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.

P Compressional
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.

                                       
Primary Wave Motion
S Shear
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.
Shear or Secondary wave

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.



L Love Wave Motion
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.
Love wave
R Rayleigh Wave Motion
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.
Rayleigh wave


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.

ã Copyright 2004-5.  L. Braile.  Permission granted for reproduction and use of files and animations for non-commercial uses.