Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 241

A tendency to a de-amplification occurs for rocks with acceleration greater than
0.4g. More recent studies of Anastasiadis and Klimis (2002) for C and D site types
have shown that the de-amplification occurs for 0.55g and 0.25g, respectively.
Therefore, the nonlinear inelastic soil behavior, even in the small acceleration
range (for instance, in low to moderate seismic regions), introduces a complexity in
evaluating the response of soil layers during the earthquakes. The coupling
between earthquake characteristics and local site conditions may play an important
role. It is essential to take into account in earthquake analysis the geotechnical
aspects for hazard mitigation (Ansal, 1995).

7.4.4 Topographic Surfaces Irregularities

The effects of topographic amplification of seismic response have been observed in
numerous earthquakes, where damage has been concentrated near cliff and ridge
crests, due to the interference of waves causing very complex patterns of
frequency-dependent amplifications. A characteristic example of increased
earthquake damage close to the crest of a step-like topography is illustrated in
Figure 7.20. In this case, during the 1980 earthquake in Italy, the damage of Ipirna,
an Italian village, sitting at the top of a hill, was concentrated close to the crest of a
steep slope, whereas it was insignificant in the direction away from the crest
(Athanasopoulos et al, 1998). The main types of topographical surface
irregularities are (Faccioli and al, 2002): slopes and cliffs, valleys and hills and
ridges. Amplification of seismic waves due to topographical irregularities may be
ascribed to different causes:

Figure 7.20 Effect of surface topography in the 1980 Irpina (Italy) earthquake
(after Athanasopoulos et al, 1998)