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surface geometries of the basement, or velocity changes within the basement (broken white
lines in
Figure 9.2b
)
. The boundaries between each stepwise and tilted block structure are
mainly characterized by west-dipping faults with high dip angles. Near the center of the
cross-sections (X
−
10 km), the top surface of the basement reaches its deepest depth
(
25 km). This
concave basement structure forms a deep central trough (DCT) filled with thick sediments
between the source areas of the two Niigata earthquakes. In contrast, on the western side
of the DCT (X
<
10 km thickness of sediments) and then rises sharply to the coastline (X
=−
20 km), the stepwise block structures are primarily tilted eastward with
low dip angles (eastward-dipping low-velocity zones located just above the 2007 aftershock
alignments along Y
−
10 km), although westward-tilted structures also developed,
especially in the north sections (Y
>
=−
5 and
−
5 km) showing complete inversion; in short, the
west block appears to hang over the footwall. The stepwise and tilted block structures are
observed along all of the cross-sections
(
Figure 9.2b
)
, with block widths ranging from 5 to
10 km.
Furthermore, a very fine scale (
+
1 km grid) seismic tomography using a linear seis-
mic array in the southwestern edge of the source region in the 2004 Chuetsu earthquake
confirmed that the aftershocks appear to be aligned along pre-existing boundaries of the
step-like array of tilted block structures (Kato
et al
.,
2010b
;
Figure 9.3
)
. With deepening
of the basement-cover contact to the west, the sedimentary succession in the hanging wall
deepens to about 9 km. Indeed, the westward-tilt of the sedimentary strata was delineated by
sedimentary basin correlates well with the seismic reflection profile. The surface elevation
increases towards the southern part of the SKTL (Muikamachi fault), which illustrates that
the cover sequence has been deformed by upward movements of the step-like tilted blocks
in the basement. There seem to be three plateaus within the topography to the west of the
Muikamachi fault trace (gray horizontal lines in the top graph in
Figure 9.3
)
. We therefore
hypothesize that these plateaus might be created by both upward movements of the three
tilted blocks and compressive deformations of the sedimentary strata in the hanging wall
through compressional inversion.
The stepwise and tilted block structures as described above are clear evidence of the
buried Miocene rift system formed during the spreading of the Japan Sea. It is worth-
while noting that the aftershock distributions associated with the two Niigata earthquakes
correlated well with these complex and heterogeneous structures. Most aftershocks are
aligned along the NW-dipping faults with high dip angles (
60
°
) or along the SE-dipping
faults with low-dip angles (
planes are orthogonal to each other, and correspond roughly to the boundaries of the tilted
blocks. These results suggest that the seismogenesis of the two Niigata earthquakes was
due primarily to compressional inversion tectonics involving pre-existing structures related
to the Miocene rift system. Since pre-existing faults within the ancient rift system are weak
35
°
