Abstract
Mechanisms that control fluctuation of long-term eruptive activity are not well understood. Since 1955, Sakurajima volcano has experienced repeated Vulcanian eruptions accompanied by deflation. However, ground deflations sometimes occurred even if no eruption took place. Characteristics of the non-eruptive deflation (NED) have not been investigated because of the related ambiguous ground vibration, infrasound and visual phenomena. In this study, we categorize deflation events contained in continuous deformation records covering eruptive activities at the Sakurajima volcano Minamidake crater during the period from November 2017 to December 2020. The deflation events are classified into eruptive and NED types by examining the seismic data, infrasound generations, and visual phenomena. Their event type occurrence characteristics are then further clarified based on their relationships to pressure source volume changes. Comparing the pressure source volume changes with the monthly mass of volcanic ash expelled from the crater, which is estimated from deposition data around the volcano, we found the contribution of NED events to volcanic ash mass eruption amounts to be much smaller than that for Vulcanian events, and visual observations confirm that NED events are phenomena that primarily emit volcanic gas. The NED event changes during the episode are interpreted by the magna transition from bubble-rich to bubble-poor conditions and are supported by the sulfur dioxide (SO2) discharge rate. The activity of NED events is helpful to understand long-term variations in eruptive activity.
Highlights
Eruptive activity at volcanoes with the open-vent systems often continues for a long time, changing its intensity and the style of the eruptions and location of vents
Volume changes and depths of pressure source inducing ground deformation events associated with Ex- and Er-type eruptions and non-eruptive deflation (NED) events are estimated according to Eq 6, fixing the pressure source location at Minamidake crater
The volume change amounts in November 2019 are the largest in terms of both eruptive and NED events, while the NED event peaks in December 2017 March 2018, and February 2020 are significantly larger in relation to eruptive volume amounts changes, which stayed at normal or lower levels
Summary
Eruptive activity at volcanoes with the open-vent systems often continues for a long time, changing its intensity and the style of the eruptions and location of vents. The eruption center shifted to the Showa crater, which had been dormant for 58 years, and in June 2006, a series of eruptive activities commenced that lasted until October 2017, at which time the eruption center returned to the Minamidake crater where eruptive events were ongoing as of May 2021 These eruptive activity transitions are influenced by long-term magma intrusions from a large magma chamber beneath the Aira caldera to the north of Sakurajima volcano
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