Estimation of seismic observation records in the vicinity of large earthquakes and its impact on geological disposal pro
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Estimation of seismic observation records in the vicinity of large earthquakes and its impact on geological disposal program Hiroyuki Tsuchi 1, Taishi Oouchi 1,Yoshikazu Ichikawa2, Kazuo Okutsu2 and Toru Sasaki 2 1 Nuclear Waste Management Organization of Japan (NUMO), Mita NN Bldg. 1-23, Shiba 4-Chome, Minatoku Tokyo, Japan 2 Kajima Corporation, 6-5-11, Akasaka, Minato-ku, Tokyo, Japan ABSTRACT Japan is located at the converging plate boundaries and is one of the most earthquake-prone zones in the world. In order to ensure the safety of a geological repository against earthquakes, a site with a high possibility of direct destruction by active faults should be excluded, and the relationship between the characteristics of seismic ground motion, subsurface structures, geological disposal system and the propagation characteristics of seismic ground motion should be fully investigated. Earthquake-resistant design based on the latest technology is also very important for ensuring the safety of a geological repository. Following rapid improvement of seismic observation networks after the Hyogoken Nanbu earthquake in 1995, numerous seismic observation records have been obtained in the vicinity of large earthquakes. According to these seismic observations, some phenomena that might affect the safety of a geological disposal system have occurred. Some earthquakes occurred in the areas where active faults had not been identified, while some records showed that seismic motion in the deep underground environment was greater than that at the surface. We have identified the implications from the latest information concerning large earthquakes for the geological disposal program. This study made it clear that detailed investigation incorporating state-of-the-art technologies could reduce the likelihood of missing active faults to an extremely low level and a more practical analysis of seismic ground motion could be achieved by taking the latest information into account. INTRODUCTION Possible impacts of earthquakes on the geological disposal system include displacement or deformation of geological formations due to faulting, earthquake ground motion and changes in groundwater flow and hydrochemical properties. Of these, earthquake ground motion is not included in the siting factors for qualification of preliminary investigation area for the reason that earthquake ground motion deep underground is generally smaller than at the surface (for example[1]) and the safety of a deep repository during operation can thus be assured by appropriate earthquake-resistant design. After backfilling and closure, the repository will move together with the surrounding rock. Table I shows NUMO’s fundamental policy regarding rock deformation including the effects of earthquakes. Considerable development of the earthquake observation networks has provided sufficient reliable data for the surface as well as in the deep underground environment. The fact that severe earthquakes have occurred comparatively close together near existing nuclear power plant lo
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