Fig. 1

Observation sites and crater locations. a Iwo-yama volcano (summit is indicated by the red triangle) and the locations of the observation sites. The contour interval is 20 m. The small red circles are the vents of the 2018 small phreatic eruption. The region surrounded by the rectangle is enlarged in b. Map inset shows the location of Kirishima Volcanic Complex (KVC) on Kyushu Island, southwest of Japan. b Craters and vents of the 2018 eruption. Their locations and naming were cited from Tajima et al. (2020). The photograph was taken from GSI Tiles, of the Geospatial Information Authority of Japan. There are two newly formed main craters, Iwo-yama South Craters (the South Crater) and Iwo-yama West Crater (the West Crater). The craters were composed of several vents, namely S1 to S7 in the South Crater and W1 to W7 in the West Crater. After the onset of the eruption, the vents in the South Crater merged into three sub-craters (Y2a, Y2b, and Y3); the vents S1–S3 to Y3, S4–S6 to Y2a, and S7 to Y2b. In this study, we call the sub-craters as vents because our analysis does not have a resolution to discriminate the individual vents in the South Crater (S1–S7). Y1 appeared on April 7, 12 days before the eruption. We excluded it from the discussion because it did not participate in the eruption