Probabilistic Evaluation of Near-Field Ground Motions due to Buried-Rupture Earthquakes Caused by Undefined Faults

The Nuclear Safety Commission of Japan (NSC) was discussing the revision of Safety Examination Guideline for Seismic Design of Nuclear Power Plants of Japan, and finished the discussion and revised them in September 19, 2006. In the discussion, one of the main issues was the design earthquake motion due to close-by earthquakes caused by undefined faults. This paper proposes a probabilistic method for evaluating ground motions and their predicted frequencies by covering variations of earthquake magnitude and location of undefined faults by strong motion simulation technique (hybrid technique) based on fault models, and evaluated probabilistic response spectra (uniform hazard spectra) due to close-by earthquakes. These uniform hazard spectra evaluated by the hybrid technique are compared with those evaluated by an empirical approach. The response spectra with a damping factor of 5% at 0.02 s simulated by the hybrid technique are about 160, 340, 570, and 800 cm/s/s for annual exceedance probabilities of 10-3, 10-4, 10-5, and 10-6, respectively, which are in good agreement with the response spectra evaluated by the empirical approach. It is also recognized that the response spectrum proposed by Kato et al. (2004) as the upper level of the strong motion records of buried-rupture earthquakes corresponded to the uniform hazard spectra of annual exceedance probability of 10-5 - 10-4. (authors)