Geoscience Reference
In-Depth Information
boundary, reducing shear strengths and facilitating ruptures there. Given these consid-
erations, we hypothesize that fluid migrations along the fault, along with relative
mechanical weaknesses within the fault zone, were the principal factors that caused the
2007 Noto-Hanto earthquake by reactivating a pre-existing normal fault created during the
opening of the Japan Sea.
9.5 Conclusions
We imaged the entire crustal structure and stress field of buried ancient rift systems including
heterogeneity within the crust through the seismic-tomographic analysis utilizing a dense
seismic network deployed immediately after three recent large intraplate earthquakes along
the eastern margin of the Japan Sea. We discovered that stepwise and tilted block structures
of the basement, which are geophysical evidence of a Miocene rift system, are widely
distributed beneath the thick sedimentary basin in the Niigata region. A similar structure
associated with the ancient rift system was imaged in the source area of the Noto-Hanto
earthquake. Mainshock fault planes with high dip angles are far from an optimal orientation
against the regional stress field, which means that those faults are mechanically weak. Most
aftershocks following recent intraplate earthquakes align roughly along the tilted block
boundaries of the basement and are controlled by weaknesses associated with buried rift
systems.
Furthermore, the structural coincidence between the stress axis distribution and the
velocity structure observed in the Niigata region raises the possibility that ductile deforma-
tion of the sediments can partially accumulate elastic strain in the brittle parts of the fault
zone. In addition, low-velocity anomalies are localized beneath the seismogenic zones,
indicating that fluids may have locally weakened the crust. This study therefore suggests
that reactivation of pre-existing faults within ancient rift systems by stress loading through
ductile flow in the upper crust and creeping of the locally weakened lower crust is a plau-
sible mechanical explanation for intraplate earthquakes. We believe that this mechanical
explanation is applicable to other intraplate earthquakes in compressional inverted basins,
beneath which ancient rift systems may exist, such as the NewMadrid seismic zone (NMSZ)
In the NMSZ, a local low-velocity anomaly in the lower crust and upper mantle is imaged
stress to the upper crust when loaded, thus leading to repeated shallow earthquakes in the
NMSZ, which is a similar picture to that illuminated along the eastern margin of the Japan
Sea.
In order to deepen our understanding of intraplate earthquake generation, a more quan-
titative approach such as numerical simulation is quite important. Although Shibazaki and
of fault zones and the heterogeneous crustal structure deduced from seismic tomography,
more information is required on the distribution of pre-existing weak fault zones in order
