A model for the 1975–1976 Tolbachik eruption, Kamchatka, as deduced from geodetic data

Abstract

Abstract Geodetic measurements made between 1971 and 1978 in the region of the large Tolbachik fissure eruption of 1975–1976 in Kamchatka revealed two general types of crustal deformations: 1. (1) uplift and extension of a large area occurring during the eruption. 2. (2) subsidence and compression of the zone near the newly-formed volcanoes occurring after the eruption. Furthermore, rapid horizontal and vertical displacements of the area adjacent to the eruption region occurred during the eruption. Displacements both preceding and occurring after the magma issue to the surface have been measured. Their duration was found to be from a few minutes to several days. Crustal deformations associated with volcanic activity may be caused by pressure sources located at certain depths below volcanoes. The configuration and the origin of pressure sources may be different (feeding channel, peripheral chamber, intruding column or dyke), but the deformation source mechanism in all cases is the same. Based on these simulations, the depth and radius of a simple spherical pressure source were calculated. This source creates a field of elastic displacements of the earth's surface best coinciding with geodetic data. The magma overpressure was calculated from data on the width and length of a fissure during the magma issue to the surface. It proved to be not more than 300 kg/cm2. the location of pressure sources was deduced from data on vertical displacements. Based on these data, the existence and depths of two shallow magma chambers which fed the North vents eruption have been calculated. They are located at depths of about 6 and 1.5 km, one over another. Verification of the calculated deformation sources was supported by deep seismic sounding data. Two seismic shadows apparently caused by the melt occurrence in the magma chambers have been detected at just these same depths.