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whereas beneath southwest Japan, the Philippine Sea plate is subducting from the southeast
through the Nankai Trough at a convergence rate of
2-5 cm/yr (e.g., Loveless and
the Japanese islands are more frequent than in other regions in the world. Damage caused
by the intraplate earthquakes is usually devastating due to their shallow depths (less than
15 km).
Particularly, in the eastern margin of the Japan Sea, historical and recent destructive
intraplate earthquakes (e.g., the 1964 Niigata earthquake with JMA magnitude (M
JMA
)7.5;
the 1983 Japan Sea earthquake with M
JMA
7.7) have been concentrated along a zone of high
east-west contractional strain rates detected by geodetic measurements (larger than 10
−
7
per year) and geological studies (Sagiya
et al
.,
2000
;
Okamura
et al
.,
2007
)
(
Figure 9.1a
)
. In
addition, shallow microseismicity revealed by a state of the art nationwide high-sensitivity
(shaded area in
Figure 9.1b
)
. Within the contractional zone, three destructive intraplate
earthquakes showing reverse faulting with a strike of approximately N35
E most recently
occurred in the Niigata and Noto-Hanto regions. In addition, a shallow intraplate earthquake
withM
JMA
6.7 was induced by the 2011 M
w
9.0 Tohoku-Oki earthquake at the south portion
of this contractional zone.
These earthquakes are commonly located within Miocene-Pleistocene sedimentary
basins. These sedimentary basins were formed as back-arc basins in a rift structure that
developed during the opening stage of the Japan Sea (25-15 Ma) (Sato
et al
., 1994). Several
normal faults have subsequently been inverted as reverse faults owing to a change in the
tectonic stress regime from extension to compression since 3.5 Ma. This stress inversion
led to well-developed thrusts and related surface folding. The overlap between intraplate
earthquakes and ancient rift systems beneath thick sediments suggests that ancient rift
systems are important for nucleating present-day intraplate earthquakes in the compres-
sional inverted basins. However, the details of the ancient buried rift structures and their
potential effects on the seismogenesis of large intraplate earthquakes have not been fully
understood. In addition, the driving force behind the generation of intraplate earthquakes
been argued that local heterogeneities in crustal structure play an important role in con-
trolling the spatiotemporal evolution of seismicity and associated faulting behavior (e.g.,
et al
.,
2011
)
. In order to illuminate these issues, it is thus critical to fully describe the crustal
heterogeneity originating in the ancient rift system. Seismic tomography combined with a
dense seismic network are powerful tools for imaging high-resolution crustal structures as
well as precise hypocenters, and offer new insights into potential seismogenic structures.
The dense and well-covered ray-paths from the many aftershocks triggered by each large
earthquake provide us precious opportunities to (1) investigate the regional velocity struc-
ture and stress field in detail with a spatial resolution of
°
3-5 km, and (2) demonstrate that
crustal heterogeneities associated with the Miocene rift structures significantly contribute
to present-day seismogenesis along the eastern margin of the Japan Sea.
