Intraplate earthquakes induced by reactivation of buried ancient rift system along the eastern margin of the Japan Sea - Intraplate Earthquakes - page 245

of this system in the form of intraplate earthquakes. Since the width and depth of the weak
zones provide constraints on the process of stress loading to intraplate earthquakes, detailed
knowledge of the heterogeneity within the lower crust is quite important.

9.3.4 Numerical modeling of development of fault zones

Using finite element modeling incorporating visco-elasto-plasticity, Shibazaki and Kato
( 2012 ) numerically simulated the development of fault zones in a geological setting with
thick sedimentary layers and weak zones in the basement inferred from the tomographic
study in the Niigata region ( Figure 9.2b ; Kato et al ., 2009 ) . It is assumed that values of elastic
constants and friction coefficients in the sedimentary layer are smaller than those in the
basement. Furthermore, they assumed that the frictional coefficient in the basement is low
in areas of low P-wave velocity, examining how the present rheological structures inferred
from P-wave velocity structure affect the development of fault zones. Figure 9.6 shows
the equivalent total strains defined for the deviatoric components of the total strains after
8.0 10 4 years (i.e., the amount of contraction is 1.2 km). The equivalent total strains

include viscous, plastic, and elastic strains. Fault zones are created just above concavities
along the boundary between the lower sedimentary layer and the basement. For example,
along all sections, fault zones are well developed at locations where the effective friction
coefficient is low in the basement and above the DCT, confirming the development of a
NW-dipping reverse fault zone, which was activated by the mainshock of the 2004 Chuetsu
earthquake. The model also shows the development of NW-dipping faults that were parallel
to the fault of the mainshock. These faults may correspond to the faults of large aftershocks.
In addition, the model shows the development of SE-dipping reverse fault zones that were
activated by the mainshock of the 2007 Chuetsu-Oki earthquake. Thus, the numerical results
indicate that buried rift and weak zones in the basement caused the development of the
complex fault configuration observed in the Niigata region.

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9.4 The 2007 Noto-Hanto earthquakes

A shallow M JMA 6.9 inland earthquake occurred on the west coast of the Noto Peninsula
in Japan on March 25, 2007. The focal mechanism estimated by moment tensor inversion
(NIED) revealed a dominant reverse slip component with a strike of approximately N55 E

( Figure 9.7a ) . The Noto Peninsula is situated close to the Yamato Basin in the Japan Sea,
which is a large rift basin formed during the extension stage of the Japan Sea (e.g., Shimazu
et al ., 1990 ) . Based on geological studies of early Miocene syn-rifting succession in the
northern regions of the Noto Peninsula (e.g., Kano et al ., 2002 ) , it is proposed that small-
scale rift structures were formed along the northern coast of the Noto Peninsula during the
extension stage of the Japan Sea. Following the 2007 Noto-Hanto earthquake, we deployed
a total of 89 temporary seismic stations on land in the source region. The seismic network
was densely deployed with station spacing averaged less than 2 km ( Figure 9.7a ) .

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