Characteristics of micro-earthquake swarms preceding eruptions at Showa crater of Sakurajima volcano, Japan

Abstract

The Showa crater of Sakurajima, an andesitic volcano in Japan, resumed eruptive activity in June 2006 for the first time in 58 years. Explosive eruptions at the Showa crater have been active since 2009. Micro-earthquake swarms have sometimes been observed in the 1 or 2 h preceding these eruptions since 2012 and have frequently occurred during periods of low eruptive activity. These precursory micro-earthquakes are dominated by high-frequency (5–6 Hz) components and no clear S-wave and are thus classified as BH-type earthquake swarms. The hypocenters of the earthquakes are located in a shallow region (depth of approximately 1 km) near the Showa crater. Prior to the eruptions, the onset of inflation has been observed by extensometers, followed by precursory earthquake swarms 30 min to 1 h later. In the case of insufficient ground contraction, the precursory earthquakes reoccur when the strain reaches the same level it showed just before the eruptions occurred. From the occurrence period and pattern of the precursory earthquakes, it appears they may be generated within a narrow conduit leading to the Showa crater.