A wave is a physical phenomenon that entails the transfer of energy through space or matter, typically in the form of oscillations or vibrations. This transfer of energy is often characterized by the propagation of disturbances that travel through a medium, such as air, water, or solids. Waves can be classified based on their properties, including wavelength, frequency, amplitude, and direction of propagation. The study of waves is a fundamental aspect of many scientific and engineering disciplines, such as physics, acoustics, optics, and electronics, and has numerous practical applications, ranging from telecommunications to earthquake monitoring.
Waves are a natural phenomenon that occurs frequently in the world around us.
Light is a wave
Sound is a wave
Waves have both a physical length (wavelength) and a time interval (period) that repeat periodically.
Wave Terminology
Amplitude is a measure of the height of a wave and is expressed in units of displacement, such as centimeters or meters.
Wavelength, on the other hand, is the distance between two consecutive points on a wave that have equal amplitude and phase, like crest to crest or trough to trough. It is a measure of the spatial width of a wave and is expressed in units of length, such as meters or kilometers.
Lastly, the period of a wave is the time required for one complete cycle, which is measured in seconds.
Frequency refers to the number of cycles of waves that occur per unit of time, such as the ground shaking up and down during an earthquake. It is the inverse of the period and is measured in Hertz (Hz), which is one cycle per second.
Wave speed is the rate at which a wave travels through a medium. It indicates how quickly a point on the ground shakes in response to an earthquake. Wave speed is not a dynamic quantity; rather, it is a fixed material property, similar to density.
An elastic wave is a type of motion that occurs in a medium when particles are displaced. When this happens, a force proportional to the displacement acts on the particles, causing them to return to their original position. If a material displays elasticity and the particles in a certain region are set in vibratory motion, an elastic wave will be propagated.
What are seismic Waves?
A seismic wave is a disturbance that transfers energy from one place to another. They are elastic waves generated by an impulse such as an earthquake, explosion, implosion, volcanic eruption, etc.
Seismic waves travel both:
Through the body of the earth and Over the surface of the earth.
The analogy between sound and seismic waves
Seismic waves, specifically p-waves, is a type of acoustic energy similar to sound waves and can be compared to speech.
| Speech | Earthquakes |
| 1). Vocal cords vibrate | 1). A locked fault segment fails (ruptures) |
| 2). Sound waves propagate through the atmosphere | 2). Seismic waves propagate through Earth |
| 3). Ears record these vibrations | 3). Seismometers record these vibrations |
| 4). brain processes the recordings | 4). Seismologists process recordings (seismograms) |
Various Sources of Seismic Waves
Earthquakes create seismic waves, but they are not the only phenomena that can generate them. Many other processes are also capable of producing seismic waves.
for example:
- Volcanic eruptions
- Explosions
- wind
- Surf
- Traffic
- Sonic Booms (planes, shuttles, meteorites)
- Humans
Multiple Frequency Signals
Many interesting signals are actually made up of a combination of waves with varying frequencies. This range of frequencies is commonly referred to as the “band,” and we use the term “bandwidth” to describe it. Light is an example of a multiple-frequency signal, with different frequencies corresponding to different colors.
Types of seismic waves
Two main types of seismic waves are produced during an earthquake based on the path they travel through the earth.
Body wave:
Earthquakes generate different types of waves, including P and S waves that travel through the Earth’s interior.
Surface wave:
Body waves interact with the surface and surficial layers of the earth. They travel along the earth’s surface, with amplitudes that decrease exponentially with depth. Examples include L and R waves.
Types of seismic waves
Body Waves |
Surface Waves |
|
small amplitude |
Large amplitude |
| Short wavelength | Long wavelength |
| Narrow frequency band | Wide range of frequencies (large bandwidth) |
| Travel more quickly | Travel slowly |
| produced by all earthquakes | Not produced by deep earthquakes |
A second way to distinguish between waves is based on the type of deformation they cause. Compressional waves, also known as P-waves, cause changes in volume, while Rayleigh waves, also known as compressional surface waves, move the ground up and down in a rolling motion.
Shear Waves cause changes in shape:
Love wave (shear surface wave)
S-wave (shear body wave)
Primary waves (P)
Compressional Body Waves (P-Waves)
“P” stands for primary waves, which are the fastest and arrive first. They can pass through all materials, including solids, liquids, and gases. The speed of P-waves inside Earth can reach up to 14 km/s and varies depending on the type of rock.
Compressional Wave Vibrations
P-waves cause an alternating compression and expansion of the material, resulting in ground deformation along the direction in which the wave travels. Although P waves are sound waves, most seismic P waves are too low in frequency for humans to hear. P-waves are less destructive due to their smaller amplitudes than the S-waves and surface waves that follow them.
Secondary waves (S)
Shear Body Waves (S-Waves)
The letter “S” refers to the secondary waves, which are the second fastest. These waves are also known as shear waves or transverse waves. S-waves can only travel through solids because fluids, such as liquids and gases, do not have the ability to support shear stresses. The velocity of S-waves within the Earth varies depending on the rock type and can reach up to 8 km/s (kilometers per second).
Shear Wave Vibrations
S-waves cause a shearing motion in the ground, resulting in shearing deformations as they pass through a material. They travel perpendicular to the direction of the wave. These waves are typically the most significant waves near an earthquake and can cause the most destruction.
These waves, known as S-waves, travel slower than P-waves and have a larger amplitude for earthquake sources.