Factors Controlling Strong Ground Motion and Their Associated Uncertaintiesby Walter Silva, Pacific Engineering and Analysis, El Cerrito, United States,
Abstract: Of special interest in strong ground motion modeling is a quantification of the degree of influence specific physical parameters have on the computed motions. Associated with the model sensitivity to controlling parameters is a quantification of how well the model performs in its predictive capability. For a specific ground motion model, a statistically formal assessment is made of the uncertainty in computed 5% damped acceleration response spectra for a suite of source, propagation path, and site parameters at a distance of 20 km from a M 7.0 earthquake source. The assessment is made for both a point- and a finite-source model. Both rock and a deep soil sites are considered. Results indicate that the point source provides about the same low levels of uncertainty as the finite source. The controlling source parameter for the point source is stress drop while station azimuth, location of nucleation point, and slip distribution are controlling factors for the finite source. The most significant path parameter is the crustal shear-wave velocity profile. Site effects are dominated by shallow crustal damping at rock sites and dynamic material properties at deep soil sites. Specifically shear-wave velocities at deep soil sites are a significant factor for frequencies above 1 Hz while material nonlinearities (shear modulus and damping) are important above about 10 Hz. For motions computed at a depth of 500 m due to an M 6.5 earthquake at a distance of 20 km, depth nodes or spectral minima are well developed. Interestingly, the motions are lower at depth than at the surface at all periods except at about 0.07 sec for the horizontal component and about 0.03 sec for the vertical component where surface and at-depth motions are about the same.
Subject Headings: Ground motion | Parameters (statistics) | Wave velocity | Shear waves | Material properties | Uncertainty principles | Damping | Soil dynamics
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