Seismic Ground Motion - Dynamic Behavior of Concrete and Seismic Engineering - page 116

To conclude this section, whilst theoretical convex relief focusing certainly plays
a significant part, it does not seem to be the only physical phenomenon involved.
The fact that only this phenomenon has been incorporated into the French national
para-seismic regulations is quite justified. We should, however, be aware that more
important effects that are not well delineated at present do take place, but can only
be grasped by measurement.
3.4.3.2. Effects of the sedimentary or alluvial cover
3.4.3.2.1. Observations
Earthquake damage is generally more significant in sediment-filled areas than on
rocky outcrops. The intensity local increments observed commonly reach 1 or 2
degrees in intensity (MM or MSK scale) and have sometimes exceeded three
degrees (Mexico and San Francisco for instance), indicating in certain cases the
complete control of damage distribution on the nature of the ground. These
observations have given rise to large numbers of instrumental studies that have
confirmed the existence of amplification phenomena, and to as many theoretical and
numerical works, the results of which are summarized below. The curious reader is
referred to the different synthesis articles in the bibliography.
3.4.3.2.2. Physical explanations for amplification
The basic phenomenon responsible for amplification is the trapping of seismic
waves - particularly the S waves - into low mechanical (shear) stiffness superficial
formations. For horizontally-stratified formations (“1D” structures), this only affects
the incident volume waves that travel vertically back and forth between the surface
and the substratum-sediment interface (Figure 3.5 left). When these are not very
stiff, superficial formations present thickness side variations whether they are 2D
(valleys) or 3D (basins). We can also discern generation of local surface waves
(through diffraction on lateral heterogenities) that can reflect between the valley or
basin sides.
The interference between trapped waves causes resonance phenomena, the
features of which (frequencies, natural modes, amplification) depend on the
geometric and mechanical characteristics of the structure. Diffraction and trapping
phenomena also have other consequences apart from amplifying certain frequencies.