Fig. 1

LFEs along the Nankai Trough. a LFE epicenters and 43 rectangular subregions used in this study. Red circles denote LFE epicenters from April 2004 to August 2015 (Kato and Nakagawa 2020). Blue rectangles and crosses indicate the subregions and their centers, respectively. The light green rectangles and crosses denote the subregion locations selected in Sect. “Subregion examples”. The dashed black line denotes the trench axis of the Nankai Trough. The orange areas indicate the Nankai Trough megathrust seismogenic zones (Obara and Kato 2016). The small inset map shows the location of the Nankai Trough. b LFE size-frequency distribution. The blue solid line indicates the number of events expected from Gutenberg–Richter’s relationship (Gutenberg and Richter 1944) (b = 3.40). c Goodness-of-fit value R for the goodness-of-fit test method (Wiemer and Wyss 2000; Woessner and Wiemer 2005). The arrow indicates the smallest magnitude with an R > 95% (M 0.4)