Ground deformation after the 2015 phreatomagmatic eruption at Kuchino-Erabujima Volcano, Japan detected by InSAR time series analysis with ALOS-2 data

The Kuchino-Erabujima Volcano has experienced multiple phreatomagmatic eruptions since 2015. Monitoring ground deformation at active volcanoes is essential for understanding magma supply systems and predicting volcanic activity. However, maintaining continuous ground-based observations near active craters has been challenging. To investigate the crustal deformation of the Kuchino-Erabujima Volcano after the explosive phreatomagmatic eruption in 2015, we applied an InSAR time series analysis using data of the Advanced Land Observing Satellite 2 (ALOS-2) acquired between June 2015 and November 2021. The results revealed a localized subsidence of more than 1.0 cm/y in the area within a radius of 500 m, centered on the Shin-dake crater. This deflation source is estimated to be located at a depth of approximately 300 m beneath the crater, indicating a shallow magma and/or fluid reservoir. The deformation shows temporal variations, with stagnation and inflation preceding the explosive phreatomagmatic eruptions in 2018 and 2020, respectively. These fluctuations were correlated with changes in the Global Navigation Satellite System (GNSS) baseline length, SO₂ emissions, and volcanic seismicity. This suggests that pressure changes in the shallow reservoir source are linked to those in the deep magma chamber and volcanic activity. In particular, stagnation and inflation may represent eruption preparation processes in shallow reservoirs. Our findings shed light on the behavior of the shallow hydrothermal system in the precursor to phreatomagmatic eruptions and demonstrate the effectiveness of InSAR time series analysis for monitoring deformation in inaccessible volcanic areas.