A Recipe for Microzoning Aimed to Earthquake Disaster Prevention
Kojiro Irikura, Aichi Institute of Technology
Aichi Institute of Technology, 1247 Yachigusa, Yakusa, Toyota, Aichi 470-0392, Japan“National Seismic Hazard Map” has been making as one of the national projects integrating all fields ofearthquake researches such as active fault, earthquake forecast and strong motion prediction studies after
the 1995 Kobe earthquake. The map consists of two parts made from two different approaches,
probabilistic and deterministic. The probabilistic seismic hazard map is shown as the predictedlikelihood of a ground motion level such as PGA, PGV, and seismic intensity occurring in a given area
within a set period of time. It provides important information for land planning, design standards ofstructures and people’s enlightening as to seismic risks. The deterministic seismic hazard map is shownas the distribution of the ground motion level predicted for individual specific earthquakes assuming
their fault models. The strong ground motions at specific sites near each source fault are estimated astime history, based on a “recipe” characterizing the source and numerical synthesis of waveforms with ahybrid scheme.
The source fault model for the map is defined based on a “recipe” which has been made withinformation about active fault surveys scaling relations of fault parameters, and characterized source
model from the waveform inversion of rupture processes using strong motion data. Verification andapplicability have been examined by comparing synthesized ones estimated using the “recipe” withobserved ground motions from recent disastrous inland-crustal-earthquakes such as the 2005 Fukuoka
Seiho-oki earthquake, the 2007 Niigaka-ken Chuetsu-oki earthquake (Mw 6.6), the 2008 Iwate-Miyagi
Nairiku earthquake (Mw 6.9) and so on. Ground motions from these earthquakes are found to bepredictable as long as the source fault is specified through investigation of active folds and faults.
Further, we attempt to simulate strong ground motions during the 2008 Wenchuan earthquake of Mw 7.9
assuming the characterized source model.
The technique of the strong motion prediction mentioned above is introduced to the “Regulatory
Guide for Aseismic Design of Nuclear Power Reactor Facilities” revised in 2006 by the Nuclear Safety
Commission of Japan (NSCJ). The NSCI decided “Seismic Reevaluation Recommended on Existing
NPPs Based on the Revised Seismic Design Guide,“ for the existing nuclear power reactor facilities toverify their seismic safety against the revised version of the Seismic Guide (back-checks).
The Niigataken Chuetsu-oki Earthquake occurred on July 16, 2007 near the Kashiwazaki-Kariwa
Nuclear Power Station has given significant damages to non-safety related equipment, although the units
in operation were safely shut down as designed. Specially to note is that this earthquake has reminded
the regulatory bodies, utilities, etc, of quite a number of challenging issues to tackle in ensuring seismic
safety of nuclear facilities. There was recorded ground acceleration of 680 gals on the base mat of Unit
No.1 reactor, 2.5 times more than the ground motion levels for aseismic design of facilities. The activefaults caused to the earthquake have not been specified in evaluating input ground motions for theaseismic design. We made careful examinations why the active faults have been missed and how theground motions have been underestimated. We found that the ground motions from the earthquake werepredictable as long as the source fault is specified through investigation of active folds and faults.
In the growing interest in nuclear power worldwide, this earthquake raised significant concerns evenin other countries. It is important to provide the experiences in Japan to share worldwide scientific andtechnical knowledge on seismic safety which has been obtained so far.