Journal of Seismology and Earthquake Engineering
Volume:5 Issue: 3, Autumn 2003

In this paper, an advanced formulation of time-domain two-dimensional Boundary Element Method (BEM) for linear elastodynamics is used to carry out site response analysis of topographic structures subjected to incident P-, SV-, and Rayleigh waves. A modified set of well behaved full space two-dimensional elastodynamic convoluted kernels is presented and employed, that has a higher degree of accuracy than those presented by the previous researchers. Numerical results are presented, including cases of half-plane, canyon and ridge sections, subjected to the different body and surface waves.

The south Caspian Basin is a seismic block within the Alpine-Himalayan Belt. The source time function of 31 events of Caspian earthquakes have been obtained from teleseismic body waveform modeling. The duration of each subevent with magnitude larger than 5 (Mw>5.0) and depth between 4≤ h≤ 76 km was determined from source time function. Corner frequency and stress drops have been calculated for each of 31 events by using pulse duration from source time function. When viewed over the entire depth range, the total duration (tt) is related to Mo by logtt = (0.2642 ± 0.001) log MO-8.9119 ( ± .0.194). Corner frequencies range is from 0.038Hz to 0.16Hz. Static stress drops calculated from the pulse shapes for each event studied in this paper changed between 0.07 bars to a maximum of 46 bars. Minimum and maximum displacements are 0.79m and 3.3m respectively. The variation in stress drop is considerable, but no evidence is seen for a scaling relation in which stress drop increase with moment. These relative source durations do not show any clear depth dependence.

The work done in the framework of a large international cooperation, showing the very recent numerical experiments carried out within the framework of the EC project "Advanced methods for assessing the seismic vulnerability of existing motorway bridges" (VAB) to assess the importance of non-synchronous seismic excitation of long structures have been illustrated. The definition of the seismic input at the Warth bridge site, i.e. the determination of the seismic ground motion due to an earthquake with a given magnitude and epicentral distance from the site, has been done following a theoretical approach. In order to perform an accurate and realistic estimate of site effects and of differential motion it is necessary to make a parametric study that takes into account the complex combination of the source and propagation parameters, in realistic geological structures. The results for the final local model, characterized by an exaggeratedly thick and low velocity layer, demonstrate that a deep source excites lower frequencies than a shallow one and that the effect of increasing the epicentral distance is to attenuate high frequencies, making the resonant peaks, present at frequencies around 0.8 Hz, the dominant features of the entire spectra. The main practical conclusion of our analysis, verified by laboratory experiments, is that the Warth bridge is likely to well stand the most severe seismic input compatible with the seismic regime of the Eastern Alps.

In this paper, the effects of higher modes on seismic response of multi degree of freedom (MDOF) steel moment-resisting frames (SMRF) are investigated. Modification factors to the response of first mode SDOF system are presented in order to estimate seismic MDOF system demands. The study is based on spectral analysis and linear and nonlinear dynamic time history analysis of 4, 10, 15, 20 and 25 storey SMRFs under Elcentro, Tabas, Naghan and Manjil earthquake loading. A modification factor is defined in order to estimate roof elastic displacement demands of an MDOF frame from first mode elastic displacement spectra. Base shear modification factor (to compute MDOF strength reduction factors), maximum story drift demand modification factor , and maximum story dynamic ductility demand modification factor are defined and presented for SDOF system responses in order to estimate the main MDOF system nonlinear seismic responses, including higher mode effects.

The design of facilities to resist seismic loads requires selection of an appropriate safety level. This paper will discuss the requirements of different international standards and will suggest that the criteria might be established on the basis of a quantitative risk analysis. The paper will, furthermore, discuss the experience oil companies have gained through working in Norway and North Western Europe in designing facilities to withstand the relevant seismic loads. Although this area of the world is an intraplate area with relatively low seismicity, it is suggested that the experience gained in using risk based seismic design criteria and quantitative risk analysis could be of value to those working in more seismic regions of the world. The paper will mainly refer to onshore terminal facilities on the Western Coast of Norway, which is the area of the North Western Europe with highest seismic hazard, and to offshore platforms in the Northern North Sea. Finally, the paper will present considerations related to upgrading of existing facilities with particular emphasis on facilities in the oil and gas industry.