Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 220

Traveled path. The propagation of seismic waves depends on the percentage of
the path travel through rock or soft sediment, with the presence of mountains and
valleys from the source to the site. The results of the traveled paths are the
amplification, attenuation or modification, more or less, of the source basic
characteristics.
Site conditions. Multi-layers with different properties and thicknesses compose
the site soil profile. The alternations of these layers, the presence in the site of hills,
valleys or important buildings, are very important factors, which can modify the
source characteristics.
Therefore the seismicity of a site depends on some fundamental input
parameters and some additional factors. In order to establish the seismic hazard for
a site, the first step is the identification of the source type. The second step is the
evaluation of the influence of the distance from the source and the site and the
study of the local conditions. For the determination of the seismic risk, these data
must be represented by means of values referring to accelerations, velocities,
displacements, durations and number of pulses in the site, as a function of the
source type, magnitude, duration, directivity and the local influence as attenuation
or amplification, site stratification, relief, etc. These factors will be analyzed in the
following sections.
7.2 SOURCE CHARACTERISTICS
7.2.1 Surface Fault Rupture
A sudden movement of rocks along a fault plane causes earthquakes. The
movement or displacement is called fault rupture. When the fault rupture
progresses upward, it creates a surface fault rupture. This surface can remains
beneath the surface, being a blind fault , or reach the Earth's surface, creating a
surfacefaulting.
The rupture begins at some depth called focus or hypocenter (Fig. 7.2) . The
rupture then spreads outward in all directions along the fault plane, but there is a
dominant direction.
The characteristics of the fault rupture depend on the following factors:
- rupture surface;
- type of fault;
- depth of the fault;
- amount of the fault slip;
- age of faulting.
The earthquake magnitude depends on the length of the fault rupture. The highest
magnitude earthquake, in Chile 1960 , has a length about 1300 kilometers.
The formation of a rupture surface along a fault is a very complex
phenomenon, which raises many questions. There are regions where great
earthquakes occur along some plate boundaries, while there are also quiet regions
along the same fault. Why does it happen? Why are there segments of fault very
active in some periods and quiet in others? The best example is the Mexico coast,