Time-dependent model for volume changes in pressure sources at Asama volcano, central Japan due to vertical deformations detected by precise leveling during 1902–2005

Abstract

A time-dependent model for volume changes in pressure sources at Asama volcano is developed from precise leveling data collected since 1902. The optimal source model is determined by comparing five different models (a model with three types of spherical sources, single dike, and dike with a spherical source) during three periods: 1935 to 1939, 1939 to 1950/1953, and 1990 to November 2004. The optimal parameters for each model are estimated by employing a genetic algorithm (GA). The model with two spherical sources was selected as the optimal model. In this model, one of the sources was estimated to be located at a depth of approximately 6 km beneath Kurofu volcano; the other source was estimated to be located close to Maekake volcano at a depth of approximately 2 km. The volume changes in the two spherical sources are calculated using the Akaike's Bayesian information criterion (ABIC) for the period from 1902 to 2005. During the period from 1902 to 1943, a large inflation was estimated in a source at a depth of approximately 6 km beneath Kurofu volcano, which is an older volcano at the base of Asama. After 1943, a rapid deflation continued until 1967. This rapid deflation changed to a marginal inflation that continued from 1967 until 2005. The temporal change in the pressure source beneath Kurofu volcano exhibits a strong positive correlation with the eruption frequency. The leveling data does not suggest a significant volume change in the source beneath Kurofu volcano during the quiet period between 1962 and 2005. During the 20th century, magma appears to have been episodically supplied beneath Asama volcano. In particular, the inflation of the source beneath Kurofu volcano, and probably the magma supply, reached a peak from 1930 to 1940.