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Fig. 3 | Earth, Planets and Space

Fig. 3

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. 3

Cross-sectional views of b values and seismicity since 2018 until the 2021 M6.9 earthquake. a Left panel: b values computed based on seismicity along the line segment “A” in Fig. 1b. To create this figure, we project sampled earthquakes onto a vertical plane extending from P1 to P2 in Fig. 1b, roughly perpendicular to the Japan trench axis (Fig. 1a). At each grid node (1 km grid spacing), we selected all events within a search radius of 10 km and considered a swath width of 10 km, resulting, at each grid node, in a cylindrical sample volume of 20 km diameter and 10 km depth. Black curve indicates the upper surface of the Pacific plate (Nakajima and Hasegawa 2006). Grey segments indicate the rupture areas of the 2021 M6.9 and M6.8 earthquakes (Tohoku University 2021a) (hypocenters indicated by stars). Right panel: black, red, and blue circles represent seismicity before the 2021 M6.9 earthquake, between the 2021 M6.9 and M6.8 earthquakes, and after the 2021 M6.8 earthquake, respectively. b Same as a for the cross-section along the line segment “B” in Fig. 1b. c Events within the three circles of radius r = 10 km, indicated by “1”, “2”, and “3”, are used to show a fit of the GR law to observations. Color corresponds to the b value indicated by the color bar. The uncertainty estimates are according to Shi and Bolt (1982). logPb1,3 is the logarithm of the probability that the b values for the circles 1 and 3 are not different and logPb2,3 is the same as that for circles 2 and 3. d Plot of b values as a function of time for circle 1

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