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The main purpose of the paper is the analysis of seismic site effects invarious alluvial basins. The analysis is performed considering a numericalapproach (Boundary Element Method). Two main cases are considered : a shallowdeposit in the centre of Nice (France) [1] and a deep irregular basin inCaracas (Venezuela) [2]. The amplification of seismic motion is analysed interms of level, occuring frequency and location. For both sites, theamplification factor is found to reach maximum values of 20 (weak motion). Siteeffects nevertheless have very different features concerning the frequencydependence and the location of maximum amplification. For the shallow depositin Nice, the amplification factor is very small for low frequencies and fastlyincreases above 1.0 Hz. The irregular Caracas basin gives a much differentfrequency dependence with many different peaks at various frequencies. Themodel for Caracas deep alluvial basin also includes a part of the localtopography such as the nearest mountain. One can estimate seismic site effectsdue to both velocity contrast (between the basin and the bedrock) and localtopography of the site. Furthermore, the maximum amplification is located onthe surface for Nice, whereas some strong amplification areas also appearinside the basin itself in the case of Caracas. One investigates the influenceof this focusing effect on the motion vs depth dependence. This is of greatinterest for the analysis of seismic response of underground structures. Theform and the depth of alluvial deposits are then found to have a greatinfluence on the location of maximum amplification on the surface but alsoinside the deposit for deep irregular basins. It is essential for the analysisof the seismic response of both surface and underground structures.

Seismic Site Effects for Shallow and Deep Alluvial Basins: In-Depth Motion and Focusing Effect