Ground Motion Amplification as a Function of Surface Geology

by Jonathan Stewart and Andrew Liu

Stewart, Jonathan, and Andrew Liu (2000). Ground Motion Amplification as a Function of Surface Geology. SMIP2000 Seminar on Utilization of Strong-Motion Data, p. 1 - 22.

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Empirical relationships are developed to predict amplification factors for 5% damped spectral acceleration as a function of surface geology. Amplification factors are derived for spectral periods T = 0.01 – 5 s by assigning a reference spectrum to > 700 recordings from shallow crustal earthquakes. The reference spectrum is derived from soft rock attenuation relations modified to account for event-specific source/path peculiarities and rupture directivity effects. Strong motion sites are classified according to three geologic classification schemes: age only, age + depositional environment, and age + material gradation. Within each scheme, amplification is regressed against ground motion amplitude, and for one scheme, against amplitude and duration. The material gradation scheme is found to produce the least scatter in the amplification functions. The results of the regression indicate significant nonlinear ground response effects, and pronounced variations in the levels of amplification across geological categories. Amplification is also found to be sensitive to the duration of strong shaking. Due to the soft rock reference spectra used in this study, amplification levels are smaller than had been identified in previous studies employing reference motions from relatively firm rock sites.