Fig. 2

b-value analysis based on seismicity before the 2021 M6.9 earthquake. a Maps of b values obtained from seismicity during the three periods; top panel: from 2012 to 2014, middle panel: from 2015 to 2017, and bottom panel: from 2018 to immediately before the 2021 M6.9 earthquake. In making these maps, we selected earthquakes along the plate interface (Nakajima and Hasegawa 2006): earthquakes were chosen if their depths were within a range from 5 km above the interface to 15 km below it. We calculated the b value and simultaneously M_c (Figures S1a-c of Additional file 1) for each grid node (0.02° spacing) selecting all events within a search radius of 10 km. Other symbols are the same as in Fig. 1. Epicenters of M ≥ 6.0 earthquakes that occurred in the corresponding periods are shown by open stars. b Top panel: Δb (= b_bottom—b_top), the difference in b values between the periods 2012–2014 (b_top: top panel in a) and 2018–2021 (b_bottom: bottom panel in a). Bottom panel: logP_b, the logarithm of the probability that b_bottom is not different from b_top