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Fig. 2 | Earth, Planets and Space

Fig. 2

From: Changes in the b value in and around the focal areas of the M6.9 and M6.8 earthquakes off the coast of Miyagi prefecture, Japan, in 2021

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 Mc (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 (= bbottombtop), the difference in b values between the periods 2012–2014 (btop: top panel in a) and 2018–2021 (bbottom: bottom panel in a). Bottom panel: logPb, the logarithm of the probability that bbottom is not different from btop

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