Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 228

Figure 7.8 Body (P and S) and surface (L and R) seismic waves
(Gioncu and Mazzolani, 2002)
mechanical properties. Due to the discontinuities between the layers, reflection and
refraction phenomena occur, making very complex the way of seismic wave to the
surface site.
The bodywaves are P waves (from latin Undae Primae) and S waves (from latin
Undae Secundae) (Fig 7.9a) that propagate through the Earth's crust. Richard
Dixon Oldham, a British geologist and seismologist, was the first scientist who
recognized the P and S-waves in the early Seismology. The first kind of body
waves is the P wave. They push and pull the rock and move through just like the
sound waves push and pull the air, by compressions and dilatations. The rock
particles, affected by a P-wave, oscillate backward and forward in the same
direction as the wave propagates. They are the ones that travel fastest, having
velocities 5 to 7 km/sec, and thereby are usually felt first. This wave has very high
frequency. The second type of body wave is the S wave , which moves rock up and
down, or side-to-side, the Earth vibrating perpendicularly to the direction of the
wave travel. This wave is much slower than the P waves, with a velocity of 3 to 4
km/sec, and therefore, arrives after the primary wave, having a reduced frequency
in comparison with the first wave. Due to the difference in velocities, a time delay
between P-wave and S-wave arrival exists, and this delay can serve for locating the
earthquake epicenter.
The second type of waves are the surface waves , which travel along the free
Earth's surface (Fig. 7.9 b): L waves (from the British mathematician Love, who
worked out the mathematical model for this kind of wave in 1911) and R waves
(from Lord Rayleigh, who mathematically predicted the existence of this kind of
wave in 1885). The surface waves propagate also along the discontinuities in
media of the Earth's interior. Other modes of wave propagation exist (for example
reflected and refracted waves, effects of conversion and dispersion, etc.), but they
are of comparatively minor importance. Surface waves carry the greatest amount of