Vertical ground movements in the Southern Zone Of Tolbachik Cinder Cones after the end of the 1975-1976 Great Tolbachik Fissure Eruption and possible causes of the movements

We present elevations between long-term benchmarks in the Tolbachik Geodynamic Test Site as directly measured by second-order leveling in 1976-2011. These raw data are very important for future researchers, since the measurements can be repeated between surviving benchmarks using any more accurate techniques for comparisons with past results. We show that the greatest ground subsidence occurred at the base of Cone II, at the North Vent eruption center during the 1975-1976 Great Tolbachik Fissure Eruption, with the subsidence reaching 2.33 m for the past 35 years. The subsidence nearly decays at a distance of 7 km. Assuming that subsidences propagate symmetrically, we found an approximate volume for this subsidence, which is 0.12 km(3). To make a comparison, we mention that the volume of all the three cones is 0.254 km(3). We adduce data showing that a ground uplift of 4-5 cm over an extensive area in the southeastern sector from Cone VIII after the eruption at the South Vent came to an end. We hypothesize that the uplift was caused by lateral emplacements of magma at depths of 0-5 km along numerous southeasterly directions without coming to the surface.