Intraplate earthquakes induced by reactivation of buried ancient rift system along the eastern margin of the Japan Sea - Intraplate Earthquakes - page 244

V p = 5.7 km/s undergoes a marked change around the mainshock hypocenter ( Figure 9.5 ) .

The structural coincidence between the stress axis distribution and the velocity structure
raises the possibility that rotation of the compressional stress axis in the southwest area
might be caused by lateral variation of sediment layers in the hanging wall. Sediments in
the hanging wall with low elastic modulus can potentially allow ductile flow along the fault
zone when compressional shear stress is applied. The ductile deformation of the sediments
can partially accumulate elastic strain in the brittle parts of the fault zone, and may play
a role in stress loading. Although it is difficult to directly demonstrate the stress loading
process by the ductile deformation of the sediments into source faults, there is geodetic
evidence showing ongoing ductile deformation of the sediments. For example, following
the 2007 Chuetsu-Oki earthquake, episodic growth of fault-related folds in the shallow
sedimentary layer was clearly detected by SAR interferometry, and did not accompany
any seismicity (Nishimura et al ., 2008 ) . The long-term leveling measurement supports the
episodic growth of folds therein.
In addition to the deformation of sediments, it has been proposed that ductile creeping
of the weak lower crust could cause stress loading into seismogenic faults (Iio et al ., 2002 ) .
Note that slow anomalies in the lower crust ( V p = 6.1-6.3 km/s) are localized around deep

extensions of mainshock faults for the 2004 and 2007 Niigata earthquakes ( Figures 9.4 and
9.5 ) . Furthermore, a highly conductive body was found in the lower crust (deeper than
15 km) beneath the source region of the 2004 earthquake by wideband magnetotelluric
survey (Uyeshima et al ., 2005 ) . Thus, these slow anomalies in the lower crust probably
represent crustal fluids that might be exsolved from the solidified intrusions beneath the
rift axis. According to a regional (larger-scale) tomography study conducted in the Niigata
region (Nakajima and Hasegawa, 2008 ) , these slow anomalies appear to extend to the
deeper part of the crust and connect to a distinct low-velocity zone beneath the Moho
(

30-50 km depth) under source areas of the two Niigata earthquakes. Similarly, from the
backbone mountain range to the fore-arc side of northeast Japan, several low-velocity zones
are recognized just below the source areas of large intraplate earthquakes (e.g., Okada et al .,
2010 ) . In the cases of the 2003 NorthMiyagi earthquake and the 2008 Iwate-Miyagi Nairiku
earthquake, the upper crustal structures implying an ancient rift system were imaged by
temporary dense seismic observations conducted after those mainshocks, as well as the
low-velocity zones below the source areas (Okada et al ., 2007 , 2012). Several seismic
reflection profiles also confirmed that reverse-fault reactivation of pre-existing Miocene
normal faults occurred across the entire seismogenic zone in the fore-arc side of northeast
Japan (Kato et al ., 2006b ) .
Since the strength of ductile creep, which is a dominant deformation process in the lower
crust, is weakened by crustal fluids (e.g., Carter and Tsenn, 1987 ) , the slow anomalies in the
lower crust correspond to locally weak zones (Iio et al ., 2002 ) . Concerning the deformation
process in the weak zone, Iio et al .( 2002 ) assumed that the weak zone consists of numerous
aseismic faults since the deformation is thought to be localized in ductile shear zones. Thus,
local ductile creep of the weak zone within the lower crust causes stress loading, or stress
transfer to the upper ancient rift system (Kenner and Segall, 2000 ) , leading to the reactivation