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Wave propagation
| Its now commonly accepted that wave propagation effects have
a strong effect on strong ground motion. On their way to the
surface, seismic waves undergo modifications due to reflection,
refraction and absorption. We account for these effects by the
use of Haskell propagator matrices and a complex wave number
approach for the absorption. Horizontally polarized S-waves
(SH-waves) are considered, since these are the most important
ones for earthquake engineering purposes. The geological structure
is represented by a sequence of horizontally layered plane strata
overlying an infinite halfspace, which contains the seismic
source. The seismic signal is strongly affected by the subsurface
geological structure. Considerable amplitude amplifications
occur if the velocity (and density) of the uppermost layers
are lower than the velocities (and densities) of the material
below. Thicknesses and velocities of the uppermost layers are
typically in a range that makes amplitude amplification effects
occur at frequencies between 1 and 20 Hz., i. e., the frequency
intervall of technical interest. The sketch displays the modifations
the seismic signal undergoes on its way from the source to the
receiver. Amplitudes are supposed to be large close to the source,
then they decrease as the wave travels towards the surface.
Within the layer stack the signal is refracted and reflected.
In the layers close to the surface, which we suppose to have
a low impedance, we observe amplitude amplification.
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