Rupture history of the January 1, 1996, Ms 6.6 volcanic earthquake preceding the simultaneous eruption of Karymsky and Akademia Nauk volcanoes in Kamchatka, Russia

Abstract

The intensive foreshock‐mainshock‐aftershock sequence of earthquakes preceded and accompanied the January 2, 1996, simultaneous eruption of the active Karymsky and the dormant Akademia Nauk volcanoes in Kamchatka, Russia. The finite fault, broad band teleseismic P waveform inversion was applied to the Ms 6.6 mainshock of this sequence which occurred on January 1, 1996, about 14 hours before the beginning of eruption. It was one of the strongest volcano‐tectonic earthquakes in this century after the 1912–1914 great Ms 7.0 volcanic earthquakes related to the Katmai, Alaska, and Sakurajima, Japan, volcanic eruptions. The main feature of the rupture process inferred from the inversion was the breaking of a sequence of four asperities with displacement greater than 200 cm which were situated at depths from 0 to 35 km just beneath the Akademia Nauk volcano. There were no broken asperities beneath the Karymsky volcano. The majority of small earthquake foci were located out of maximum asperities and filled a space between two zones of asperities at depths from 12 to 20 km. The position of broken asperities allows us to suggest that during the large earthquake rupturing an ancient magmatic column beneath the volcano was destroyed. Small earthquakes which were recorded mainly from 5 to 20 km depth were able only to destroy the intermediate‐depth part of this column between 12 and 20 km. The main work for opening a way to move magma to the surface was made by a large shock which had broken the upper and deep parts of the ancient magma column.