2.1 Hypocenter relocations
We relocated 21.102 events listed in the JMA unified catalogue for the
southern Kagoshima Bay region for the period from March 1, 2003 to April
8, 2018 using the Double-Difference (DD) method (Waldhauser &
Ellsworth, 2000). This relative relocation method minimizes the
residuals between the observed and theoretical travel time differences
for adjacent earthquake pairs at each station. We applied the DD method
to differential arrival time data, which were estimated from the
waveform cross-correlation, and those listed in the JMA unified catalog.
The procedure was identical to that reported in Yoshida and Hasegawa
(2018a, b), which can be briefly described as follows.
First, we obtained precise differential arrival time data using waveform
cross-correlations. We used the waveform data observed at 20 permanent
seismic stations surrounding the focal area (Fig. 1a; green stations).
At each station, the ground velocity was measured using three-component
short-period seismometers (natural period of 1s) and a sampling rate of
100 Hz. We applied a 5–12 Hz Butterworth filter to the waveforms of
each target event. We used 2.8 and 4.3 s time windows for the P- and
S-waves, respectively, starting 0.3 s before their arrival. The arrival
times were obtained from the JMA unified catalogue. If arrival times
were not available, they were estimated using the one-dimensional
JMA2001velocity model (Ueno et al., 2002) and the hypocenters, and
origin times listed in the JMA unified catalogue. We calculated the
waveform cross-correlations of event pairs with hypocenters within 3 km
from each other and obtained differential arrival times when the
cross-correlation coefficients were greater than 0.8. In total, we
acquired 17.332.318 P-wave differential arrival time data points and
27.738.043 S-wave data points. We also derived the differential arrival
data from the arrival time data listed in the JMA unified catalog:
474.670 data for P-waves and 543.226 data for S-waves. For the
mainshock, only data derived from the JMA unified catalog were used
because of its long duration.
Second, we applied the hypo-DD algorithm (Waldhauser & Ellsworth, 2000)
to the differential arrival time data. We used a spherical shell
two-layer model (Aki, 1965) for the hypocenter relocation. In this
model, the seismic velocities in each layer are proportional to the
power of the distance from the center of the Earth (Figure S3). The
medium parameters were determined for consistency with the seismic
tomography results obtained in the Kyushu region (Saiga et al., 2010).
We used the hypocenters listed in the JMA unified catalogue for the
initial locations for the relocation. Figures 2a and 3a show the
distribution of these initial hypocenters. Differential arrival time
data were weighted with respect to the square root of the
cross-correlation coefficient. The hypocenters were updated after 50
iterations of the relocation procedure. During the first ten iterations,
a higher weight was assigned to the catalogue data to constrain the
relative locations of large-scale features. In the latter 40 iterations,
a higher weight was assigned to the data derived by the
cross-correlations to delineate shorter-scale features. We evaluated the
uncertainty in the relative hypocenter locations by recalculating the
hypocenters 200 times based on bootstrap resampling of differential
arrival time data.