IV noted an increase in activity . . . in mid-March 1993, after a short period of repose, when explosions in the central crater sent an ash-and-gas cloud 1-2 km above the summit. On 15 March, volcanic tremor was noted, increasing in amplitude after 15 April.

A significant increase in seismicity beneath the volcano 24-27 April was reported by IVGG. Observers reported a glow near the summit area during the night of 25-26 April. A snowstorm prevented observation of the volcano 28-29 April, as volcanic tremor continued. Small steam and ash bursts inside the crater rose 200-300 m above the rim on 6 May. The plume extended 40 km NW from the volcano. Volcanic tremor remained above background.

IVGG reported three ash explosions from the summit crater on 10 May between 2030 and 2045, producing a plume that rose ~1 km above the crater rim and extended 7 km about SE. That same day, tremor amplitude measured by IV reached a maximum of 2.4 µm. Occasional steam and ash bursts occurred in the summit crater again 14 May; the plume rose 0.5-1 km above the crater rim and extended 1-7 km SW. Tremor amplitude had decreased by 19 May.

IV geologists note that tremor at Kliuchevskoi is common and is related to eruptive activity in the summit crater and, to a lesser degree, to flank eruptions. Tremor amplitude is largely dependent on the style of volcanic activity: amplitudes <0.5 µm are associated with steam-gas emission; 0.5-3 µm with Vulcanian explosions; and >3 µm with Strombolian explosions or lava spouting. Aircraft observations on 4 April 1993 revealed a newly formed crater at the summit with a diameter of 500 m and a depth of 200 m. A July 1992 overflight by S. A. Fedotov (IV) had previously revealed the almost complete subsidence of the 1984-90 cone. The last episode of dome collapse followed by renewed dome growth took place during 1962-68 when a new small volcanic cone was seen on the floor of the crater and minor lava fountaining was observed from its vent.

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: V. Ivanov and V. Dvigalo, IV; V. Kirianov, IVGG