When an earthquake happens deep underground a crack will start to open on a pre-existing line of weakness in the Earth's brittle crust. This crack will then grow larger and larger, relieving built-up stress as it goes.
The speed at which the crack propagates or grows is 2–3 km/sec. Eventually the rupture will cease to grow and will slow down and stop. The size or magnitude of the earthquake depends upon how much the fault has ruptured (the slip) and also the area over which the rupture has occurred.
This rupturing process creates elastic waves in the Earth that propagate away from the rupture front at a much faster speed than the rupture propagates, the exact speed depends upon the nature of the wave (a longitudinal or P-wave is faster than a transverse or S-wave), and on the elastic properties of the Earth. As you go deeper into the Earth, the density and pressure increases and so do the velocities of seismic waves.
Seismic waves are fundamentally of two types, compressional, longitudinal waves or shear, transverse waves.
Through the body of the Earth these are called P-waves (for primary because they are fastest) and S-waves (for secondary since they are slower). However, where a free surface is present (like the Earth–air interface) these two types of motion can combine to form complex surface waves.
Although often ignored in introductory texts, surface waves are very important since they propagate along the surface of the Earth (where all the buildings and people are) and usually have much higher amplitudes than the P-waves and S-waves. It is usually surface waves which knock down buildings.
Seismic waves, like all waves, transfer energy from one place to another without moving material.
|Type (and names)||Particle motion||Typical velocity||Other characteristics|
|Alternating compressions ('pushes') and dilations ('pulls') in the same direction as the wave is propagating||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.|
|Alternating transverse motions perpendicular to the direction of propagation.||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 liquid outer core 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.|
|Transverse horizontal motion, perpendicular to the direction of propagation and generally parallel to the Earth's surface||VL ~ 2.0 - 4.5 km/s in the Earth depending on frequency of the propagating wave||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, with low frequencies normally propagating at higher velocity. Depth of penetration of the Love waves is also dependent on frequency, with lower frequencies penetrating to greater depth.|
|Motion is both in the direction of propagation and perpendicular (in a vertical plane)||VR ~ 2.0 - 4.5 km/s in the Earth depending on frequency of the propagating wave||Rayleigh waves are also dispersive and the amplitudes generally decrease with depth in the Earth. Appearance and particle motion are similar to water waves.|
Arrival times and velocity models.
You can use a box and several wire helixes ('slinkies') attached to it with bulldog clips to show how an earthquake generates P- and S-waves.
This activity shows you one way to detect vibrations and display them on a computer screen.
This activity uses two microphones to represent two seismic monitoring stations.