  We address the problem of the response to a seismic wave of an urban site
consisting of $N$ non-identical, non-equispaced blocks overlying a soft layer
underlain by a hard substratum. The results of a theoretical analysis,
appealing to a space-frequency mode-matching (MM) technique, are compared to
those obtained by a space-time finite element (FE) technique. The two methods
are shown to give rise to the same prediction of the seismic response for N=1
and N=2 blocks. The mechanism of the interaction between blocks and the ground,
as well as that of the mutual interaction between blocks, are studied. It is
shown that the presence of a small number of blocks modifies the seismic
disturbance in a manner which evokes qualitatively, but not quantitatively,
what was observed during the 1985 Michoacan earthquake in Mexico City.
Disturbances at a much greater level, induced by a large number of blocks (in
fact, a periodic set) are studied in the companion paper.
