Review of the 1755 Lisbon Earthquake Based on Recent Analyses of Historical Observations - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 263

Fig. 2 Isoseismals of two large intra-plate events: a) 1755 (Levret 1991) and b) 1969 (Moreira,
1984). Note the similarities (the Guadalquibir valley) and dissimilarities (the southwestern Iberia)
in the propagation of these two events
is more regular and concentric, emphasizing the possible role of the source mech-
anism, the wave attenuation anisotropy and the site effects. This latter effect is re-
markably well illustrated in the positive irregularity (higher values than we could
expect) in the Guadalquibir valley, showing an effect of amplification due to the
presence of thick alluvial layers in that region.
In Spain, Martinez Solares (2000) was able to classify the damages occurred,
distinguishing the damages in buildings of high frequency (small buildings) from the
damages of buildings of lower frequency (monumental buildings), clearly prevailing
the energies associated to lower frequency in areas very far from the seismic source.
Pereira de Sousa (1919-1932) draws also the isoseismals from 1755 taking as
basis of his work the geological map by assuming that the intensities should corre-
late well with surface geology. And he was not particularly mistaken, because that
analogy exists, although it is not only the lithology of the superficial formations
that contributes to the amplification of seismic waves. It is well known, and this
earthquake is a clear example of that, besides the source mechanism and the wave
propagation phenomenon, the type, geometry and mechanical properties of local
geological strata are of most importance to characterize seismic action at a site.