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Naoko Kato, Earthquake Research Institute, Univ. of Tokyo (Japan)
Hiroshi Sato, Earthquake Research Institute, Univ. of Tokyo (Japan)
Susumu Abe, JGI Inc. (Japan)
Taku Kawanaka, JGI Inc. (Japan)
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The Kinki district, central Japan, has a population of 20 million, including mega-cities, such as Osaka, Kyoto and Kobe, and marked by dense distribution of active faults. The deep geometry of active faults is crucial for better estimation of strong ground motions and to understand the connectivity of active faults.
The connectivity of deeper extension of active faults provides the basic information of slip partitioning of a fault system, which is important to evaluate the risk of active faults. The direct imaging by deep seismic reflection profiling is powerful tool to detect a fault surface at depth. For the reduction of seismic hazard in the urban area, deep seismic reflection profiling were carried out across active faults in the Kinki district in 2004 and 2006. We introduce the results of seismic profiling and discuss the slip partitioning estimated from the deep geometry of active faults.
Common mid-point seismic reflection data were acquired using four vibroseis trucks or air-guns with 9700 cubic centimeter capacity. Widely spaced explosive sources up to 300 kg plus sets of 100 stationary vibroseis sweeps at several sites per seismic line were used to obtain a deeper image. The image of deeper extension of active faults were successfully obtained. The deep geometry of the Median Tectonic line (MTL) active fault system is clearly imaged down to 6 km. In spite of the late Quaternary strike-slip movement, the fault surface dips northward by 20 to 30 degrees (1-2 km in depth) and 40 degrees (2-6 km). Judging from the age of the footwall sediments, formation of shallow low-angle fault is due to reverse faulting during 3 -1 Ma. Such temporal change of fault displacement was produced by the change of motion of the Philippine Sea plate. Today, oblique subduction is observed along the Nankai trough. Dip-slip component is spent on the subduction mega-thrust along the trough, and strike-slip component is spent along the MTL active fault system. More small scale of slip partitioning is observed between the Katata (reverse) fault and Hanaore (strike-slip) faults.
The Katata fault bounds the western coast of the Lake Biwa and the Hanaore fault is located on the hanging wall of the Katata fault. Both faults run almost parallel with 7 km distance. The seismic section across the both faults portrays that the Katata fault is traced down to 6 km with 30 degrees. The Hanaore fault on the hanging wall shows vertical fault surface. Due to this high-angle nature, the Hanaore fault cannot be identified on the seismic section. However, judging from the high-angle nature of the fault, it is highly probable that the Hanaore fault is merged to the deeper extension of the Katata fault. Thus, the vertical slip is spent along the Katata fault at the rate of 1-1.6 mm/yr, and right-lateral slip accommodates along the Hanaore fault. Such connectivity of faults is crucial for the evaluation of the risk of active faults.
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