| We show that the concept of doorway states and giant resonances may be applied to very large macrosystems. We apply it to understand the seismic response of sedimentary basins covered by water-logged clays, a rather common situation for urban sites. A model is introduced in which the doorway state is a plane wave propagating in the interface between the sediments and the clay. This wave is produced by the coupling of a Rayleigh and an evanescent SP-wave. This in turn leads to a giant resonance in the soft clays contained in the basin. Our model calculations are compared with measurements during Mexico City earthquakes, showing quite good agreement. |
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