3.1 Fault structure and seismic gap
We obtained the relocated hypocenters of 20.347 events and the focal mechanisms of 61 events. Almost all events in the Kagoshima Bay earthquake sequence can be accurately relocated with the DD algorithm. The location data for 755 earthquakes were removed because their hypocenters were located above the ground surface or they contained outliers in the differential arrival time data. We computed the differences between the maximum and minimum values in the 95% confidence interval of the hypocenter locations (Fig. S6) estimated from the bootstrap resampling and obtained the medians as a measure of the estimation error of the relative location: 0.00\(13\) in longitude, 0.0011 in latitude, and 0.42 km in depth.
Figures 2b and 3b show the distribution of the relocated hypocenters. Movie 1 shows the animation of the cross-sectional views of the hypocenters along various lines. Most hypocenters are located within ~5 km from the mainshock hypocenter and are distributed along several planes. These characteristics are in contrast to the distribution of the initial hypocenters (Figs 2a and 3a), which were scattered three-dimensionally, similar to a cloud. This significant change in the hypocenter distribution is due to the improvements of the relative locations of the hypocenters in this study based on the use of many accurate differential arrival time data. Similar improvements of the relative hypocenters, from cloud-like distribution to planar structures, were previously reported for shallow earthquakes in Japan based on a similar method and data (e.g., Yoshida & Hasegawa, 2018a, b). The cloud-like distribution of the initial hypocenters reflects the errors in the hypocenter locations in the JMA unified catalog, which are due to errors in the manual selection.
Figure 4 shows the spatial distribution of the focal mechanisms. Because the reference focal mechanisms are in the northern part of the source region (Fig. 1b), newly estimated focal mechanisms are mainly located in the northern part. The figure shows that the nodal planes of most focal mechanisms are parallel to the planar structures of the hypocenters, suggesting that individual small earthquakes occurred on several macroscopic planes.