Intraplate earthquakes induced by reactivation of buried ancient rift system along the eastern margin of the Japan Sea - Intraplate Earthquakes - page 242

138.4˚ 138.6˚ 138.8˚ 139˚

b

k m

2007

Depth =15 km

0 5 0

37.4˚

2004

37.2˚

6.0 6.5 7.0

Vp (km/s)

Figure 9.4 (b) 15 km. The center of the gray scale corresponds to an average velocity calculated at
this depth (6.5 km/s). All of the relocated aftershocks (white circles) are superimposed.

that the eastward- or westward-dipping faults on the flanks of the DCT are situated along
mechanically weak structural boundaries between the thick sediment and the basement
( Figure 9.5 ) .
The geometries of the block structures associated with the ancient rift system change
along the fault strike in the vicinity of the cross-section at around Y 0kmin Figure 9.2b .

In the source region of the 2004 Chuetsu earthquake, the thickness of sedimentary layers
in the hanging wall increases markedly on the southwest side of the mainshock hypocenter
(e.g., Kato et al ., 2006a ) . For the case of the 2007 Chuetsu-Oki earthquake, a southeast-tilted
basement structure dominates in the south, whereas a northwest-tilted basement structure
gradually develops towards the north (e.g., Kato et al ., 2008a). These spatial variations
indicate the presence of a segment boundary of the block structure near the cross-section
of Y

=

=

0kmin Figure 9.2b .

9.3.3 Stress loading processes to the reactivation of the ancient rift system

Even if several ancient weak fault zones in the upper crust exist within the rift system,
a driving force to reactivate them is needed. We here discuss the mechanisms of stress
loading into source faults of intraplate earthquakes in the Niigata region.