Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 242

Figure 7.21 Spectral accelerations at characteristic points of the valley in a near-
source site (after Klimis and Anastasiadis, 2002)
- Focusing phenomenon , due to the incidence of waves on a locally convex
or concave surface profile;
- Dynamic phenomenon , producing a resonant motion of a whole hill or
mountain, if the waves incident at the base meet some values;
- Presenceoflocaldeposit of unconsolidated soil for a limited extent.
Paolucci and Rimoldi (2002) have studied the case of the presence of a hill
(Civita di Bagnoregio) in the Italian geological environment. The results show
very important amplifications of ground motion on the hill, which are double those
of the surrounding area.
The valley effect is studied by Klimis and Anastasiadis (2002). Two valley
types with different geometry and mean slopes are theoretically analyzed (Fig.
7.21). The amplification at near crest free surface is about 1.4 times for milder
slope inclination and higher, about 1.85 times, for steeper slope. Reduced
amplifications are obtained for the intermediate-source sites, and the amplification
effect disappears for the far-source sites.
7.4.5 Aluvial Basins
This aspect began to present interest after the 1976 Tangshan, 1985 Mexico City
and 2009 L'Aquila earthquakes, when the alluvial basin produced an important
amplification of the source accelerations. The finite lateral extent of soil surface
layers generates surface waves at the edge and two-dimensional resonance patterns
in the lateral direction and increases the amplitude, natural period and duration of
ground motions. The influence of the variation in the basin dimensions and the
effects of soil type and input rock motion characteristics are investigated by
Rassem et al (1997). The differences in the maximum amplitude of accelerations
for deep and shallow basins in the Beijing area are presented in Figure 7.22.