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.