HR: 08:00h
AN: T31G-01 INVITED    [Abstracts]
TI: 16 July 2007 Niigata-ken Chuetsu-Oki earthquake; its characteristics, tectonic background and significance for active fault evaluation
AU: * Sugiyama, Y
EM: sugiyama-y@aist.go.jp
AF: Active Fault Research Center, AIST, Geological Survey of Japan, 1-1-1 Higashi, Tsukuba, 305-8567, Japan
AB: On July 16, 2007, the Mw 6.6 (MJMA 6.8) Niigata-ken Chuetsu-Oki earthquake occurred along the Japan Sea coast south of Sado Island. Kashiwazaki-Kariwa nuclear power plant, situated at the eastern margin of the source area of this earthquake, recorded significantly severe ground motion, which was more than twice, at the maximum, as strong as the designed acceleration in the E-W direction at the foundation base mat level of 3 nuclear power units. Exceedance of the designed ground motion by the Chuetsu-Oki earthquake became the center of public attention. The epicenter of the 2007 Chuetsu-Oki earthquake is located about 30 km northwest of the epicentral area of the 2004 Chuetsu earthquake, and both of the two earthquakes were caused by reverse faulting. Although the source of the 2004 Chuetsu earthquake has been identified as an NW-dipping reverse fault, the dip direction of the source fault of the 2007 Chuetsu-Oki earthquake, SE or NW, has not been settled yet as of September 3, 2007. The 2004 Chuetsu and 2007 Chuetsu-Oki earthquakes occurred in the Strain Concentration Zone characterized by shortening tectonics with E-W- to NW-SE-trending compressive axis. The Strain Concentration Zone traces its origin back to the formation of Japan Sea under E-W extensional tectonics ca 20 Ma. Along the eastern margin of the Japan Sea, grabens or depositional basins were formed in association with the opening of the Japan Sea, being filled with ~5-km-thick Neogene-Quaternary volcano-sedimentary sequences. Around 3 Ma, the eastern marginal area of the Japan Sea changed into a compressional tectonic field, and reverse faulting and folding have been prevailing in the area ever since. The 2004 Chuetsu earthquake was produced by reverse faulting on a northern segment of the known Muikamachi fault system, which probably had been formed as the eastern boundary fault of the Shin'etsu depositional basin. For the 2007 Chuetsu-Oki earthquake, two possibilities exist regarding its relation to known faults. If the source fault dips southeast, a deep part of a 20-km-long submarine active fault, which had possibly been formed as a western border fault of the Shin'etsu basin, should have caused the earthquake. If the source fault dips northwest, on the other hand, west-dipping reverse faults in the coastal hilly area might have relation to the recent earthquake, although there is no known active fault that exactly corresponds to the location and extent of the 2007 earthquake source area. This presentation will brief the latest situation of the controversy on dip direction and its implication to active fault evaluation for seismic safety of nuclear power plants, based on the newest results of post-earthquake researches by related institutions.
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8005 Folds and folding
DE: 8012 High strain deformation zones
DE: 8036 Paleoseismology (7221)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting