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On 16 July 2007, a strong earthquake with a moment magnitude of 6.6 (MJMA=6.8 according to the Japanese Meteorological Agency), occurred at 10:13 h local time with its hypocentre below the seabed of the Jo-chuetsu area in Niigata prefecture in Japan, affecting the Kashiwazaki-Kariwa Nuclear Power Plant (KK NPP) located approximately 16 km south of its epicentre.
Kashiwazaki-Kariwa NPP is the biggest nuclear power plant site in the world. It is located in the Niigata prefecture, in the northwest coast of Japan. The site has seven units with a total of 7965 MW net installed capacity. The consequences of the earthquake on the plant were unique in the sense that the levels of seismic ground motion estimated in the design process were very significantly exceeded by the event. The results of the evaluation and review process presently in progress will induce changes that will be implemented in Japanese regulatory guidance and standards. It is also likely that, eventually, there will be an influence on the approaches to the seismic safety of nuclear power plants worldwide. This paper will review the role played by paleoseismological studies in the re-evaluation of seismic hazards for the Kashiwazaki-Kariwa NPP from a user perspective, i.e. from the perspective of the International Agency for Atomic Energy (IAEA). IAEA has been involved immediately following the earthquake in the discussion of the new investigations and analyses for assessing the seismic safety of the site and the plant, with the main purpose of identifying the preliminary findings and lessons learned from this event in order to share them with the international community.
There are a multitude of reasons for the exceedance of design basis ground motions. Some of the reasons are clearly related to the identification and characterization of the seismogenic sources (e.g., among others, the estimate of potential maximum magnitudes) of the seismotectonic model in the near region of the site. Paleoseismological analyses clearly plays a key role in this process. The site is in an area of where crustal deformation due to the earthquake was observed. This is seen in the INSAR representation of the uplift that has taken place near the site area during the earthquake. There is folding within the site area (both an anticline and a syncline structure) which is still continuing as well as faults that last moved more than 125000 years BP. Assessing the potential for surface faulting at the site will require investigations to correlating surface cracks with local tectonics and the understanding of local tectonics within the near regional framework.
There is much to be learned from the final results of the ongoing investigations at the site regarding the methods for the evaluation of the potential for fault displacement. Due to the extremely high quality of the geological information and the involved expertise, these results will affect the state of the art in paleoseismology.
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