Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 236

7.3.3 Path Effects for Deep Sources
The amplitude of surfaces waves is the largest at or near the surface and rapidly
decreases with the depth. Consequently, the shallow crustal earthquakes generate
large surface waves, but with the increasing of the focal depth, the surface waves
become smaller and smaller. Therefore, if surface waves usually dominate the
seismogram for crustal earthquakes, they often become insignificant for the deeper
source (intraslab earthquakes), the body waves being dominating (Kulhanek,
1990). These aspects concerning the differences in seismic wave attenuation
provide the designer with a powerful tool for a reliable discrimination of crustal
earthquakes against deep sources. Unfortunately, there are not many data about the
influence of the propagation path. Generally, it is accepted that P-waves are not
influenced by the path characteristics, having a very high frequency, very different
from the traveled path. Contrary, this traveled path influences the S-waves, having
higher frequencies (Elnashai and Papazouglou, 1997).
7.4 GROUND RESPONSE TO EARTHQUAKES
7.4.1 Significance of Local Soil Conditions
Observations of damage during numerous earthquakes have shown that local site
conditions can have a strong influence on the performance of structures during
earthquakes. Significant damage and loss of life has been directly related to the
effect of local site conditions in recent earthquakes, such as the 1964 Niigata, 1977
Bucharest, 1985 Mexico City, 1988 Armenia, 1989 Loma Prieta, 1990 Iran, 1994
Northridge, 1995 Kobe, 1999 Kocaeli and 2009 L'Aquila earthquakes. Therefore,
during the last decades the role of the local site effects has been widely discussed.
Local site effects can be responsible for micro-seismic variations, which can be
more important than the propagation-path influence. The ignorance or
inappropriate consideration of site effects, which can be still encountered in
practice, is, therefore, a professional negligence (Studer and Koller, 1995, Gioncu
and Mazzolani, 2002).
The local site characterization is a very complicated multi-disciplinary problem.
An attempt to classify the local site effects must consider the local layered
deposits, soil type and shear wave velocity of each layer, topographic surface
irregularities, alluvial valley and liquefaction.
7.4.2 Local Horizontally Layered Deposits
Local horizontally layered deposist can be identified by characterizing the multi-
layers with different mechanical properties and thickness, corresponding to some
soil categories. Figure 7.16a shows the soil layers for Eastern Bucharest (Lungu et
al, 1997), which determine an important pulse with long periods in accelerogram
due to the presence of deposits of lacustral layers (Fig. 7.16b) (Gioncu and
Mazzolani, 2002, Ifrim et al, 1986). The presence of these pulses in ground