Current status and future scope of underground velocity structure modeling for design basis ground motion at nuclear power plants

In July 2013, the Nuclear Regulatory Authority enforced new regulatory requirements and then the safety review of nuclear power plants nationwide has been conducted. In this report, we reviewed the features of the underground velocity structure model of each nuclear power plant based on a series of the documents of safety review. In addition, the future issues concerning the modeling of the underground structure velocity are summarized. Based on the review, the issues, which should be conducted for the modeling of underground velocity structure in accordance with new regulatory requirements, were able to be classified into the following six types: (1) Settings of free surface of base stratum, (2) Examination of horizontally layered modeling applicability and investigation of existence of unexpected site amplification, (3) Shallow velocity structure, (4) Deep velocity structure, (5) Exploration of 2D/3D structure and its modeling, (6) Validation of underground velocity structure model and ensuring safety of amplification characteristics. Consequently, it was clarified that the six types of issues have been considered at all investigated sites in the modeling of underground velocity structure. Moreover, the following issues were identified as tasks for verification and updating models in the future: (1) Earthquake observation at free surface of base stratum, (2) Long-term dense array earthquake observation at a site, (3) Improvement of deep velocity structure by deep borehole array observation, (4) 2D/3D velocity structure imaging by tomography technique using deep borehole, (5) Model update considering site amplification based on a ground profile including Q-values (damping factors), (6) Accumulate knowledge about relationship between seismogenic layer and velocity structure. (author)