Intraplate earthquakes induced by reactivation of buried ancient rift system along the eastern margin of the Japan Sea - Intraplate Earthquakes - page 241

138.4˚ 138.6˚ 138.8˚ 139˚

a

k m

Depth = 9 km

2007

0 5 0

37.4˚

37.2˚

2004

Anticline

DCT

Syncline

5.0 5.5 6.0

Vp (km/s)

Figure 9.4 Map views of V p structures at representative depths (Kato et al ., 2009 ) . (a) 9 km. Relocated
aftershocks distributed at depths from 7.5 km to 10.5 km (white circles) are superimposed. Gray stars
denote epicenters of the 2004 and 2007 mainshocks. Major active faults are drawn as white lines.
Anticlines and synclines are represented by white lines with arrows. A broken line indicates the
SKTL.

block changes from westward to eastward across it. Based on laboratory measurements
of exposed rocks (Christensen, 1996 ) , we interpret the high-velocity bodies beneath the
DCT to be a diabase intrusion into the Miocene rift axis. We propose that the diabase body
intruded into the upper crust, acting as a magma source within the extensional rift system
during the opening of the Japan Sea (Bjorklund et al ., 2002 ; White et al ., 2008; Figure 9.5 ) .
Indeed, andesitic and basaltic rocks are partially exposed on the surface in the study area
(GSJ, 2002).
The spatial correlation of the DCT with the surface traces of major active faults and fold
axes ( Figure 9.4a ) illustrates that the major active faults have slipped in response to upward
movements of the stepwise and tilted blocks in the basement, which moved as a result of
the horizontal compressional stress ( Figure 9.5 ) . These upward movements have led to the
active growth of anticlinal folds. The elevations of topographic hills on the eastern side
of the DCT are higher than those on the western side, due to a difference in dip angles
of faults in the basement crossing the Miocene rift axis. Additionally, it is worth noting