ANALYSIS OF LOCAL VARIATIONS IN FREE FIELD SEISMIC GROUND MOTION

Earthquake engineers are often faced with the problem of determining the temporal and spatial variation of near-surface seismic motions in a site. This type of information is needed for the evaluation of soil-structure interaction effects, liquifaction potential and the effects of local site conditions on surface motions.; Actual ground motions are due to a complicated system of body waves and surface waves. However, it is usually assumed that near-surface motions consist only of vertically propagating waves. In order to examine the validity of this assumption for engineering design a theoretical investigation has been made into the nature of near-surface motions produced by horizontally propagating waves. These include inclined P-, SV-, and SH-waves, Rayleigh waves and Love waves in horizontally layered sites over a viscoelastic half space.; The research involved five phases; (1)review of current knowledge, (2)development of a new method of site response analysis, (3)application to site response analysis, (4)application to soil-structure interaction analysis and (5)evaluation of the relative importance of horizontally propagating waves in engineering design.; The new method of site response analysis involves a finite element type discretization of the site in the vertical direction. According to this method the site is subdivided into thin sublayers and it is assumed that displacements vary linearly between layer interfaces. The method is essentially linear and works in the frequency domain. Nonlinearities are handled by the equivalent linear method and transient motions are handled by Fourier techniques. The procedures have been implemented in the two computer codes, SITE and LOVE. These codes can produce the complete transient field of motion from the knowledge of the motion at one point and the type of wave field producing the motion.; The procedure has been applied to a number of sites (rock, sand, and alluvium) assuming different types of horizontally propagating wave fields and the motions produced by these fields are compared with those produced by vertically propagating waves. The results show that realistic inclined body waves produce near-surface motions which vary with depth in essentially the same manner as those produced by vertically propagating body waves. The motions produced by surface waves are somewhat different. The study also shows that in soil sites surface wave motions decay rapidly in the direction of wave propagation and that within a few hundred feet of the control point all components of frequencies higher than 1 Hz are reduced to insignificant amplitudes. The same phenomenon occurs in rock sites but at a much slower rate. It is therefore questionable whether high frequency surface waves are important for engineering design.; Examples of soil-structure interaction analyses are provided for a structure on rock, a structure on sand, and a large retaining wall on an alluvial site. The results show that for realistic wave fields the motions of structures on soil sites depend only slightly on the type of wave field assumed.; The final conclusions of the study are that the current assumption of vertically propagating waves is probably sufficient for many practical purposes. However, surface waves may be important in rock sites and in the determination of dynamic earth pressures.