AMPLIFICATION AND INCREASED DURATION OF EARTHQUAKE MOTION ON UNEVEN STRESS-FREE GROUND
Résumé
When a flat stress-free surface (i.e., the ground in seismological applications) separating air from a isotropic, homogeneous or horizontally-layered, solid substratum is solicited by a SH plane body wave incident in the substratum, the response in the substratum is a single specularly-reflected body wave. When the stress-free condition, equivalent to vanishing impedance, is relaxed by the introduction of a spatially-modulated, non-vanishing impedance, the response turns out to take the form of a spectrum of plane body waves and surface waves. It is shown that, in a great variety of situations, resonances are produced at the frequencies of which one or several surface wave amplitudes can become large. Furthermore, at resonance, the amplitude of the motion on the surface is shown to be amplified with respect to the situation in which the surface impedance vanishes. A subsidiary, but all-important, effect of this resonant response is that, when the solicitation is pulse-like, the peak value of the time history of surface motion is larger, and the duration of the signal is considerably longer, for a spatially-modulated impedance surface than for a constant, or vanishing, impedance surface.