Intraplate earthquakes induced by reactivation of buried ancient rift system along the eastern margin of the Japan Sea - Intraplate Earthquakes - page 235

at the sampling rate of 200 or 100 samples/second. Each station is equipped with a GPS
receiver, which maintains the accuracy of the internal clock. After retrieving the waveforms,
we then merge the huge earthquake datasets recorded by the temporary seismic network
together with the corresponding data from permanent stations in each target region. We
then manually pick P- and S-wave arrival times from earthquake waveforms detected based
on the JMA catalog. We determine high-resolution three-dimensional velocity structures
as well as precise hypocenters, applying the double-difference tomography method (Zhang
and Thurber, 2003 ) to both the arrival time data and the differential arrival times obtained
by the manually picked and waveform correlation method (e.g., Kato et al ., 2006a ) . In
addition, we analyze high-resolution stress fields using focal mechanisms determined by
the polarity data of P-wave first motion.

9.3 The 2004 Niigata-ken Chuetsu and 2007 Chuetsu-Oki earthquakes
Two neighboring destructive intraplate earthquakes (both with M JMA 6.8) showing reverse
faulting with a strike of approximately N35 E recently occurred in the Niigata region: the

first, Niigata-ken Chuetsu earthquake on October 23, 2004, and the second, Niigata-ken
Chuetsu-Oki earthquake on July 16, 2007 ( Figure 9.1 ) . The focal areas of the 2004 Chuetsu
and 2007 Chuetsu-Oki earthquakes were located within a thick (locally > 6 km deep)
deformed Miocene-Pleistocene sedimentary basin (the Niigata Basin), which is character-
ized by NNE-SSW-trending faults and anticlinal fold hinges that form topographic hills
( Figure 9.2a ) . This sedimentary basin was formed as a back-arc basin in a rift structure that
developed during the opening stage of the Japan Sea (25-15 Ma). This basin is bounded
to the east by the Shibata-Koide Tectonic Line (SKTL), where basement rocks dating
back to more than 30 Ma are widely exposed. Geological studies (e.g., Sato, 1994 ) have
inferred that parts of the normal faults within the rift system have subsequently been reac-
tivated as a reverse fault system since the extensional tectonic stress regime changed to
a compressional one in the late Pliocene (2-3 Ma), through a process of compressional
inversion (Williams et al ., 1989 ) . Although these shallow large earthquakes generated
many fissures and landslides on the surface, only minor surface faulting was observed
(Maruyama et al ., 2005 ) .
We conducted a series of temporary seismic observations through a dense deployment
of 145 portable stations after the 2004 earthquake (from October to November, 2004) (Kato
et al ., 2006a , 2007) and 108 portable stations including a linear-seismic array on land and
20 ocean-bottom seismometers after the 2007 earthquake (from July to August, 2007) (Kato
et al ., 2008a, 2009; Shinohara et al ., 2008 ; ) ( Figure 9.2a ) .

°

9.3.1 Aftershock distribution and dynamic rupture process

Depth sections of P-wave velocity ( V p ) models along W35 ° N-E35 ° S lines are shown in

Figure 9.2b . Relocated aftershocks (gray circles) distributed within ± 2.5 km of each line