A seismic wave is a vibration generated by an earthquake, explosion, or similar energetic source and propagated within Earth or along its surface. Earthquakes generate four principal types of elastic waves; two, known as body waves, travel within Earth, whereas the other two, called surface waves, travel along its surface. Seismographs record the amplitude and frequency of seismic waves and yield information about Earth and its subsurface structure. Artiﬁcially generated seismic waves recorded during seismic surveys are used to collect data in oil and gas prospecting and engineering.
Back and forth motion
P-waves, also called compressional or longitudinal waves, give the transmitting medium—whether liquid, solid, or gas—a back-and-forth motion in the direction of the path of propagation, thus stretching or compressing the medium as the wave passes any one point in a manner similar to that of sound waves in air. Within Earth, P waves travel at speeds from about 6 km (3.7 miles) per second in surface rock to about 10.4 km (6.5 miles) per second near Earth’s core some 2,900 km (1,800 miles) below the surface.
As the waves enter the core, the velocity drops to about 8 km (5 miles) per second. It increases to about 11 km (6.8 miles) per second near Earth’s centre. The speed increase with depth results from increased hydrostatic pressure as well as from changes in rock composition. In general, the increase causes P waves to travel in curved paths that are concave upward.
Perpendicular to direction of seismic wave propagation
S-waves, also called shear or transverse waves, cause points of solid media to move back and forth perpendicular to the direction of propagation; as the wave passes, the medium is sheared ﬁrst in one direction and then in another. Within Earth the speed of S waves increases from about 3.4 km (2.1 miles) per second at the surface to 7.2 km (4.5 miles) per second near the boundary of the core, which, being liquid, cannot transmit them; indeed, their observed absence is a compelling argument for the liquid nature of the outer core. Like P waves, S waves travel in curved paths that are concave upward.
Of the two surface seismic waves, Love waves—named after the British seismologist A.E.H. Love, who ﬁrst predicted their existence—travel faster. They are propagated when the solid medium near the surface has varying vertical elastic properties. Displacement of the medium by the wave is entirely perpendicular to the direction of propagation and has no vertical or longitudinal components.
The energy of Love waves, like that of other surface waves, spreads from the source in two directions rather than in three, and so these waves produce a strong record at seismic stations even when originating from distant earthquakes.
The other principal surface waves are called Rayleigh waves after the British physicist Lord Rayleigh, who ﬁrst mathematically demonstrated their existence. Rayleigh waves travel along the free surface of an elastic solid such as Earth. Their motion is a combination of longitudinal compression and dilation that results in an elliptical motion of points on the surface. Of all seismic waves, Rayleigh waves spread out most in time, producing a long wave duration on seismographs.
An earthquake starts suddenly and may last from a few seconds to a few minutes. Different kinds of earthquake waves cause different kinds of ground motion (figure above). The nature and severity of the shaking at a given location depend on four factors:
- magnitude of the earthquake, because larger magnitude events release more energy;
- distance from the focus, because earthquake energy decreases as waves pass through the Earth;
- nature of the substrate at the location (that is, the character and thickness of different materials beneath the ground surface) because earthquake waves tend to be ampliﬁed in a weaker substrate; and
- “frequency” of the earthquake waves (where frequency equals the number of waves that pass a point in a speciﬁed interval of time).
By J.P.Rafferty; M.Qasim