Geoscience Reference
In-Depth Information
a
2007 M6.8
1828 M6.9
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.6˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.4˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
37.2˚
2004 M6.8
km
Y
0510
X
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.4˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.6˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
138.8˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
139.2˚
Survey-line in Figure 9.3
Figure 9.2 Dense seismic observations and regional V p model in the Niigata region (modified after
Kato et al ., 2009 ) . (a) Map of relocated aftershocks of the 2004 Chuetsu and the 2007 Chuetsu-Oki
earthquakes (gray circles observed in 2004, and yellow circles in 2007). Moment tensors for the
two earthquakes are from the NIED. Blue squares denote the linear seismic array, and red and blue
triangles represent temporary stations deployed after the earthquakes in 2004 and 2007, respectively.
Black squares denote permanent stations. The grid used in the tomography analysis is plotted with
crosses, and horizontal dashed lines correspond to the cross-sections in Figure 9.2 b . Major active
faults are drawn as red lines; a blue broken line indicates the SKTL. The gray broken line denotes a
seismic survey line shown in Figure 9.3 .
are superimposed. The striking aspect of the relocated aftershocks is a multi-segmented
and complex distribution. The aftershock distributions associated with the 2004 mainshock
reveal that the mainshock and the largest aftershock (A1) occurred on two 60
northwest-
ward (NW) dipping planes, located approximately 5 km apart. Conversely, the October 27
event (A2) occurred on a southeast-ward (SE) dipping plane with a dip angle of 25
°
°
that
was conjugate to the mainshock fault plane (Kato et al ., 2006a , 2009).
Similar conjugate sets of fault planes are also imaged in the aftershock sequences
following the 2007 Chuetsu-Oki earthquake (e.g., Kato et al ., 2008a) ( Figure 9.2b ) . From
the center to the southern area, most of the aftershocks aligned along a SE-dipping plane
with a low dip angle. Conversely, aftershocks in the north aligned along both NW- and
SE-dipping planes. These planes were virtually normal to each other or conjugate. The
 
 
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