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Hiroshi Sato, The University of Tokyo (Japan)
Susumu Abe, Japex Geoscience Institute (Japan)
Hideo Saito, Japex Geoscience Institute (Japan)
Toshihiko Kanazawa, The University of Tokyo (Japan)
Takahiro Miyauchi, Chiba Univerisity (Japan)
Fumihiro Anada, Hokuriku Electric Power Co., Ltd. (Japan)
Susumu Yoshida, Chiba Univerisity (Japan)
Takeo Noguchi, Hokuriku Electric Power Co., Ltd. (Japan)
Naoko Kato, The University of Tokyo (Japan)
Taku Kawanaka, Japex Geoscience Institute (Japan)
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In 2007, the northwestern part of the Noto peninsula, central Japan, was attacked by damaging earthquakes; Noto Hanto earthquakes (Mj 6.9). To correlate seismogenic source faults with geologic structure is important for the better estimation of devastative earthquake before rupturing. We conducted seismic profiling of focal areas, using controlled source. In this paper, we introduce the results of seismic survey and discuss the relationship between a seismogenic source fault and geologic structure. The Noto Hanto earthquake occurred at 9:42 am, local time on March 25, 2007, with a magnitude (Mj) of 6.9. Focal depth of the main shock is about 11 km, the source fault, trending WNW-ESE for 20 km in length, dipping to the south at 60 degrees, shows reverse and right-lateral displacement. Soon after the mainshock, seismologists organized the Joint University Group and deployed total 76 off line recorders to observe the aftershocks (Hirata et al., 2007). The obtained aftershock distribution shows the strong correlation with the traces of active faults reported by Katagawa et al.(2005). Deep seismic reflection profiling was carried out across the estimated source faults in the on shore-offshore focal area. We used two vessels; a gun-ship with 1500 cu. inch air-gun and a cable-ship with a 1.2-km-long, streamer cable with 96 channels and 480 cu. inch air-gun. Common-mid point reflection data were acquired along the 150-km-long seismic line. On land, high-resolution reflection data were obtained using single vibroseis truck. The obtained seismic sections portray the faults, which displaced shallowest part of the reflectors.
Deeper extension of such active fault was traced down to 4 km in depth on the seismic section. Deeper extension of the imaged fault accords well to the linear distribution of hypocenters of aftershocks, providing a direct evidence of fault reactivation at depth. Seismic sections suggest the three tectonic phases; the early Miocene rifting, late Miocene and Quaternary shortening deformation. The main ENW-WSW-trending, Miocene normal faults are parallel to the trend of the Yamato basin, suggesting that they were formed with backarc opening and spreading. Dip angle of the source fault, delineated by the aftershock distribution, accords well to the common dip angle of normal faults. Thus, the 2007 Noto Hanto earthquake was generated by the reverse and strike-slip faulting of Miocene normal fault under a compressional stress regime with a WNW-ESE rending P-axis. The ruptured fault segment is strongly controlled by transfer faults, which was formed during the rifting. Based on the detailed 3D geometry of source fault, crustal deformation data, the co-seismic slip distribution has been determined on the fault surface.
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