Geoscience Reference
In-Depth Information
9
Intraplate earthquakes induced by reactivation of
buried ancient rift system along the eastern
margin of the Japan Sea
AITARO KATO
Abstract
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 formed during the spreading of
the Japan Sea, are widely distributed beneath the thick sedimentary basin in
the Niigata region. A similar structure associated with the ancient rift system
is imaged in the source area of the Noto-Hanto earthquake. Most aftershocks
following the recent intraplate earthquakes align roughly along the tilted block
boundaries of the basement and are controlled by weaknesses associated with
buried rift systems. Furthermore, we discuss the stress loading mechanisms for
source faults of intraplate earthquakes. The structural coincidence between the
stress axis distribution and the velocity structure observed in the Niigata region
raises the possibility that ductile deformation 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 plausible mechanical explanation for intraplate earthquakes.
9.1 Introduction
Many intraplate earthquakes have occurred in the Japanese islands as a result of internal
deformations of overlying plates. The Japanese islands are situated in a tectonically active
zone, where two oceanic plates are subducting. Beneath northeast Japan, the Pacific plate
is subducting from the east through the Japan Trench at a convergence rate of
8.5 cm/yr,
