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It is well known that local soil conditions play a key role in the amplification of earthquake waves. In particular, a liquefiable shallow soil layer may produce a significant influence on ground motion during strong earthquakes. In this paper, the response of a liquefiable site during the 1995 Kobe earthquake is studied using vertical array records, with particular attention on the effects of nonlinear soil behaviour and liquefaction on the ground motion. Variations of the characteristics of the recorded ground motions are analysed using the spectral ratio technique, and the nonlinearity occurring in the shallow liquefied layer during earthquake is identified. A fully coupled, inelastic finite element analysis of the response of the array site is performed. The calculated stress–strain histories of soils and excess pore water pressures at different depths are presented, and their relations to the characteristics of the ground motions are addressed.

Nonlinear time domain site response analysis is used to capture the soil hysteretic response and nonlinearity due to medium and large ground motions. Soil damping is captured primarily through the hysteretic energy dissipating response. Viscous damping, using the Rayleigh damping formulation, is often added to represent damping at very small strains where many soil models are primarily linear. The Rayleigh damping formulation results in frequency dependent damping, in contrast to experiments that show that the damping of soil is mostly frequency independent. Artificially high damping is introduced outside a limited frequency range that filters high frequency ground motion. The extended Rayleigh damping formulation is introduced to reduce the overdamping at high frequencies. The formulation reduces the filtering of high frequency motion content when examining the motion Fourier spectrum. With appropriate choice of frequency range, both formulations provide a similar response when represented by the 5% damped elastic response spectrum.

The proposed formulations used in non-linear site response analysis show that the equivalent linear frequency domain solution commonly used to approximate non-linear site response underestimates surface ground motion within a period range relevant to engineering applications. A new guideline is provided for the use of the proposed formulations in non-linear site response analysis.

Large modifications of seismic waves are produced by variations of material properties near the Earth's surface and by both surface and buried topography. These modifications, usually referred to as "site response", in general lead to larger motions on soil sites than on rock-like sites. Because the soil amplifications can be as large as a factor of ten, they are important in engineering applications that require the quantitative specification of ground motions. This has been recognised for years by both seismologists and engineers, and it is hard to open an earthquake journal these days without finding an article on site response. What is often missing in these studies, however, are discussions of the uncertainty of the predicted response. A number of purely observational studies demonstrate that ground motions have large site-to-site variability for a single earthquake and large earthquake-location-dependent variability for a single site. This variability makes site-specific, earthquake-specific predictions of site response quite uncertain, even if detailed geotechnical and geological information is available near the site. Predictions of site response for average classes of sites exposed to the motions from many earthquakes can be made with much greater certainty if sufficient empirical observations are available.

In recent years, H/V measurements have been increasingly used to map the thickness of sediment fill in sedimentary basins in the context of seismic hazard assessment. This parameter is believed to be an important proxy for the site effects in sedimentary basins (e.g. in the Los Angeles basin). Here we present the results of a test using this approach across an active normal fault in a structurally well known situation. Measurements on a 50 km long profile with 1 km station spacing clearly show a change in the frequency of the fundamental peak of H/V ratios with increasing thickness of the sediment layer in the eastern part of the Lower Rhine Embayment. Subsequently, a section of 10 km length across the Erft-Sprung system, a normal fault with ca. 750 m vertical offset, was measured with a station distance of 100 m. Frequencies of the first and second peaks and the first trough in the H/V spectra are used in a simple resonance model to estimate depths of the bedrock. While the frequency of the first peak shows a large scatter for sediment depths larger than ca. 500 m, the frequency of the first trough follows the changing thickness of the sediments across the fault. The lateral resolution is in the range of the station distance of 100 m. A power law for the depth dependence of the S-wave velocity derived from down hole measurements in an earlier study [Budny, 1984] and power laws inverted from dispersion analysis of micro array measurements [Scherbaum et al., 2002] agree with the results from the H/V ratios of this study.

Experimental and numerical simulations are performed to evaluate the modification of ground response resulting from either the presence of soft layers or occurrence of partial liquefaction. Results from two densely instrumented dynamic centrifuge tests are presented to show the ambiguous role played by the presence of a soft layer. It was found that the lateral extent of the soft layer has significant influence on the overall response of the layered strata and any structure founded on it. The experimental observations are supported by simplified numerical analysis. The amplification or deamplification of the input motion is found to be a function of the ratio of the width of soft layer to the wave length. Based on the numerical analysis, a general function describing the site amplification is presented which may be used as a guide in seismic design of foundations in such layered strata.

A wavelet-based procedure is presented for the determination of site frequencies from the analysis of historic earthquake accelerograms recorded at the surface. The acceleration time histories are analysed with continuous wavelet transform and the complex Morlet wavelet is used for this purpose. The procedure, which is based on a zoom-in of the wavelet map after the strong motion part of the earthquake fades away, is able to identify the first natural frequency of a layered soil deposit (and in a few occasions also the second frequency). A methodology to calibrate the model of the soil profile with the identified site frequency is also presented. The thickness of a layer that extends from the last known layer to the bedrock is determined such that the fundamental frequency of the new soil system equals the measured value. The proposed procedures are illustrated with numerical examples and applications to real data.

EURO-SEISTEST is a European test site established in the Mygdonian basin, near Thessaloniki, in Northern Greece. Its aim is to allow detailed and well documented studies in many fields of engineering seismology, earthquake engineering, soil dynamics and seismology. Theoretical and experimental seismic ground response analysis along the valley is one of the most important tasks of the project. The very detailed knowledge of the geological and geotechnical structure of the site as well as numerous data from a continuous seismological and geodetical survey and dense semi-permanent and permanent instrumentation, offer the possibility of conducting detailed empirical and theoretical studies on the effects of surface geology on seismic ground motion. EURO-SEISTEST is a unique test site for checking the reliability of existing theoretical models of site effects, to develop new ones, and to interpret the physics of seismic ground response. The paper presents: (a) the general layout of the test site, (b) the main results of the extensive geophysical and geotechnical surveys, to determining the geometry and the dynamic properties of soil formations, and (c) the validation of valley's structure with instrumental estimates of the site effects along the valley. Emphasis is given to comparisons and correlations between results derived from different methods of seismic prospection (refraction, surface wave inversion, borehole seismics) and geotechnical in situ tests. In order to better constrain the structure of the valley and the dynamic properties of the soil formations, specific criteria and procedures are developed for soil characterisation.

The fundamental mode shape of layered soil profiles is a key site response parameter, it has been adopted into the Japanese seismic code to represent the shape of the soil displacement response along the vertical direction. In this study, a simple approach for estimating the fundamental mode shape of layered soil profiles is developed. The proposed approach can directly model the fundamental mode shape and can be conveniently implemented using arithmetic operations, thus making it suitable to be used by the engineers. The assessments of the proposed approach using a series of layered soil profiles demonstrate that it can produce results in close agreement with the actual results.

This paper presents the selected case studies of seismic microzonation and the lessons learned from the microzonation studies in India. India has experienced major damages and loss of life due to earthquakes. Macrozonation map in Indian seismic code BIS-1893 is frequently revised soon after a major earthquake in the country. The latest revision, which was published in 2002 after Bhuj earthquake in 2001, contains four macro zones. These zones are delineated based on geology and past seismic activity and without considering geotechnical aspects such as site effects and liquefaction. The government of India has initiated microzonation work of 63 cities in India to evaluate the earthquake vulnerability of major urban centers and prepare new zonation map. Most of these microzonation studies are under progress and a few of them have been completed. This paper presents a brief overview of some of these studies. Most of the microzonation studies done in India have not given due consideration to the geotechnical aspects in microzonation studies. The geotechnical aspects were fully incorporated in the recently completed Microzonation work of Bangalore and the ongoing microzonation study of Chennai. A detailed of description of the microzonation study of Bangalore urban centre is included in this paper.