REVIEW FOR QUIZ
1. Introduction: Seismic
wave animations are used to
illustrate different types of wave propagation through elastic
materials. Animations are provided to view
wave
propagation in a 3-dimensional solid for Compressional (P), Shear (S),
Rayleigh
(R) and Love (L) waves. Wave
characteristics and particle motions of these wave types can be easily
illustrated using the seismic wave animations.
2. Seismic Waves: The main seismic wave types are Compressional (P), Shear (S), Rayleigh (R) and Love (L) waves. P and S waves are often called body waves because they propagate outward in all directions from a source (such as an earthquake) and travel through the interior of the Earth. In the animations shown here, the P and S waves are shown propagating horizontally, parallel to the Earth’s surface. Love and Rayleigh waves are surface waves and propagate approximately parallel to the Earth’s surface. Although surface wave motion penetrates to significant depth in the Earth, these types of waves do not propagate directly through the Earth’s interior. Descriptions of wave characteristics and particle motions for the four wave types are given in Table 1.
Primary Wave
Body wave
Longitudinal
The deformation propagates.
Particle motion consists of alternating
compression and dilation (extension). Particle motion is parallel
to
the direction of propagation (longitudinal). Material returns to
its
original shape after the wave passes.
Secondary Wave
Body wave
Transverse
The deformation propagates. Particle motion consists of alternating transverse motion. Particle motion is perpendicular to the direction of propagation (transverse). The transverse particle motion shown here is vertical but can be in any direction; however the Earth’s approximately horizontal layers tend to cause mostly SV (in the vertical plane) or horizontal (SH) shear motions. Material returns to its original shape after the wave passes.
Table 1: Seismic Waves
|
|||
|
Wave Type (and names) |
Particle Motion |
Typical Velocity |
Other Characteristics |
|
P,
Compressional, Primary, Longitudinal |
Alternating compressions (“pushes”) and
dilations (“pulls”) which are directed in the same direction as the
wave is propagating (along the ray path); and therefore, perpendicular
to the wavefront. |
VP
~ 5 – 7 km/s in typical Earth’s crust; >~ 8 km/s in Earth’s mantle
and core; ~1.5 km/s in water; ~0.3 km/s in air. |
P motion travels fastest in materials, so the
P-wave is the first-arriving energy on a seismogram. Generally
smaller and higher frequency than the S and Surface-waves. P
waves in a liquid or gas are pressure waves, including sound waves. |
|
S,
Shear, Secondary, Transverse |
Alternating transverse motions (perpendicular
to the direction of propagation, and the ray path); commonly
approximately polarized such that particle motion is in vertical or
horizontal planes. |
VS
~ 3 – 4 km/s in typical Earth’s crust; >~ 4.5 km/s in Earth’s
mantle; ~ 2.5-3.0 km/s in (solid) inner core. |
S-waves do not travel through fluids, so do
not exist in Earth’s outer core (inferred to be primarily liquid iron)
or in air or water or molten rock (magma). S waves travel slower
than P waves in a solid and, therefore, arrive after the P wave. |
|
L,
Love, Surface waves, Long waves |
Transverse horizontal motion, perpendicular
to the direction of propagation and generally parallel to the Earth’s
surface. |
VL ~ 2.0 - 4.4 km/s in the
Earth depending on frequency of the propagating wave, and therefore the
depth of penetration of the waves. In
general, the Love waves travel slightly faster than the Rayleigh waves. |
Love waves exist because of the Earth’s
surface. They are largest at the surface and decrease in
amplitude with depth. Love waves are dispersive, that is, the
wave velocity is dependent on frequency, generally with low frequencies
propagating at higher velocity. Depth of penetration of the Love
waves is also dependent on frequency, with lower frequencies
penetrating to greater depth. |
|
R,
Rayleigh, Surface waves, Long waves, Ground
roll |
Motion is both in the direction of
propagation and perpendicular (in a vertical plane), and “phased”
so that the motion is generally elliptical – either prograde or
retrograde. |
VR ~ 2.0 - 4.2 km/s in the
Earth depending on frequency of the propagating wave, and therefore the
depth of penetration of the waves. |
Rayleigh waves are also dispersive and the
amplitudes generally decrease with depth in the Earth. Appearance
and particle motion are similar to water waves. Depth
of penetration of the Rayleigh waves is also dependent on frequency,
with lower frequencies penetrating to greater depth. |
Surface Wave
Long waves
The deformation propagates. Particle motion consists of alternating transverse motions. Particle motion is horizontal and perpendicular to the direction of propagation (transverse). To best view the horizontal particle motion, focus on the Y axis (red line) as the wave propagates through it. Amplitude decreases with depth. Material returns to its original shape after the wave passes.
Surface Wave
Long waves
The deformation propagates. Particle motion consists of elliptical motions (generally retrograde elliptical as shown in the figure) in the vertical plane and parallel to the direction of propagation. Amplitude decreases with depth. Material returns to its original shape after the wave passes.
The web page
for
this document is: http://www.eas.purdue.edu/~braile/edumod/waves/WaveDemo.htm
Partial funding for this development
provided by the National Science Foundation.