Seismic waves

1. SEISMIC WAVESSEISMIC WAVES
لايخ ميهاربإ حبص دمحأ
Sec. 8
B.N. 14

2. Seismic WavesSeismic Waves
•Seismic waves are waves of energy that travel through the
core of the earth or other elastic bodies generated from
earthquake, explosion, or some other process that imparts
low-frequency acoustic energy.
•The propagation velocity of the waves depends on
the density and elasticity of the medium.
•Velocity tends to increase with depth, and ranges from
approximately 2 to 8 km/s in the Earth's crust up to 13 km/s
in the deep mantle.
•Various types travel at different velocities.
•Refraction or reflection of seismic waves is used for research
of the Earth's interior, and artificial vibrations to investigate
subsurface structures.

3. Seismic WavesSeismic Waves
•Seismic waves are
predicted during the 19
th
century.
•It is similar to sound waves
except that the periods of
oscillations are far longer.
•The frequency range of
these waves is large from
as high as the audible
range to as low as the free
oscillations of the entire
Earth
•There are two types of
seismic waves: body and
surface waves

4. Body WavesBody Waves
•travel through the interior of the
Earth
•follow ray paths refracted by the
varying density and modulus (stiff
ness) of the Earth's interior
(density and modulus, in turn,
vary according to temperature,
composition, and phase similar to
the refraction of light waves)
•the two types are P-waves and S-
waves

5. P wavesP waves
•P waves, also called primary or pressure waves, are longitudinal or
compressional in nature.
•These waves are composed of alternating compressions and
rarefactions.
•P waves can travel through any medium.
•In solids, these waves generally travel almost twice as fast as S
waves. In air, these pressure waves take the form of sound waves,
hence they travel at the speed of sound.
•These waves travel at ~6 km/s near the surface to ~10.4 km/s near
the Earth’s core about 2900km below the surface.
•As the waves enter the core, the velocity drops to ~8 km/s
increasing to ~11 km/s near the center.
•These results from increased hydrostatic pressure as well as from
changes in rock composition and phase.

6. P wavesP waves
•The transmitting rocks are alternately compressed and
expanded giving the rock particles a back-and-forth motion
along the path of propagation.

7. P wave shadow zoneP wave shadow zone
•Almost all the information available on the structure of the
Earth's deep interior is derived from observations of the
travel times, reflections, refractions and phase transitions of
seismic body waves, or normal modes. Body waves travel
through the fluid layers of the Earth's interior, but P-waves
are refracted slightly when they pass through the transition
between the semisolid mantle and the liquid outer core. As a
result, there is a P-wave "shadow zone" between 104° and
140° from the earthquake's focus, where the initial P-waves
are not registered on seismometers.

8. P wave shadow zoneP wave shadow zone

9. S wavesS waves
•S waves, also called secondary or shear waves, are transverse
in nature
•Unlike P waves, S waves can only travel through solids.
•These waves travel from ~3.4 km/s near the surface to ~7.2
km/s near the boundary of the liquid core (Gutenberg
discontinuity).
•Also, these waves travel at a slower rate but with greater
amplitude.
•S waves travel transversely to the direction of propagation
and involves the shearing of the transmitting rock causing the
rock particles to move back and forth perpendicular to the
direction of propagation.

10. S wavesS waves
•As the waves pass, the rock is distorted first in one direction
and then in another.

11. S wave shadow zoneS wave shadow zone
•Unlike the P-wave, the S-wave cannot travel through the
molten outer core of the Earth, and this causes a shadow
zone for S-waves opposite to where they originate. They can
still appear in the solid inner core: when a P-wave strikes the
boundary of molten and solid cores, called the Lehmann
discontinuity, S-waves will then propagate in the solid
medium. And when the S-waves hit the boundary again they
will in turn create P-waves. This property allows seismologists
to determine the nature of the inner core.

12. S wave shadow zoneS wave shadow zone
S-waves don't penetrate the outer core, so they're shadowed
everywhere more than 104° away from the epicenter.

13. Surface WavesSurface Waves
•Surface waves travel only on the surface of the Earth.
•These waves are guided by the free surface of the Earth.
•Surface waves are analogous to water waves.
•Because of their low frequency, long duration, and large
amplitude, they can be the most destructive type of seismic
wave.
•They follow along after the P and S waves have passed
through the body of the planet.
•S waves disperse into long wave trains and at substantial
distance from the source, they cause much of the shaking felt
during earthquakes.

14. Surface WavesSurface Waves
•There are two types of surface waves: Rayleigh waves and
Love waves.

15. Love WavesLove Waves
•Love Waves are named after A. E. H. Love who predicted their
existence in 1911.
•Love waves travel with a slower velocity than P- or S- waves,
but faster than Rayleigh waves.
•These waves are propagated in a surface layer that overlies a
solid layer with different elastic properties.
•The displacement of the rock particles is entirely
perpendicular to the direction of propagation and has no
vertical or longitudinal components.
•The energy created by these waves spreads from the source
in two directions rather than in three.

16. Love WavesLove Waves
•These produces a strong record at seismic equations even
when originated from distant earthquakes.

17. Rayleigh WavesRayleigh Waves
•Rayleigh waves are named after Lord Rayleigh (John William
Strutt, 3rd Baron Rayleigh, OM) who predicted their existence
in 1885.
•The motion in this kind of wave is a combination of
longitudinal and vertical vibration that give elliptical motion
to the rock particles.
•These waves have the strongest effect on seismographs.
•Rayleigh waves are generated by the interaction of P- and S-
waves at the surface of the earth, and travel with a velocity
that is lower than the P-, S-, and Love wave velocities.

18. Rayleigh WavesRayleigh Waves
•The intensity of Rayleigh
wave shaking at a particular
location is dependent on
several factors:
•The size of the earthquake.
•The distance to the
earthquake.
•The depth of the
earthquake.
•The geologic structure of the
crust.
•The focal mechanism of the
earthquake.
•The rupture directivity of the
earthquake.