Figure 4. Estimated focal mechanisms plotted on the hypocenter
distribution. The left figure is a map view and the nine figures (A–I)
on the right are cross-sectional views along vertical sections indicated
by the red lines in the left figure.
Based on Figs 2b, 3b and 4, the fault structures of the 2017 Kagoshima
Bay earthquake sequence are complex, consisting of several subparallel
planes. However, the distribution of the hypocenters was relatively
simple before the mainshock. Figure 5 shows an enlarged view of the
spatial distribution of the hypocenters of the foreshocks (red dots).
Most hypocenters are evenly distributed in one plane, with a strike
parallel to those of the nodal planes of the focal mechanisms of the
mainshock and individual small earthquakes, suggesting that the
mainshock and most of the foreshocks occurred on this plane.