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.