Investigation of the spatial variation of earthquake ground motions in the Hamaoka Nuclear Power Station using the dense seismic observations

Abstract

In the Hamaoka Nuclear Power Station (NPS), a significant difference of earthquake records between near the No.3 reactor and near the No.5 reactor was observed in the frequency range of approximately 2.0 - 5.0Hz during the Suruga Bay earthquake (MJMA6.5) on August 11 2009. Such a significant difference had not been discussed by using the previous records of the Hamaoka NPS. Since this phenomenon might be caused by an effect of 2-D/3-D subsurface structure, we installed seismic observation stations densely at 14 points in/around the Hamaoka NPS in order to obtain the fundamental information about the 3-D subsurface structure of the NPS. Through analyzing the ground motion records of 21 selected earthquakes, the peak ground accelerations in the northeastern area and the southeastern area including No.5 reactor of the NPS were significantly larger than those in the western area including No.3 and No.4 reactors for several earthquakes. As for the stronger ground motions in the northeastern area, an effect of shallow sedimentary layers was suggested based on the peak periods of the H/V spectral ratio of microtremors at observation stations. Meanwhile, in the area including No.4 and No.5 reactor, this effect was relatively lower. Therefore, it was suggested that the differences of the ground motions within this area as seen during the main shock, were induced by not the shallow sedimentary layers but the relatively deeper subsurface structure. Additionally, it was found that the significant differences between near the No.3 and the No.4 reactors and near the No.5 reactor only was being appeared during the azimuth with a range from approximately 35 to 60 degrees including the epicenter of the main shock. The azimuth dependency suggested that the differences of the ground motions were induced from not only the characteristics of the 1-D subsurface structures but also the existence of 2-D/3-D heterogeneity of subsurface structure.