On Earthquake Ground Motion and Structural Response inAlluvial

This paper is concerned with the problem of soil ampliication and structural damage due to local site conditions in sedimentary valleys during earthquakes. It focuses on a small valley in Kirovakan, for which one dimensional (1D) wave propagation analyses have failed to provide adequate answers for the large extent and spatial distribution of damage during the 1988 Armenia Earthquake. A more realistic two-dimensional nite element analysis is performed herein in search of an explanation for the observed behavior. Using as input an inferred rock accelerogram, the response of the valley is calculated for a vertically incident SH-wave. Synthetic accelerograms of the surface ground motion are presented for diierent sites; these accelerograms are then used to determine the ampliication ratios of the surface response with respect to that of the free-eld motion of the rock outcrop, for diierent frequencies and for a continuous set of sites. In addition, response spectra are evaluated for simple oscillators representing structures located at various sites. Results of the 2D simulations show striking diierences with respect to those from 1D analyses. In particular, (a) while the resonant frequencies exhibited by a 1D model for a given site also appear in the 2D model, the peak ground response and structural response are almost twice as large for the 2D as for the 1D model; (b) the 2D model exhibits, in addition, a new set of resonant frequencies and concomitant \mode shapes" across the valley, which are directly related to its nite width; (c) due to these additional resonances the ground ampliication ratio tends to oscillate very rapidly, both spatially and with frequency, leading to the observation that two identical structures located in the same vicinity or two slightly diierent structures located essentially on the same site can be subjected to signiicantly diierent seismic forces, even if the underlying soils have very similar characteristics. These results provide a meaningful explanation for the observed damage, and thus, serve to exemplify a situation in which site eeects caused by the nite lateral extent of a valley must be taken into consideration in order to model satisfactorily seismic behavior.