Characterizing thermal transitions of the 2018 Shinmoe-dake eruption in Japan using Himawari-8 high-frequency infrared imagery and seismic data

Discrete explosive eruptions, including vulcanian eruptions, occur at various stages during the effusion of viscous lava. However, the differences in occurrence time and mechanisms are not yet well understood or categorized. We investigated the high-frequency infrared observation data from Himawari-8 and seismic data for the 2018 Shinmoe-dake activity and also the 2019 Bezymianny activity for comparison. Each stage of the 2018 Shinmoe-dake activity showed characteristic thermal variations: the ash-plume emission stage had no thermal anomalies, the lava effusion stage had continuous large thermal anomalies, and the vulcanian eruption stage had a series of spike-like thermal anomalies. The timing of individual thermal anomaly spikes coincided with the onset of vulcanian eruptions. Each spike exhibited an asymmetric thermal pulse, consisting of an abrupt thermal increase followed by a gradual decrease, corresponding to the deposition of hot ejected materials and subsequent cooling, without any precursory thermal anomaly. In contrast, the 2019 Bezymianny activity generated a symmetric or two-sided thermal pulse, involving a precursory anomaly; the thermal anomaly increased with lava effusion, peaked with an explosive eruption, and decreased due to cooling. Thus, this study identified two types of explosive eruptions. In the post-effusion type (2018 Shinmoe-dake), the lava effusion continues without explosive eruptions until the cessation of effusion, eventually forming a lava bed with a solid surface covering the vent. This forms a cap rock and is the cause of subsequent vulcanian eruptions. These eruptions show an asymmetric thermal pulse with no precursory anomaly. In the syn-effusion type (2019 Bezymianny), in contrast, the lava effusion activity transitions to an explosive eruption while lava is still effusing. Accordingly, the discrete explosive eruption shows a symmetric thermal pulse involving a precursory thermal anomaly. Their difference can be related to whether the volatile components are efficiently released from the ascending magma during the lava effusion.