Fig. 1
Seismicity in the study region. a Epicenter distribution of earthquakes before the 2019 Yamagata-Oki earthquake. The yellow star is the epicenter of the 1964 Niigata earthquake. The black star is the epicenter of the Mj 5.9 aftershock that occurred immediately after the 1964 Niigata earthquake (redetermined by Miyaoka et al. (2019)). Blue circles denote the aftershocks that occurred within 24 h of the 1964 Niigata earthquake. Gray circles are the earthquakes that occurred before the 2019 Yamagata-Oki earthquake (1922–2019). Gray lines are the surface traces of active Quaternary faults. b Epicenter distribution of the earthquakes that occurred before and within 24 h of the 2019 Yamagata-Oki earthquake. The black star, southern yellow star, blue and gray circles, and gray lines are the same as in a. The northern yellow star denotes the epicenter of the 2019 Yamagata-Oki earthquake. Red circles denote the earthquakes that occurred within 24 h of the 2019 Yamagata-Oki earthquake. Focal mechanisms were estimated by JMA (2019 Yamagata-Oki earthquake) and Abe (1975) (1964 Niigata earthquake). c Regional map of Japan, with the study region indicated by the blue square. d Magnitude–time plot of the earthquakes (1922–2019) that occurred in the study region. Mj ≥ 1.8 earthquakes are shown in green. The two-way arrow indicates the time period used to estimate the HIST-ETAS parameters