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mainshock hypocenter appeared to be close to the bottom of a steeply NW-dipping plane. It
has been proposed that the mainshock rupture initiated near the bottom of the NW-dipping
fault plane and propagated to the southwest and then transferred to the SE-dipping plane
across the central crosscutting area of the two faults (Kato
et al
., 2008a; Takenaka
et al
.,
along the inferred segmented fault system using a boundary integral equation method
(BIEM). For angles between these fault planes from 80
, the possibility of rupture
transfer from the NW-dipping fault plane to the SE-dipping is numerically demonstrated
for any frictional level along fault planes, independently of the crosscutting distance in the
center, suggesting two rupture modes. Simultaneous rupture transfer along the overlapping
part is possible only under a high-stress load; however, this rupture mode yields an exces-
sively large amount of coseismic slip. Otherwise, where regional stress is relatively low but
pore pressure is high enough to cause the rupture (described as the low frictional coefficient
case), the rupture transfer to the other fault segment does not occur until rupture terminates
on the first fault segment regardless of the crosscutting distance between the two faults.
Similarly, the 2009 Suruga Bay intraplate earthquake (M 6.4) in Japan, which occurred
within the Philippine Sea plate and had a reverse fault component, showed a similar rup-
These recent studies of dynamic rupture process associated with large intraplate earth-
quakes suggest that rupture transfer or simultaneous ruptures are more common features
than previously thought.
°
to 95
°
9.3.2 Ancient rift system buried beneath thick sedimentary basin
9.2b
)
, especially orthogonally to the fault strike (Kato
et al
.,
2006a
,
2009). Seismic velocities
in the hanging wall above the 2004 mainshock fault are lower than those in the footwall at
depths shallower than 8 km. We consider that the low-velocity body in the hanging wall
corresponds to soft sediments that have accumulated in half-grabens formed by crustal
stretching during the opening of the Japan Sea. Conversely, the high-velocity body in the
footwall is thought to correspond to the old basement rocks (30 Ma). Here, we define the
basement as high-velocity bodies in which
V
p
is greater than 5.7 km/s (bounded below by
the white curves in
Figure 9.2b
)
. Most aftershocks seem to be bounded by the basement.
The centroid depth of the relocated aftershocks associated with the 2007 mainshock is
slightly deeper than that of the 2004 mainshock. This lateral variation of aftershock depths
well correlates with that of thickness of sedimentary layers, which on average increases
with distance towards the west.
It is important to note that the top surface of the basement on the eastern side (
−
5km
<
X
<
15 km) shows clear stepwise structures that gradually deepen in the westward direction.
In addition, westward-tilted block structures are interpreted by aftershock streaks, the top
+