Nonlinear Site Response at Differential Array Station During1979 Imperial Valley Earthquake

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The objective of this research program is to investigate evidence for the time-dependent nonlinear behavior of soil resulting from earthquake-induced strong ground shaking. The evidence is being sought in the accelerograms recorded during the 1979 Imperial Valley earthquake. During this earthquake, accelerogram records were obtained of the strong ground motion at the Differential Array station and at the Array No.1 free-field location. These records are being analyzed in detail to determine the time-dependent character of the apparent stiffness reduction of the underlying soil during this event. The analysis consists of: (1) Dividing each of the two accelerograms into several time windows representing (a) the weak motion prior to the S-wave arrival, (b) the initial portion of strong shaking beginning with S- wave arrival, (c) the middle and latter portions of strong shaking, and (d) the weaker motions at the ends of the records; (2) computing the Fourier Amplitude Spectra of each time segment in each accelerogram; (3) calculating the ratio (transfer function) of the Differential Array and Array No.1 Fourier Amplitude Spectra for each time segment; (4) noting any changes in the frequency at which the first prominent peak in the transfer function occurs; (5) estimating the associated shear-modulus reductions implied by the frequency changes; and (6) estimating the soil-shearing strain from the integrated accelerograms and shear- wave velocity profile of the underlying soil. The results of this analysis will indicate the presence of any time-dependent nonlinear behavior; and, if present, it will suggest whether this behavior occurred gradually or suddenly; and whether during the end of the shaking the shear-modulus of the soil returned to its initial state prior to the strong ground shaking. These effects are very relevant to earthquake hazard mitigation.