Influences of subsurface geological structure neighboring geological repository on earthquake motion of the site
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Influences of subsurface geological structure neighboring geological repository on earthquake motion of the site Taishi Oouchi 1, Hiroyuki Tsuchi1, Tetsuya Ota2, Koji Hane2 and Toru Sasaki2 1 Nuclear Waste Management Organization of Japan (NUMO), Mita NN Bldg. 1-23,Shiba 4-Chome, Minatoku, Tokyo, Japan 2 Kajima Corporation, 6-5-11, Akasaka, Minato-ku, Tokyo, Japan ABSTRACT According to recent seismic observation records, there are some cases where unexpectedly large seismic motion was observed deep underground and that was larger than at the surface. The factors influencing such phenomena are assumed to be deep geological structures with topographic irregularity, velocity structure and non-linearity of subsurface layers. These factors should be taken into account in the earthquake-resistant design of a geological repository. The influence of a deep underground geological structure with topographic irregularity on ground motion has been studied and it has been confirmed that such a structure have a significant impact on ground motion and the constructive interference of waves may result in strong earthquake ground motion in the vicinity of a structural boundary deep underground. INTRODUCTION Amplification of ground motion by a deep underground irregular structure has been identified as one of the main causes of the “heavily damaged belt” in Kobe City during the 1995 Hyogo-ken Nanbu earthquake (for example [1], [2], [3] and [4]). Recently, the Tokyo Electric Power Company has indicated that strong ground motion at the Kashiwazaki-Kariwa nuclear power plant site during the 2007 Niigataken Chuetsu-oki earthquake was due to the amplification of ground motion caused by old fold structures deep underground, in addition to the mechanism of fault rupture processes (for example [5]). The objective of this study was therefore to investigate the influence of a deep underground geological structure with topographic irregularity on the ground motion at a geological repository site. In this study, we considered the characteristic shape of the underground geological structure to be a simple structure. METHODS AND ANALYTICAL CONDITIONS Table I shows the soil profiles used in the 2D model. The subsurface layers were assumed to be mudstone or sandstone and the bedrock was assumed to be granite. The unit weight and velocity of the elastic waves referred to the average physical properties for these soils. The quality factor (Q) of the medium referred to that used with the system for automated seismic moment tensor determination using FREESIA [6]. The region of the deep irregular underground structure in Figure 1 was modeled using the two-dimensional (2D) FEM (Finite Element Method), energy-transmitting boundaries were assumed at both sides and a viscous boundary was assumed at the bottom. The soil property was assumed to be linearity. In order to compare the 2D FEM analysis with the one-dimensional (1D) analysis, A, B, C and D were assumed to be a virtual flatly
layered region. The 2D FEM analysis of the deep irregular underground str
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