Figure 1

Map showing the location of atmospheric radon observation sites, Sapporo Medical University (SMU) and Fukushima Medical University (FMU). Black circles indicate the hypocenters of earthquakes, e1–e14 (e8: N46.7°, E154.1°, e9: N47.0°, E155.0°, details in Table 1). Dashed squares mark Area 1 (N41.0°–44.0°, E138.0°–146.0°) and Area 2 (N36.0°– 39.0°, E136.0°–144.0°), delineated to calculate cumulative seismic moment³⁹ described in Fig. 2c,g. This map was generated using generic mapping tools (version 5.4.2) from an open source collection (http://www.soest.hawaii.edu/gmt/).