What are Primary Waves?
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.
 |
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.
 |
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.
You must cCreate an account to continue watching
Register to view this lesson
Are you a student or a teacher?
Create Your Account To Continue Watching
As a member, you'll also get unlimited access to over 84,000 lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed.
Get unlimited access to over 84,000 lessons.
Try it now
It only takes a few minutes to setup and you can cancel any time.
Already registered? Log in here for access
BackResources created by teachers for teachers
Over 30,000 video lessons
& teaching resources‐all
in one place.
I would definitely recommend Study.com to my colleagues. It’s like a teacher waved a magic wand and did the work for me. I feel like it’s a lifeline.
Jennifer B.
Teacher
Back
Your next lesson will play in
10 seconds
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.
 |
The ability of seismic waves to be refracted enables scientists to study the earth's internal structure. One of the earth's boundaries that were discovered through the use of seismic waves is the mantle-core boundary, occurring at a depth of 2900 km. A specific zone, known as the P-wave shadow zone, opposite the source of seismic waves, did not receive any direct P-waves. Scientists concluded that the presence of the shadow zone occurred because the speed of P-waves decreases as they encounter the core-mantle boundary. The sudden decrease in velocity resulted in the inward refraction of P-waves until they passed through the boundary.
The P-wave shadow zone, in general, is the angular distance between 104{eq}^{\circ} {/eq} to 140{eq}^{\circ} {/eq} from the source of an earthquake that did not receive P-waves. Scientists interpreted this as a result of the varying composition of the mantle from the core. The observation was also used to deduce that the outer core is molten since S-waves were not able to pass through. In addition, the discovery also helped determine the diameter of the core.
Properties of P-Waves
P-waves have the following distinct properties compared to other seismic waves:
- P-waves or primary waves are compressional waves. The direction of its propagation is parallel to the motion of the particles within the material.
- As a compressional wave, it can travel through solids, liquids, and gas.
- It is also called primary waves because it has the fastest velocity among the other seismic waves generated during an earthquake. P-waves are the first ones that are detected by seismic stations.
- P-waves can travel along the Earth's surface at speeds ranging from 5-8 kilometers per second. This range increases as P-waves move deeper into the Earth's interior.
- Unlike other types of seismic waves, P-waves do not cause much damage due to their limited push-pull motion.
In addition to all these properties, P-waves have a wide range of frequencies, ranging from the audible range (>20 Hz) to very small frequencies. The typical frequencies of P-waves produced from an earthquake are between 1 to 0.1 Hz. The difference in the arrival times P-waves and S-waves is used to determine the earthquake's origin (hypocenter).
Lesson Summary
Primary (P) waves are types of seismic waves produced from the sudden release of energy during an earthquake. It is a compressional or longitudinal wave since its direction of propagation is parallel to the push-pull of the particles in the material. Similar to other examples of longitudinal waves, P-waves can also travel in liquids, gases, and solids. It also has the fastest speed among the seismic waves, allowing it to reach the seismic stations first in the occurrence of an earthquake. Except for very large earthquakes, P-waves do not cause that much damage, unlike the other types of seismic waves (e.g., secondary (S) waves and surface waves).
The speed of P-waves varies depending on the composition of the material, such as its rigidity, compressibility, and density. For example, P-waves move faster in solids than in liquids and gas. In addition, its speed is also affected by variations in temperature and pressure. P-waves move at speeds between 5-8 kilometers per second along the surface. Its speed generally increases with depth, except for it slowing down when it enters the fluid outer core. As the seismic waves slow down, it bends or refracts, allowing them to be used to determine the planet's internal structure. One of which is the discovery of the mantle-core boundary due to the P-waves shadow zone. A zone at an angular distance of 104{eq}^{\circ} {/eq} to 140{eq}^{\circ} {/eq} from the source of the earthquake that did not receive direct P-waves. It occurred due to the refraction of P-waves as they traveled from the solid mantle to the molten inner core.
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.
Subsequent earthquakes occurred, and the seismographs worked as they were supposed to, recording the P-waves. To the surprise of scientists however, there were two distinct areas that wouldn't record P-waves. From the epicenter, P-waves were recorded on seismographs, until 110 degrees away from the epicenter and remain absent until 150 degrees, where they would reappear. Seismologists soon realized the seismic shadow zone was due to the P-waves refracting while traveling through the different layers of the earth.
Here is an example to help. When you put your hand behind a glass of water and it distorts or looks like it's farther away, that is the light bouncing off your hand and refracting (or slowing down) through the water and then refracting (or speeding up) again once out of the water. The same thing is happening with the P-waves through the different layers of the earth.
So, it turns out as the P-waves travel through the earth, they get to the outer core and refract back outwards toward the surface. This causes some areas of the earth, specifically 110 - 150 degrees away from the epicenter, to not experience any seismic activity, otherwise known as a seismic shadow zone.
Lesson Summary
P-waves, or primary waves, get their name from being the first wave felt or recorded after an earthquake occurs. While they travel faster than other seismic waves, they are the least destructive of all seismic waves. P-waves are unique in their ability to travel through water; this ability has allowed us to study the interior composition of the earth. Refraction of P-waves through the inner and outer core cause a seismic shadow zone at certain areas around the globe.
To unlock this lesson you must be a Study.com Member.
Create your account
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.
Subsequent earthquakes occurred, and the seismographs worked as they were supposed to, recording the P-waves. To the surprise of scientists however, there were two distinct areas that wouldn't record P-waves. From the epicenter, P-waves were recorded on seismographs, until 110 degrees away from the epicenter and remain absent until 150 degrees, where they would reappear. Seismologists soon realized the seismic shadow zone was due to the P-waves refracting while traveling through the different layers of the earth.
Here is an example to help. When you put your hand behind a glass of water and it distorts or looks like it's farther away, that is the light bouncing off your hand and refracting (or slowing down) through the water and then refracting (or speeding up) again once out of the water. The same thing is happening with the P-waves through the different layers of the earth.
So, it turns out as the P-waves travel through the earth, they get to the outer core and refract back outwards toward the surface. This causes some areas of the earth, specifically 110 - 150 degrees away from the epicenter, to not experience any seismic activity, otherwise known as a seismic shadow zone.
Lesson Summary
P-waves, or primary waves, get their name from being the first wave felt or recorded after an earthquake occurs. While they travel faster than other seismic waves, they are the least destructive of all seismic waves. P-waves are unique in their ability to travel through water; this ability has allowed us to study the interior composition of the earth. Refraction of P-waves through the inner and outer core cause a seismic shadow zone at certain areas around the globe.
To unlock this lesson you must be a Study.com Member.
Create your account
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.
Register to view this lesson
Are you a student or a teacher?
Unlock Your Education
See for yourself why 30 million people use Study.com
Become a Study.com member and start learning now.
Become a MemberAlready a member? Log In
BackResources created by teachers for teachers
Over 30,000 video lessons
& teaching resources‐all
in one place.
I would definitely recommend Study.com to my colleagues. It’s like a teacher waved a magic wand and did the work for me. I feel like it’s a lifeline.
Jennifer B.
Teacher
Back
P-Waves Overview & Velocity | What are Primary Waves?
Related Study Materials
Explore our library of over 84,000 lessons
Browse by subject