4.4 Comprehensive interpretation of the seismic activity in Kagoshima Bay
Here, we summarize our simple model that comprehensively explains the observed results of the foreshock-mainshock-aftershock sequence of the 2017 M5.3 Kagoshima Bay earthquake.
First, fluids that have infiltrated the mainshock fault plane caused the foreshock activity. The hypocenter migration of the foreshock activity can be interpreted considered to be a reflection of fluid movement and possibly triggered aseismic slip on the plane. Second, the occurrences of foreshocks and aseismic slip increased the shear stress in the future source region of the mainshock (seismic gap in Fig. 6). The mainshock finally occurred in this region due to the gradually increasing pore pressure and shear stress. Third, the change in the stress associated with the occurrence of the mainshock primarily triggered aftershocks in the area surrounding the mainshock including regions outside the mainshock fault plane. Fluids started to move upward due to the deformation and shaking associated with the mainshock. Together with the fluids, the aftershock hypocenters moved to shallower regions. Thus, the overall sequence of the 2017 M5.3 Kagoshima Bay earthquake can be explained by consistent upward fluid movement.