Environmental Engineering Reference
In-Depth Information
An indication of the compressive effect that seismic waves can have, even at great dis-
tances, has been reported by Rainer (1974). He made a comparison between the incidence
of rock bursts occurring in a mine in Bleiberg, Germany, with earthquakes on a worldwide
basis, and found a strong correlation between rock bursts at Bleiberg and the large shocks
that occurred in Agadir (1960), Skopje (1969), San Fernando (1971), and Nicaragua (1972).
Shear, transverse, or secondary waves (S waves)
are generated where the initial pressure
pulse, or the P wave that it generates, strikes a free surface or a change in material in a
direction other than normal. The shape of the transmitting material is then changed by
shear rather than compression. S waves can travel only in a solid because their existence
depends on the ability of the transmitting medium to resist changes in shape (the shear
modulus). P waves can travel in any matter that resists compression or volume change to
solid, liquid, or gas. The S waves move at slower velocities than P waves and arrive later
at a distant point even though they are both generated at the same instant. Both P and S
waves travel through the Earth in direct, refracted, or reflected paths, depending upon the
material through which they are traveling.
Surface Waves or Long Waves (L Waves)
Long waves travel along the free surface of an elastic solid bounded by air or water. They
are defined by the motion through which a particle in its path moves as the wave passes.
Rayleigh (R) waves
cause the particles to move vertically in an elliptical orbit, or to “push
up, pull down” in the direction of propagation.
Love (Q) waves
cause the particles to vibrate transverse to the direction of wave advance,
with no vertical displacement.
Both Rayleigh and Love waves move at slower velocities than P or S waves, and as they
travel they disperse into rather long wave trains (long periods). (The comparative arrivals
of P, S, and L waves are shown on the seismogram given in
Figure 11.6.)
Propagation Velocity
The velocity with which seismic waves travel through the Earth is termed the propagation
velocity, which can be expressed in terms of elastic moduli and material density for P
waves (
V
p
) and S waves (
V
s
) as follows:
V
p
√
[
K
(4/3)
G
]/
ρ
m/sec
V
s
√
G
/
ρ
m/s
where
K
is the dynamic bulk modulus, G the dynamic shear modulus and
ρ
the material
bulk density.
P
S
L
FIGURE 11.6
Seismograph from a long-period, vertical instrument for an event of September 26, 1959, located off the coast of
Oregon and recorded in Tucson, Arizona. The long-period waves are well represented on the record and the P, S,
and L waves are noted. (Compare with the synthetic records obtained from a refraction seismograph,
Figure 2.22.)
The P and S waves have been damped (attenuated) as they traveled through the Earth. (From Neuman, 1966.)
