Fig. 2

Time series of a the surface area (black circle) and surface temperature (blue square) of the crater lake and b plume SO₂ flux. The surface area of the crater lake is indicated by the ratio of the area to the crater bottom. The value of 0 in the surface area corresponds to the situation where the crater lake does not exist. The value of 0.1 in the surface area includes the situation where although the crater lake exists, the amount of water cannot be estimated because the crater is mostly covered by the volcanic plume. At the upper part of a, volcanic alert levels issued by the Japan Meteorological Agency (JMA) are presented with a corresponding color code, and the dates of occurrence of eruptions and quite minor mud emissions from the crater bottom are depicted with red and gray dots, respectively. The enlarged dots of eruptions correspond to the phreatomagmatic eruptions on 14 September 2015, 23 October 2015, and 8 October 2016. The text “Strombolian eruptions” corresponds to the period when magmatic eruptions were observed (i.e., intermittent ash emissions and Strombolian eruptions). The data are from monthly reports by the JMA (available at http://www.data.jma.go.jp/svd/vois/data/tokyo/STOCK/monthly_v-act_doc/monthly_vact_vol.php?id=503)