Environmental Engineering Reference
In-Depth Information
the location on the surface directly above the focus. Depending upon geologic conditions,
the epicenter may or may not be the location where surface effects are most severe.
Earthquakes are classified on the basis of depth of focus as follows:
Normal or shallow: 0 to 70 km (generally within the Earth's crust)
●
Intermediate: 70 to 300 km
●
Deep: Greater than 300 km (None has been recorded greater than about 720 km,
and no magnitude greater than 8.6 has been recorded below 300 km.)
●
Some Depth Relationships
Southern California events generally occur at about 5 km depth, whereas Japanese events
generally occur at less than 60 km with more than half at less than 30 km.
Foci depths for a number of events apparently define the edge of a subducting plate in
some locations, as shown in
Figure 11.2.
Focus depth can be significantly related to surface
damage. The Agadir event (1960) of magnitude 5.8 had a very shallow focal depth of about
3 km, but since it was essentially beneath the city, its effects were disastrous. A magnitude
5.8 event is usually considered to be moderate, but the shallow focus combined with the
very weak construction of the city resulted in extensive damage, although the total area of
influence was small. The Chilean event of 1960 with a magnitude of 8.4, however, had a
focal depth of about 65 km, and although it was felt over a very large area, there was no
extreme damage.
11.2.2
Seismic Waves
Origin
Earthquake occurrence causes an energy release which moves as a shock front or strain
pulse through the Earth, which is considered as an elastic medium. The pulse becomes an
oscillatory wave in which particles along the travel paths are “excited” and move in orbits
repeating cyclically. In a simple two-dimensional diagram, the oscillation is shown as a
wave shape with a crest and a trough as given in Figure 11.4.
If the wave energy produces displacements in the geologic materials within their elastic
limits, the materials return to their original volume and shape after the energy wave has
passed. The waves are then termed elastic waves. They propagate through the Earth and
along its surface as various types of seismic waves as shown in
Figure 11.5.
Wave Types
Body Waves
Primary or compression waves (P waves)
are generated by the initial shock, which applies a
compressive force to the materials, causing a wave motion in which the particles move back
y
(displacement)
Period (
I
,
s
)
Wavelength (
Crest
(amplitude)
x
(distance)
t
(time)
λ
,
m
)
FIGURE 11.4
Characteristics of elastic waves. (From Hunt,
R.E.,
Geotechnical Engineering Investigation
Manual,
McGraw-Hill Book Co., New York,
1984, 983 pp.)
A
Trough
