Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 219

from one site to another, making very difficult to distinguish the main
characteristics of ground motions.
Due to these uncertainties, there are many adverse arguments in using only the
recorded ground motions for design purposes. An alternative way to face this
situation is to use the numerical computation, in which the influence of the main
factors can analyzed. But these factors are so various, that a credible result is very
difficult to obtain. Therefore, a rational combination between recorded data and
numerical results seems to be the best way to follow for the definition of the design
ground motion.
7.1.3 Factors Influencing the Ground Motions
In the last few years, the understanding of earthquake characteristics has
considerably increased, due to the tragic experience from several recent
earthquakes. As a consequence of the direct observations and subsequent studies,
now it is possible to quantitatively predict strong motions for dangerous
earthquakes, provided that the source mechanism, wave travel path and site
geological conditions are correctly modeled (Huang, 1983, Kawase, 2004).
Therefore, a clear representation of an earthquake can be obtained, by considering
the source typology as a matrix, where the influence of some factors connected to
traveling path and site conditions are overlapped (Fig. 7.1). This procedure gives
the opportunity to separately analyze the influence of each basic factor in the frame
of the anatomy of an earthquake.
Source. Earthquakes occur in a fault, in a network of faults or in an area of
faults. In the first case, the positions of faults are known due to the seismological
studies; in the second case, the fault is poorly or not known and the earthquakes
can occur in a large region having uniform seismic potential. Each source has some
specific characteristics, in function of the fault type and the rupture area.
Figure 7.1 Basic factors influencing the recorded ground motions
(Gioncu and Mazzolani, 2002)