Seismic waves are waves of energy that travel through thecore of the earth or other elastic bodies generated fromearthquake, explosion, or some other process that impartslow-frequency acoustic energy.

The propagation velocity of the waves depends onthe density and elasticity of the medium.

Velocity tends to increase with depth, and ranges fromapproximately 2 to 8 km/s in the Earth's crust up to 13 km/s inthe 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 investigatesubsurface structures.

Seismic waves are predicted during the 19th 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 theentire Earth

The waves of energy caused by the sudden breaking of rock within the earth or an explosion.

These energy that travels through the earth and recorded on seismographs.

There are several different kinds of seismic waves, and they all move in different ways.

The two main types of waves are body waves and surface waves.

Body wavesTravel through the earth's interior similar to sound waves

Surface WavesTravel along the earth's surface - similar to ocean waves

Types of Earthquake waves

•When an earthquake occurs, it starts at depth with in the BODY of the earth.

•At that depth there are only two types of particle motion possible.

•P and S waves are known as BODY waves because they always originate inside the earth.

• These are not the most damaging but they are the first to be detected because they travel at very fast speeds ( e.g. >6km/s.)

• P waves are the fastest, and first waves to arrive.

•S waves are the second fastest (~50%) and

•Surface waves (45%) are the slowest of the three major types of earthquake waves.

• (Surface) The most damaging earthquake vibrations occur when the P and S waves reach the surface.

•At the surface they convert into horizontal S waves and a combination of vertical S and P waves called Rayleigh waves.

Travel through the interior of theEarth

Follow ray paths refracted by thevarying density and modulus (stiffness) of the Earth's interior

(density and modulus, in turn,vary according to temperature,composition, and phase similar tothe refraction of light waves)

The two types are P-waves andS-waves

A type of seismic wave that compresses and expands

The first wave to arrive at an earthquake

Primary or compressional (P) waves

The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave.

The P wave can move through solid rock and fluids, like water or the liquid layers of the earth.

It pushes and pulls the rock it moves through just like sound waves push and pull the air.

Highest velocity (6 km/sec in the crust)

Particle motion consists of alternating compression and dilation. Particle motion is parallel to the direction of propagation (longitudinal). Material returns to its original shape after wave passes.

Particle motion

Deformation propagates

P waves, also called primary or pressure waves, are longitudinal orcompressional in nature.

These waves are composed of alternating compressions andrarefactions.

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 nearthe Earth’s core about 2900km below the surface.

As the waves enter the core, the velocity drops to ~8 km/sincreasing to ~11 km/s near the center.

These results from increased hydrostatic pressure as well as fromchanges in rock composition and phase.

The transmitting rocks are alternately compressed andexpanded giving the rock particles a back-and-forth motionalong the path of propagation.

Ships at sea off the California coast in 1906 felt theearthquake when the P-wave traveled through the water andstruck the ship (generally the crews thought they had strucka sandbar).

Almost all the information available on the structure of theEarth's deep interior is derived from observations of thetravel times, reflections, refractions and phase transitions ofseismic body waves, or normal modes. Body waves travelthrough the fluid layers of the Earth's interior, but P-wavesare refracted slightly when they pass through the transitionbetween the semisolid mantle and the liquid outer core. As aresult, there is a P-wave "shadow zone" between 104° and140° from the earthquake's focus, where the initial P-wavesare not registered on seismometers.

A type of seismic wave that moves the ground up and down or side to side

http://daphne.meccahosting.com/~a0000e89/insideearth2.htm

Secondary or shear (S) waves

The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake.

An S wave is slower than a P wave and can only move through solid rock. (3.6 km/sec in the crust)

This wave moves rock up and down, or side-to-side.

Particle motion

Particle motion consists of alternating transverse motion. Particle motion is perpendicular to the direction of propagation (transverse). Transverse particle motion shown here is vertical but can be in any direction. Material returns to its original shape after wave passes.

Deformation propagates

S waves, also called secondary or shear waves, aretransverse 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 (Gutenbergdiscontinuity).

Also, these waves travel at a slower rate but with greateramplitude.

S waves travel transversely to the direction of propagationand involves the shearing of the transmitting rock causingthe rock particles to move back and forth perpendicular tothe direction of propagation.

As the waves pass, the rock is distorted first in one directionand then in another.

Unlike the P-wave, the S-wave cannot travel through themolten outer core of the Earth, and this causes a shadowzone for S-waves opposite to where they originate. They canstill appear in the solid inner core: when a P-wave strikes theboundary of molten and solid cores, called the Lehmanndiscontinuity, S-waves will then propagate in the solidmedium. And when the S-waves hit the boundary again theywill in turn create P-waves. This property allowsseismologists to determine the nature of the inner core.

S wave shadow zone

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

Scientists calculate the difference between arrival times of the P waves and S waves

The further away an earthquake is, the greater the time between the arrival of the P waves and the S waves

Move along the Earth’s surface Produces motion in the upper crust

Motion can be up and down

Motion can be around

Motion can be back and forth

Travel more slowly than S and P waves More destructive

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 seismicwave.

They follow along after the P and S waves have passedthrough the body of the planet.

S waves disperse into long wave trains and at substantialdistance from the source, they cause much of the shakingfelt during earthquakes.

There are two types of surface waves: Rayleigh waves andLove waves.

Love Waves The first kind of surface wave is called a Love wave,

named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911.

It's the fastest surface wave and moves the ground from side-to-side.

Love Waves are named after A. E. H. Love who predictedtheir 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 asolid layer with different elastic properties.

The displacement of the rock particles is entirelyperpendicular to the direction of propagation and has novertical or longitudinal components.

The energy created by these waves spreads from the sourcein two directions rather than in three.

These produces a strong record at seismic equations evenwhen originated from distant earthquakes.

Rayleigh waves are named after Lord Rayleigh (John WilliamStrutt, 3rd Baron Rayleigh, OM) who predicted theirexistence in 1885.

The motion in this kind of wave is a combination oflongitudinal and vertical vibration that give elliptical motionto 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 velocitythat is lower than the P-, S-, and Love wave velocities.

The intensity of Rayleighwave shaking at a particularlocation is dependent onseveral 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.

Rayleigh Waves

The other kind of surface wave is the Rayleigh wave, named for John William Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885.

A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.

Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving.

Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.

Particle motion consists of elliptical motions (generally retrograde elliptical) in the vertical plane and parallel to the direction of propagation. Amplitude decreases with depth. Material returns to its original shape after wave passes.

Compressional Wave (P-Wave) Animation

Deformation propagates. Particle motion consists of alternating

compression and dilation. Particle motion is parallel to the

direction of propagation (longitudinal). Material returns to its

original shape after wave passes.

Shear Wave (S-Wave) Animation

Deformation propagates. Particle motion consists of alternating

transverse motion. Particle motion is perpendicular to the direction of

propagation (transverse). Transverse particle motion shown here is

vertical but can be in any direction. However, Earth’s layers tend to

cause mostly vertical (SV; in the vertical plane) or horizontal (SH) shear

motions. Material returns to its original shape after wave passes.

Rayleigh Wave (R-Wave) Animation

Deformation propagates. Particle motion consists of elliptical motions

(generally retrograde elliptical) in the vertical plane and parallel to the

direction of propagation. Amplitude decreases with depth. Material

returns to its original shape after wave passes.

Love Wave (L-Wave) Animation

Deformation propagates. Particle motion consists of alternating

transverse motions. Particle motion is horizontal and perpendicular to

the direction of propagation (transverse). To aid in seeing that the

particle motion is purely horizontal, focus on the Y axis (red line) as the

wave propagates through it. Amplitude decreases with depth. Material

returns to its original shape after wave passes.