Site Response Study of Weak and Strong Motion Including Nonlinearity

by Feng Su, Yuehua Zeng, John Anderson and Vladimir Graizer

Su, Feng, Yuehua Zeng, John Anderson and Vladimir Graizer (1999). Site Response Study of Weak and Strong Motion Including Nonlinearity. SMIP99 Seminar on Utilization of Strong-Motion Data, p. 1 - 22.

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Site response from both weak and strong ground motion recorded at co-located sites were estimated and compared. We find weak and strong motion site responses differ significantly at stations where peak acceleration is above 0.3g, peak velocity is above 20 cm/sec, or shear strain is above 0.06% during the mainshock. The nonlinearity is present across the entire frequency band that we analyzed, from 0.5-14 Hz, and it occurred on sediment sites as well as on soft rock sites. We than compared these observations with a standard engineering model of nonlinear soil response. The model works well for the frequency range from 1.5 to 10 Hz. It diverged from data in frequencies below 1.5 Hz and above 10 Hz, but it is premature to assign much significance to this divergence because the engineering model we used was generic rather than site specific. Finally, we estimated the spectral attenuation parameter Kappa ( k ) and compare it between weak and strong motion data at co-located sites. Our result suggests that some of the variability in measurements of K comes from variability at the source. Kappa may be reduced from weak motion values at sites where nonlinearity is strong, but the source variability has the effect of reducing our confidence in that conclusion.