  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 surface impedance, is relaxed by the introduction of a {spatially-
constant, non- vanishing surface impedance}, the response in the substratum is
again a single reflected body wave whose amplitude is less than the one in the
situation of a stress-free ground. When the stress-free condition is relaxed by
the introduction of a a {spatially-modulated surface impedance}, which
simulates the action of an uneven (i.e., not entirely-flat) ground, the
frequency-domain response takes the form of a spectrum of {plane body waves}
and {surface waves} and {resonances} are produced at the frequencies of which
one or several surface wave amplitudes can become large. It is shown, that at
resonance, the amplitude of one, or of several, components of the motion on the
surface can be amplified with respect to the situation in which the surface
impedance is either constant or vanishes. Also, when the solicitation is
pulse-like, the integrated time history of the square of surface displacement
and of the square of velocity can be larger, and the duration of the signal can
be considerably longer, for a spatially-modulated impedance surface than for a
constant, or vanishing, impedance surface.
