Response spectra for design purpose of stiff structures on rock sites

For seismic design of nuclear power facilities, we propose an empirical method for evaluating response spectra and time-dependent features of horizontal and vertical earthquake ground motions on free rock surfaces. A response spectrum of horizontal motion on seismic bedrock is given by a control point in the matrix by four magnitudes M and four equivalent hypo-central distances X{sub eq}. The response spectra for other M and X{sub eq} are determined by interpolation between values in the matrix. The response spectra of the horizontal and vertical motions on the free rock surface are determined by multiplying the horizontal motion on the seismic bedrock by the amplifications of horizontal and vertical motions due to surface layers, which are given as functions of S- and P-wave velocities on the free rock surface, respectively. The proposed evaluation method adequately explains near-source observation records. Earthquake ground motion evaluation methods used for seismic design of structures are roughly classified into empirical, semi-empirical, and theoretical methods. Empirical methods have been widely used as a standard for calculating the response spectrum to be used for seismic design of nuclear power facilities. This paper discusses and proposes an empirical method for evaluating the response spectra of horizontal and vertical earthquake ground motions on the surfaces of rock mostly composed of Tertiary or older strata, as a reasonable method for establishing a design-basis earthquake ground motion for seismic design of nuclear power facilities. The earthquake ground motion evaluation method proposed here uses the earthquake magnitude, equivalent hypo-central distance, and elastic wave velocity on the ground at the evaluation point as evaluation parameters. The method is organized so as to reflect, as accurately as possible, actual earthquake ground motion observations. The major features of the method are that (1) it empirically evaluates earthquake ground motion amplification by the surface layers overlaying seismic bedrock using the elastic wave velocity on the ground at an evaluation point, (2) it includes the effect of the extension of the fault on earthquake ground motion so that it can be applied to near-source regions, (3) it can evaluate earthquake ground motion with periods from 0.02 to 5 seconds, and (4) it can evaluate both horizontal and vertical motions.