Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 230

Figure 7.10 Typical recorded accelerogram (FEMA, Session 5, 2005)
energy from shallow shock and are usually the primary cause of destruction which
results from earthquakes affecting densely populated areas (Kulhanek, 1990).
The first kind of surface wave is the L wave , which moves the ground from
side-to-side, having a velocity of 2 to 4.4 km/sec. The second kind of surface wave
is the R wave , which is a combination of P and S waves and moves the ground up
and down and side-to-side in the same direction in which the wave is moving. The
velocity of the surface waves is practically the same as the one of the L waves,
which, generally, arrives to the site at the same time, with a gentle time in
comparison with the first type.
Figure 7.10 illustrates some of the basic properties of recorded body and surface
waves. The first is the P wave, followed by the S wave after some time. After a
short time, one can see the increasing of ground motion amplitudes due to the
arriving surface waves, L and R. Therefore, the surface waves are the largest signal
in a seismogram.
7.3.2 Path Effects for Crustal Sources
The seismic waves lose energy as they propagate through the Earth along the travel
path. The rate, at which the earthquake ground motion decreases with the distance,
is a function of the source, seismic wave types, regional geology and inherent
characteristics of the earthquake. These major factors affect the severity of the
ground shaking at the site. The attenuation is very different, depending on the
source type.