3.2 Foreshock and aftershock migration
Figures 7a–c show the occurrences of the foreshocks on a color scale. In Figs 8a–c, the occurrence of each earthquake is compared with the longitude, latitude, and depth, respectively, to illustrate their migration behavior. In the longitudinal direction (Fig. 8a), the hypocenters expand nearly symmetrically in the first 230 days of foreshock activity and concentrate in the east close to the hypocenter of the mainshock during the last ~70 days. In the latitudinal direction (Fig. 8b), the hypocenters migrate from north to south. In the depth direction (Fig. 8c), the hypocenters migrate both in the shallow and deep directions, indicating that most earthquakes occurred in the deeper part surrounding the mainshock hypocenter during the last ~70 days of activity.