE on 3-4 March. For the previous week, seismicity was again at background. Ash deposits were identified both on the summit and in the saddle to Avachinsky. At the latter area on 4 March, the deposits reached 1-2 mm thickness.

In a 5 March report, KVERT noted plumes containing small amounts of ash rising to 3.7 km and extending over 220 km. They blew to the ENE, E, and SE on 3-5 March. Koryaksy's N flank contained fresh ash deposits ~ 4.0 cm thick. The crater contained a weak, new fumarole.

Geological Summary. The large symmetrical Koryaksky stratovolcano is the most prominent landmark of the NW-trending Avachinskaya volcano group, which towers above Kamchatka's largest city, Petropavlovsk. Erosion has produced a ribbed surface on the eastern flanks of the 3430-m-high volcano; the youngest lava flows are found on the upper W flank and below SE-flank cinder cones. Extensive Holocene lava fields on the western flank were primarily fed by summit vents; those on the SW flank originated from flank vents. Lahars associated with a period of lava effusion from south- and SW-flank fissure vents about 3900-3500 years ago reached Avacha Bay. Only a few moderate explosive eruptions have occurred during historical time, but no strong explosive eruptions have been documented during the Holocene. Koryaksky's first historical eruption, in 1895, also produced a lava flow.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanology and Seismology (IV&S) Far East Division, Russian Academy of Sciences (FED RAS); Kamchatka Branch of the Geophysical Service of the Russian Academy of Sciences (KB GS RAS), Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs, http://www.emsd.ru/~ssl/monitoring/main.htm); Alexander Sokorenko, IV&S; Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/).