Geoscience Reference
In-Depth Information
Elevation (m)
S
K
T
L
1000
800
600
400
200
0
-200
Basement
dept
h
(km)
W
e
a
k
fa
u
l
t z
o
n
e
s
0
5
D
CT
10
σ
1
2
00
4
E
Q
Fl
ui
d
s
S
o
li
di
fi
ed
In
tr
u
si
on
s
F
lu
id
s
2
0
0
7
EQ
Figure 9.5 Three-dimensional perspective views of active tectonics in the study area (modified after
Kato
et al
.,
2009
)
. (Top) Topography, over which active faults are drawn as solid lines. The broken line
indicates the SKTL. (Bottom) Perspective image of the depth to
V
p
=
5.7 km/s with interpretations.
Gray rectangles denote potential weak fault zones along the flanks of the DCT. Dominant reactivated
normal faults are drawn as broken lines with arrows showing slip directions during compressional
inversion. White arrows indicate the inferred direction of the maximum stress
σ
1
. Note that the
σ
1
axis rotates from W30°N-E30°S in the central area of the source region of the 2004 Chuetsu
earthquake to the E-W horizontal directions in the southwestern area. For color version, see Plates
section.
The spatial variation of stress fields in the source area of the 2004 Chuetsu earthquake
provides us with a valuable opportunity to understand the deformation of sedimentary layers
aftershock data (focal mechanisms) show that the maximum principal stress rotates from
W30
S in the central area of the source region to the E-W horizontal directions in
the southwestern area (
Figure 9.5
)
. In short, the compressional stress axis in the southwest
area is oblique to the fault strike of N35
°
N-30
°
E, even though the compressional stress axis
in the central part is almost perpendicular to the fault strike. Near the hypocenter of the
mainshock rupture, the thickness of sediment layers in the hanging wall increases towards
E35
°
°
S, as well as increasing in the S35
°
Wdirection. As a result, the iso-velocity contour of
