Fig. 1

a Location of Kusatsu-Shirane volcano. Black triangles denote active volcanoes defined by JMA. b Locations of V-net stations at Kusatsu-Shirane. Pink diamonds correspond to major thermal spring sites. A black square is the map area of c. c A detailed map including Yugama, Ainomine, and Motoshirane. Blue and white squares show observation stations operated by KSVO and JMA. Vent locations of the 2018 eruption and unidentified past events are denoted by red and violet colors (GSI, 2018). Seismic data at YNE and KSJ are adopted in hypocenter determinations by KSVO, although we do not adopt these into the ASL analysis in this paper