Session 5.3 - Site Effects  

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Waves, Layers, Microtremors, Earthquakes – Lessons from Four Soft Sites

W.R. Stephenson, H. Flores and C. Lomnitz

Soil-to-rock spectral ratios for earthquake motions and horizontal-to-vertical spectral ratios for microtremors ("Nakamura ratios"), are compared for four sites which are all composed of thick soft alluvium. The range of amplifications within the class "soft site" is large. The differences in amplification can sometimes be identified by studying horizontal-to-vertical spectral ratios for microtremors.

Paper 065: [Read] [Print]

Keywords: soft soil, amplification, Texcoco, Wainuiomata, Parkway, Alfredton

Evaluation of Parameters of Future Earthquakes for Purposes of Seismic Zonation and Hazard Assessment

V.Y. Sokolov, A.V. Ovcharenko, C-H. Loh and K-L. Wen

The method for evaluation of seismic hazards, which is based on models of dynamic deformation of the Earth’s crust, is described. The 4D-model (geographic coordinates, depth, and time) of deformation is constructed on the basis of observed geophysical data, such as data from GPS networks, Sea Water Level monitoring, seismic catalogues, and other data reflecting the dynamic process of the deformation. The process is considered as the interaction of slowly propagating waves (fronts) of deformation, the moving velocities of which vary between 0.05-300 km per year (effective widths about several tens of kilometres). The possible earthquakes could be revealed by analysis of distribution of deformation inside the crust. The case of recent 1999 Chi-Chi, Taiwan, earthquake of M=7.6 is described as an example. On the other hand, the seismic events are considered as the peculiar points of dynamic deformation – the moments of interaction of four and more fronts of deformation. Earthquake magnitude is evaluated using statistical 5D-model (geographic coordinates, depth, time, magnitude). The 4D/5D-models are applied jointly for the compilation of a theoretical seismic catalogue for the nearest tens of years that is used for the purposes of seismic zonation. The results of the modelling are described for the case of Taiwan region. Theoretical catalogues are also compared with observed data.

Paper 049: [Read] [Print]

Keywords: Modelling, Crust Deformation, Seismicity, Earthquake Prediction

Testing the Extensibility of an Earthquake Vulnerability Microzonation Methodology by Application at Bargara, Queensland

M. Turnbull

The author has developed an extensible methodology for conducting microzonation surveys for vulnerability to earthquake shaking. This methodology was used to survey the City of Bundaberg, Queensland. The methodology has now been used to conduct a similar survey of the adjoining town of Bargara. The geology of the two survey areas is quite different. This has allowed the methodology’s extensibility to be tested. The results of that test are provided in this paper.

Paper 010: [Read] [Print]

Keywords: microzonation, risk, vulnerability, Nakamura, site effect, spectral ratio

The Influence of Ground Motion Characteristics on Site Response Coefficients

R.A. Green and W.I. Cameron

Site response coefficients are used by many building codes to relate rock motions to corresponding motions for site conditions other than rock. The site response coefficients in the latest edition of the NEHRP Provisions were derived from observed earthquake motions, largely supplemented by data from numerical site response analyses. The input motions used in the site response analyses were wide ranging in characteristics, with the derived site amplification coefficients specified universally across the US. However, the amplification of seismic waves varies as a function of the motions' frequency content relative to the elastic dynamic characteristics of the soil profile, in addition to factors such as impedance ratio and soil non-linearity. Given the difference in the frequency characteristics of the ground motions observed in the various tectonic regimes across the US, it should not be expected that similar soil profiles located across the US will amplify motions similarly, if subjected to regionally-characteristic earthquakes, irrespective of soil non-linearity. Examined herein is the influence of the frequency characteristics of ground motion on the ratio of the computed soil surface pga to the corresponding rock outcrop pga, with a clear trend being identified, providing credence for the development of region specific site response coefficients.

Paper 090: [Read] [Print]

Keywords: Site amplification, Building Codes, Site Response, Geotechnical, Earthquake

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