Sulfur dioxide flux monitoring using a regular service ferry after the 2014 eruption of Kuchinoerabujima Volcano, Japan

The volcanic island of Kuchinoerabujima recently erupted on Aug. 3, 2014 and on May 29, 2015. After the 2014 eruption, a gradual increase of sulfur dioxide (SO2) flux from the volcano was observed. In response to the increase, we built a UV spectrometer system that can be operated semi-automatically without an experienced observer for repetitive flux monitoring. The monitoring was carried out based on a traverse method using a regular service ferry as a carrier. With the repetitive measurements, we detected a sudden flux increase to over 2500 ton/day at the end of Nov. 2014, a high and variable flux after mid-March to early-May 2015, and a continued decreased flux (<500 ton/day) before the May 29, 2015 eruption. The detected SO2 flux variations showed that a large amount of magma was involved in gas emissions at the volcano, the conduit system became unstable from mid-March, and the conduit system became partially sealed about one week before the 2015 eruption, which was probably the turning point toward the eruption. The repetitive SO2 flux monitoring using a regular public transportation made it possible to detect important changes of flux before the eruption and was very effective for understanding the process toward the 2015 eruption.