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.