Figure S1. An example showing stability of local seismicity. (a) Average seismicity rate (number of earthquakes per year in each grid) for 10 years between 2005 and 2014. (b) Average seismicity rate for 2 years between 2015 and 2016.
135.0˚ 135.1˚ 135.2˚ 135.3˚ 135.4˚ 135.5˚
33.9˚
34.0˚
34.1˚
34.2˚
34.3˚
34.4˚
135.0˚ 135.1˚ 135.2˚ 135.3˚ 135.4˚ 135.5˚ 0 25 50 75 100 125 150 175 200
Number of earthquakes per year
2005/01/01 - 2014/12/31 2015/01/01 - 2016/12/31
a b
Figure S2. (a) Original hypocenter distribution from the JMA unified catalog. (b) Relocated hypocenter distribution using the double-difference algorithm of Waldhauser and Ellsworth (2000) with the JMA2001 seismic velocity model (Ueno et al. 2002). The distribution of seismograph stations used in the relocation is shown in the inset of (a).
The two dotted red lines delineate the boundaries between the spotted, non-spotted, and Mikabu zones from north to south.
Mj 4.0 Mj 3.0 Mj 2.0 Mj 1.0
1 2 3 4 5 6 7 8 9 10
Depth (km)
Mj 4.0 Mj 3.0 Mj 2.0 Mj 1.0
Depth (km)
1 2 3 4 5 6 7 8 9 10
:Station
a b
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
Depth (km)
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
Depth (km)
Figure S3. Bouguer anomalies across the study region. The inset map shows the Bouguer anomalies across the entire study region and the locations of survey lines; the colors and
33.9 34.2 34.3 34.4
33.9 34.2 34.3 34.4 33.933.9 34.234.2 34.334.3 34.434.4 33.933.9 34.234.2 34.334.3 34.434.4
33.9 34.2 34.3 34.4
33.9 34.2 34.3 34.4 33.933.9 34.234.2 34.334.3 34.434.4
Original Data Detrend Data
Latitude (°N) Latitude (°N)
Latitude (°N)
Latitude (°N) Latitude (°N)
Bouguer anomaly (mGal)Bouguer anomaly (mGal)
a a’ b b’ c c’
e
d d’ e’
Figure S4. Depth distribution of the F-net (NIED) focal mechanism solutions (Mj ≥ 4.0).
The presented focal mechanism solutions span the period from 1 January 2000 to 15 March 2021.
0
2
4
6
8
10
Depth (km)
Figure S5. Best-fit 2-D gravity models along survey lines a–a’ to e–e’. Two panels are shown for each survey line: the upper panels provide a comparison of the detrended observed anomalies (red cross symbols) and calculated anomalies (cyan plus symbols), and the lower panels show the best-fit density structure models that were used to obtain the calculated Bouguer anomalies. Assumed densities of 2.67 g cm–3 and 2.83 g cm–3 are used for the background and green regions, respectively. The inset map shows the observed Bouguer anomaly map for the entire study region and the locations of survey
