Abstract
Three amplification mechanisms of large accelerations over 20m/s2 are related to various non-liner behaviors and explained by using the non-stationary Fourier spectra. The frequencies of dominant components are compared with the natural frequencies with the ratios (1:3:5:7) of a shear-spring model of the underground soil. Thus, one of the amplification mechanisms is supposed to result from the natural modes in the underground soil. Furthermore the similarity of pulse waves between MYG012 (EW) and AKTH04 (EW) is pointed out in the non-stationary Fourier spectra and the double-integrated displacement profiles. The pulse waves are identified by the Ricker wavelet. This amplification mechanism is supposed to be caused by the collision between the observation house and the side soil. The modulation of dominant component is also detected in the high frequency range. The shear strain of AKTH04 is calculated using the relative displacement profiles between the surface and the borehole. The peak shear strain is fitted to the G-γ relation and the deterioration ratio of shear stiffness is evaluated. The third amplification mechanism is related to the change of depth of isolated surface soil. The depth of the isolated surface soil is evaluated from the deterioration area shallower than 20m. It is pointed out finally that the seismic records at the ground surface are unsuitable for the database of seismic design wave.
Highlights
The strong-motion Seismograph Networks (K-NET, KiK-net) was established to mitigate the seismic hazard after the Hyogo-ken Nanbu earthquake in 1995, and it has been operated by the National Research Institute for Earth Science and Disaster Resilience [National Research Institute for Earth Science and Disaster Prevention (NIED), 2017]
The seismic records at the borehole of KiK-net are recommended to be used in the database for the seismic design wave
The authors introduced the concept of isolated surface soil to explain the modulation to the higher frequency as follows: (1) The arrived seismic wave at the bedrock propagated to the surface accompanying with the amplification of the natural vibration modes in the underground
Summary
The database of seismic records promoted the revision of seismic design (Kamae et al, 2004; Fuketa, 2013; Ohsuga, 2013; Kobayashi et al, 2014; Goda et al, 2015). The revision of the seismic design wave (Ss) was required for each nuclear power plant in Japan as “Back-Fit” after three seismic events, i.e., the Niigata-ken Chuetsuoki earthquake in 2007, the Suruga-Bay earthquake in 2009 and the 2011 off the Pacific coast of Tohoku earthquake. The tentative design basis earthquake ground motion (SS) at the Hamaoka nuclear power plant has a peak value of 20 m/s2 (Chubu Electric Power Co., Inc., 2015). The upper bound of the seismic design wave may be important from the viewpoint of seismic engineering [Nuclear Regulation Authority (NRA), 2013]
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