The 2007 Chuetsu-oki, Japan, earthquake is the world’s first major earthquake upon a source fault that extends beneath a nuclear power plant and is also characterized by difficulty determining the source fault plane. Centroid Moment Tensor solutions indicate an Mw 6.6 reverse-faulting crustal earthquake with conjugate fault planes dipping to the northwest and southeast. Early results of aftershock locations suggest that either northwest-dipping plane or southeast-dipping plane can be the source fault plane of this earthquake. We carried out source inversions and empirical Green’s function simulations of observed seismograms; however, they resulted in similar waveform residuals for the two fault planes. We then determined the relative locations of earthquake asperities to the hypocenter using travel-time differences of strong-motion pulses and relocated the aftershocks observed by ocean bottom seismometers deployed in the source region. These results imply that slips mainly occurred on the southeast-dipping fault plane. This implication was later confirmed by results of reflection surveys. During the earthquake, the Kashiwazaki-Kariwa nuclear power plant experienced stronger ground motions than those anticipated at the time of design. The ground motions consist of three seismic pulses that correspond to three asperities. The first and second pulses arose from rupture propagation to the plant, while the compact asperity on the distant southeast-dipping fault plane and its S-wave radiation pattern are responsible for the significant third pulse.

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