Earthquake observation records were observed in reactor buildings of Kashiwazaki-Kariwa Nuclear Power Plant Site during the Niigata-ken Chuetsu-oki earthquake in 2007 (NCO). Some studies on the seismic response and simulation analyses were performed to investigate dynamic characteristics of the structures. In particular, it was clarified that the vertical motions of the reactor building of Unit 6 were greater than those of adjacent reactor buildings of Unit 5 and Unit 7. This paper discusses the causes of this by earthquake observation records and simulation analyses of the reactor buildings. In general, seismic response of vertical motions is relatively well-simulated using lumped mass model with stick elements that have vertical stiffness and with the soil spring between the basemat and support ground. However, vertical motions are influenced by rocking motions with horizontal response in some cases. This paper focuses on relationships between the vertical responses and the vertical motions induced by the rocking motions with simulation analyses and observation records during both NCO earthquake and aftershock of NCO. The reasons why the vertical motions of the reactor building of Unit 6 were larger are discussed.
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