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RANDOM VIBRATION THEORY BASED SITE RESPONSE

GRADUATE RESEARCH ASSISTANTS: Cem Ozbey, Albert Kottke

FUNDED BY: USGS National Earthquake Hazard Reduction Program, Southern California Earthquake Center, Pacific Gas & Electric Co.

DESCRIPTION

Figure 1

Site response analysis is typically performed using a suite of rock acceleration-time histories prescribed at the base of a soil column and propagated to the ground surface (Figure 1). However, to develop statistically stable estimates of the site response, a large number of input motions must be used. Seismologists have used random vibration theory (RVT) to predict statistically stable estimates of peak ground motions, such as peak ground acceleration and spectral acceleration, at rock sites using only the power spectrum of the motion (Figure 1). This RVT procedure can be applied to site response analysis.

A single-corner frequency Brune source spectrum is generally used in the RVT procedure to describe the power spectrum of the input motion. RVT site response predictions are compared with those from traditional site response analyses that incorporate a large suite of input rock motions. Results indicate that one RVT site response analysis can provide a surface response spectrum that is similar to the median response spectrum from analyses performed using a large suite of input rock motions (Figures 2 and 3). However, the favorable comparison is obtained only when the seismological source parameters used to describe the Brune source spectrum are carefully chosen to produce an input rock response spectrum that is consistent with the spectrum used to choose the suite of input rock motions.

Figure2

Figure 3

Due to its stochastic nature, RVT analysis can provide median estimates of the site response with a single analysis. Therefore, RVT is a potentially powerful tool for site response analysis that can provide fast and more accurate estimates of the surface ground motion at a site. This procedure can be applied to probabilistic seismic hazard analysis to develop soil specific hazard curves.

PUBLICATIONS
Rathje, E.M., and Ozbey, M.C. Submitted. "Validation of Random Vibration Theory-Based Site Response Analysis," submitted to ASCE Journal of Geotechnical and Geoenvironmental Engineering.

Ozbey, M.C., and Rathje, E.M. 2004. “Comparison of Time Domain and Random Vibration Theory-Based Site Response Simulations,” Seismological Society of America Annual Meeting, Abstract only.