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.