Multiple-pressure-source model for ground inflation during the period of high explosivity at Sakurajima volcano, Japan — Combination analysis of continuous GNSS, tilt and strain data —

Abstract

We herein propose a three-pressure-source model for ground inflation with highly eruptive activity at the Showa crater of Sakurajima volcano, Japan. We applied a model of stacked spherical sources to continuous combined geodetic data from a global navigation satellite system, tilt and strainmeters. The data were recorded during ground inflation throughout an eruptive episode that began in October 2011 and ended in March 2012. Using a genetic algorithm, we obtained the locations and volumes of three sources. A pressure source analysis of ground inflation during the period from October 2011 to March 2012 revealed inflation sources to be located at a depth of 9.6km beneath Aira caldera (A-source) and 3.3km beneath Kita-dake (K-source), and a shallow deflation source is located at a depth of 0.7km beneath Minami-dake (M-source). The A-source corresponds to the main magma reservoir at a depth of 10km beneath the Aira caldera inferred by previous geophysical studies. The K-source is a-reservoir of Sakurajima volcano, where magma intrudes from the main magma reservoir beneath the Aira caldera during the first stage of eruptive activity. The M-source is the uppermost part of a conduit from the K-source to the summit and the Showa crater. Magma injection into the A-source started in mid-November 2011, instantly triggering the migration of increased volumes of magma from the A-source to the K-source. Approximately one month later, in mid-December 2011, an increased volume of magma started migrating from the K-source to the M-source and finally erupted at the surface. The accumulation rate for the A-source is comparable to the magma supply rate for the past 100years (0.8 to 1.6×107m3). The three-pressure-source model was applied to inflation events before the 2011 event in order to reconstruct the magma migration process. Applying our source model to earlier activity phases, we found that injected magma from the A-source remained at the K-source and a small amount of magma began to move from the K-source to the M-source before the start of eruptive activity at the Showa crater in June 2006; furthermore, a large amount of magma was found to have moved toward the crater during this violent volcanic activity. We also found that the magma injection rate into the A-source sometimes reached the level of the average injection rate of the past 100years during the ground inflation events.