*3.1. Geothermal Observations*

Geothermal features were ordinarily mapped to monitor changes in geothermal activity and were used to understand precursory signals in Ebinokogen Ioyama Volcano. We monitored changes in the geothermal area from December 2015 by measuring the temperatures of the springs and fumaroles at fixed points. A thermal anomaly was observed in the area about 10 cm below the surface where the temperatures were higher than 50 ◦C. In the early stage of this observation, the area of geothermal anomaly was measured by trilateration using measuring tapes. The measurements were plotted on a plan map to calculate the area that was 50 ◦C and above in temperature. However, the anomaly area rapidly expanded, and we put colored markers at 50 ◦C points on the ground and took aerial photographs using drones (DJI Phantom 3 and 4). The markers, 18 cm in diameter, were visible from tens of meters above. The areas over 50 ◦C in temperature were plotted on the ortho-image to measure the area based on the drone observation made (Figure 3). A Google Earth [45] map was also used for the area uncovered by the drone observation. Simultaneously, we measured the temperatures and pH values of water at the spring points west of the Ioyama.

**Figure 3.** Drone image showing the distribution of geothermal features, ejecta and craters formed during the April 2018 eruption. Yellow areas: fumaroles, as observed on 3 February 2018 and jet fumarole vents (thin red circles). Orange circles: Eruption and hydrothermal vents appeared as Y1 (7 April 2018), S1 to S7 (19 to 20 April 2018) and W1 to W7 (20 to 21 April 2018). The distribution of ballistics is shown for over ϕ8 cm. Gray dash lined: ashy deposit with a thickness of 0.1 cm. Bold red lines and texts: eruption craters Y2a, Y2b, Y3, Y1, W3 and W4. Blue areas: hydrothermal ponds and streams. R is a mud pot that appeared after the eruption. The red line on the road is a crack.
