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What are Primary Waves?

Rochelle Enrera, Jeff Fennell
  • Author
    Rochelle Enrera

    Rochelle has a bachelor's degree in Physics for Teachers from Philippine Normal University-Manila and has completed 30+ units in MS Geology at University of the Philippines-Diliman. She is a licensed teacher and has taught Grade 10 Physics for three years. She has been a science content writer and copywriter for over three years now.

  • Instructor
    Jeff Fennell

    Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

Explore P-waves. Learn the definition of a P-wave and understand what it travels through. Discover the velocity of P-waves and find the factors it depends on. Updated: 01/27/2022

What are Primary Waves?

Earthquakes occur when two blocks of earth slip past one other. Earth is composed of tectonic plates that are constantly moving. However, there are instances when their edges get stuck as they move while the other parts of the plate continue to move. The continuous interaction between the plates builds stress over time. When the maximum stress is reached, the blocks suddenly move and slip past one another, which occurs on the faults found in tectonic plate boundaries. This sudden movement releases a huge amount of energy, known as an earthquake. It produces seismic waves that can travel throughout the earth's interior and along the surface, which may result in considerable damage to human lives and properties.


Earthquakes are produced from the sudden release of energy when two blocks of earth slip past one another.

a diagram showing two blocks slipping past one another, producing an earthquake


Seismic waves produced during an earthquake travel outward from the source. They are broadly categorized into body waves and surface waves. Body waves can travel through the earth's interior, while surface waves travel along the surface and cause the most damage. Body waves are divided into two types — P-waves and S-waves. Primary (P) waves travel faster than secondary (S) waves. Except for very large earthquakes, P-waves cause less damage than S-waves and surface waves. In this lesson, learn more about P-waves definition, distinct properties, and what materials P-waves travel through.

What are primary waves? P-waves, also known as primary waves, are one of the types of seismic waves produced during an earthquake. It is the first wave detected by a seismometer during an earthquake, hence, the name "primary wave." P-waves are also called compressional waves or longitudinal waves since the direction of their propagation is parallel to the compressions and dilations of the particles in the material. Notice in the diagram how P-waves push and pull the material as it moves through it, similar to how sound waves are produced in the air. Just like other examples of longitudinal waves, P-waves can travel through liquids, gas, and solids.


P-waves propagate parallel to the motion of the particle in the material.

P-waves propagate parallel to the motion of the particle


Definition of P-Waves

During an earthquake, energy is released as one side of a fault slips past the other. The pressure that had been stored is released and travels radially away from the focus of the earthquake in the form of primary (P) and secondary (S) seismic waves.

P-waves are compressional waves that move in a back and forth motion. They are the fastest traveling seismic waves, and therefore, the first to be felt or recorded during an earthquake. If you have ever experienced an earthquake, you may remember the first jolt, this is the P-wave. When compared to other seismic waves, P-waves are the least destructive of all seismic waves.

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How Fast Do P-Waves Travel?

How fast do P-waves travel? From the definition of the primary waves, they have the highest velocity among all the types of seismic waves. They can reach speeds ranging between 5-8 kilometers per second along the earth's surface. S-waves, on the other hand, are only 60% faster than P-waves, allowing them to move between 3-4.8 kilometers per second along the surface. It is essential to note that these speeds vary when they move through the earth's interior, depending on the material they are moving through. P-waves' speeds and their ability to travel in all types of matter allow them to reach the seismic stations first than other types of seismic waves.

Factors Affecting the Speed of P-Waves

The speed of P-waves depends on three factors — rigidity, compressibility, and density of the medium.

  • Rigidity refers to the resistance of the material to any changes in size and shape due to an external force. The more rigid the material, the faster the P-waves.
  • Compressibility, on the other hand, refers to the ability of the material to make its volume smaller or more compact when an external force is applied. The higher the compressibility of the material, the faster the P-waves.
  • Lastly, density refers to the amount of mass an object contains per unit volume. The higher the destiny of the material, the slower the P-waves.

These characteristics explain why P-waves generally move faster in solids than in fluids (e.g., liquids and gas).

P-waves' speed varies more or less 8 km/s in the crust-mantle boundary. The earth's mantle, specifically below the asthenosphere, becomes more rigid and compressible as depth increases, which means that the speed of P-waves increases with depth in the earth's interior. Note that the density of the material slows down the P-waves, and the mantle's density also increases with depth. However, the effect of rigidity and compressibility of the mantle has a more significant effect on the wave velocity than the density. The speed of P-waves at the core-mantle boundary is about 13.6 km/s. P-waves slow down as they enter the molten outer core, where their speed varies from about 8 km/s to 10.3 km/s just above the inner core boundary. At the solid inner core, the P-wave velocity is 11.3 km/s.

Aside from the component of the material, variations in temperature and pressure also affect the speed of P-waves. An increase in temperature decreases the speed of the seismic waves. In contrast, an increase in pressure results in an increase in speed. The speed of P-waves generally increases with depth since the effect of increasing pressure has a more significant effect on the magnitude of the P-waves' speed, especially if the rock has a uniform composition. This holds true even if the increase in temperature with depth tends to slow down seismic waves.

What Do P-Waves Travel Trough?

What do p waves travel through? P-waves travel through all types of matter, including solids and liquids. This allows them to travel throughout the earth's interior. On the other hand, S-waves only travel through the solids since fluids cannot support shear stress.

The earth's internal structure was determined through seismic waves and their speed variations in the different boundaries. Seismic waves refract or bend when they encounter boundaries due to the changing composition of the earth's layers. If the seismic waves slow down as it enters another rock interface, they will bend downward relative to their original path. If, however, the seismic waves' speed increases with depth, it will continue to be refracted or bent until it reaches the surface again, following a curve path.


Velocity

P-waves are the fastest moving seismic wave. The velocity of the wave depends on the type of density of the material it is traveling through. At the surface, a primary seismic wave travels at about seven kilometers per second - that's nearly 15,000 mph. As the density increases deeper inside the earth, the speed of the wave also increases. At around 3,000 kilometers, the P-wave travels at nearly 14 kilometers per second.

Then something changes. At around 3,000 kilometers, the crust that we are standing on ends, and the liquid outer core begins; although the P-wave can travel through the liquid, it does so much more slowly. At the outer edge of the outer core, the speed of P-waves drops back down to 7 kilometers per second, slowly increasing until it jumps up to 10 kilometers per second through the inner core.

Of the seismic waves, only P-waves have the ability to move through liquid. While this may not seem too important, this property of P-waves has given us a peek into the earth's center. After the 1906 San Francisco earthquake, seismographs, an instrument designed to record seismic waves, were positioned around the globe to better understand seismic waves.

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Video Transcript

Definition of P-Waves

During an earthquake, energy is released as one side of a fault slips past the other. The pressure that had been stored is released and travels radially away from the focus of the earthquake in the form of primary (P) and secondary (S) seismic waves.

P-waves are compressional waves that move in a back and forth motion. They are the fastest traveling seismic waves, and therefore, the first to be felt or recorded during an earthquake. If you have ever experienced an earthquake, you may remember the first jolt, this is the P-wave. When compared to other seismic waves, P-waves are the least destructive of all seismic waves.

Velocity

P-waves are the fastest moving seismic wave. The velocity of the wave depends on the type of density of the material it is traveling through. At the surface, a primary seismic wave travels at about seven kilometers per second - that's nearly 15,000 mph. As the density increases deeper inside the earth, the speed of the wave also increases. At around 3,000 kilometers, the P-wave travels at nearly 14 kilometers per second.

Then something changes. At around 3,000 kilometers, the crust that we are standing on ends, and the liquid outer core begins; although the P-wave can travel through the liquid, it does so much more slowly. At the outer edge of the outer core, the speed of P-waves drops back down to 7 kilometers per second, slowly increasing until it jumps up to 10 kilometers per second through the inner core.

Of the seismic waves, only P-waves have the ability to move through liquid. While this may not seem too important, this property of P-waves has given us a peek into the earth's center. After the 1906 San Francisco earthquake, seismographs, an instrument designed to record seismic waves, were positioned around the globe to better understand seismic waves.

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Frequently Asked Questions

What does P wave mean?

P-wave means "primary" wave. It is a type of seismic wave produced from the sudden release of energy during earthquakes. It is also categorized as a compressional or longitudinal wave.

Why is it called a primary wave?

P-waves are called primary waves because they are the first seismic waves detected by seismic stations. They have the fastest velocities among the types of seismic waves, ranging between 5-8 km/s along the Earth's surface.

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