The Difference Between P waves and S waves

Tiffany Leonard, Jeff Fennell
  • Author
    Tiffany Leonard

    Tiffany has worked on science curriculum and lesson writing since 2015. She has her Master's in Geology from the University of Illinois and a Bachelor's in Geology and Physics from Carleton College. She taught geology courses while she was getting her MS and was a TA while at Carleton.

  • Instructor
    Jeff Fennell

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

Discover P and S waves. Learn the P and S wave definitions, also known as primary and secondary waves, and learn the difference between P waves and S waves. Updated: 09/23/2021

What are P and S Waves?

Primary (P) and secondary (S) waves are two types of waves caused by earthquakes. They are defined based on when they arrive and are felt on the surface. P waves, or primary waves, arrive first while S waves, or secondary waves, arrive second. Both waves cause the ground to shake when an earthquake occurs. S waves cause more intense shaking than P waves and are typically much more destructive.

Secondary Waves Defined

Secondary waves, or S-waves, are seismic waves produced by an earthquake. 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.

S-waves are lateral waves that move side to side as a sine wave perpendicular to the direction of the wave. They are the second seismic wave to be felt or recorded during an earthquake, after the conveniently named primary wave. If you have ever experienced an earthquake, the intensity of the earthquake grows after the first initial jolt. When compared to other seismic waves, S-waves are the most destructive of all seismic waves.

S-waves are more destructive than P-waves. The lateral movement of the wave produces a rolling effect along the surface that can cause damage to all types of structures.

During the 1989 Loma Prieta earthquake in San Francisco, S-wave were especially destructive. Parts of San Francisco were built on wet, sandy soils. When the S-wave passed through these soils, a process called liquefaction occurred that shook the soils so much it essentially acted as a liquid.

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Types of Waves: Mechanical Body Waves:

P and S waves are mechanical body waves.

A mechanical wave is a wave that must travel through a medium, such as the ground or air. The energy of a mechanical wave is transferred from one molecule to another as it propagates (or travels) through its medium. They differ from electromagnetic waves, which can travel through a vacuum.

Types of mechanical Waves

  • Sound Waves
  • Water Waves
  • Waves on a Spring
  • Seismic (Earthquake Waves)

A body wave is a type of seismic wave that moves through the Earth's interior. They differ from surface waves, which travel along the Earth's surface.

Characteristics of Body Waves

  • Move through deep layers of rock
  • Have more energy than surface waves
  • Arrive at seismometer location first

Body waves are the first waves to be felt on the surface after an earthquake. Since they can move through deep layers of rock, they follow a shorter ray path than a surface wave. A ray path is the direction the wave follows as it propagates away from the locus (or origin) through the Earth.

Cause and Origin of P and S Waves:

P and S waves are caused by the initial rupture of an earthquake. The rupture occurs when two sides of a fault slip past one another and energy is released. The stored pressure then travels away from the hypocenter radially in the form of P and S waves. Thus, P and S waves both originate or begin at the earthquake loci , where ruptures first occurs at depth. The locus is different than the epicenter, which is the surface location of the earthquake.

Primary Waves: What are P Waves?

P waves are called primary waves because they are the first waves to arrive at any location after an earthquake.

Characteristics of P waves (also known as Primary Waves)

  • Compressional or lateral waves
  • Arrive at seismometer first
  • Cause upward or downward motion when they reach the surface
  • Least destructive of all seismic waves

P Wave Motion

P waves are compressional. They vibrate parallel to the direction the wave is moving. The first upward or downward jolt felt is the P wave arrival.

These can be created at home by using a spring or slinky with fixed ends. If one person gathers a few coils and compresses them at one end before releasing them, then a compressional wave will travel from one end of the spring to the other.

P waves are compressional and vibrate parallel to the direction of propagation

P Wave particle motion

P Wave Speed

P waves are the fastest moving waves. Their velocity is generally given by:

{eq}V_P=\sqrt{\frac{k+4/3μ}{ρ}} {/eq}

Where k is the bulk modulus (compressibility), μ is the shear modulus (rigidity), and ρ is the density of the rock or medium the wave is being transmitted through. Compressibility is the ability of a medium to reduce in volume under pressure, and rigidity is the ability of a medium to be bent or shaped.

P waves travel at approximately 7 km/s at the surface of the Earth.

Where do P waves move? Medium and Depth:

P waves move faster in less dense rocks and slower in denser ones.

Which Media can P-waves propagate through? How fast do they move?

Ability to propagate Velocity Range (km/s)
Rock yes 4.5 (unconsolidated sedimentary rock) to 14 (deep in the mantle)
Gas yes 0.331 (P waves move through the air as sound waves)
Liquid yes 1 (through water) to 10 (within the outer core)

P waves also increase velocity with depth until they reach the core-mantle boundary, where the wave slows down to about 7 km/s before increasing again. This is due to the fact that the outer core is a liquid and the shear modulus (or rigidity) is zero. The P- wave increases in speed to about 11 km/s when it passes into the solid inner core.

Due to the difference in speed through a liquid, P waves are refracted within the outer core and leave something known as a Shadow Zone on the Earth's surface. The P wave shadow zone is the area on the surface where no P waves are felt or measured after the occurrence of an earthquake. This zone is located at angular distances of 104 to 140 degrees from the earthquake location.

P waves leave a shadow zone on the Earth located 104 to 140 degrees away from the epicenter. The existence of the shadow zone helped scientists discover that the outer core was liquid.

P Wave Shadow Zone

Secondary Waves: What are S waves?

S Waves are called Secondary waves because they arrive second after the P wave.

Characteristics of S Waves (also known as Secondary Waves)

  • Shear or waves
  • Sine-shaped
  • Arrive at seismometer second to the primary P wave
  • The most destructive of all seismic waves.

S waves can also sometimes cause liquefaction to occur. Liquefaction is a phenomenon that happens when loosely packed sediment loses its strength due to strong ground shaking at or near the surface.

Earthquakes with Liquefaction include:

  • 1964 Niigata Earthquake in Japan
  • 1989 Loma Prieta Earthquake in California

S Wave Movement

S waves are called transverse waves because they vibrate perpendicular to the direction the wave travels.

Velocity

S-waves travel slower than the primary waves. We can use the time difference between the arrival of the P-wave and S-wave to determine how far the epicenter is located from the location recording.

The farther away a location is from the epicenter of an earthquake, the greater the arrival time difference is between the of the P & S waves.

  • One location can determine its distance to an epicenter (A)
  • Two locations can narrow the epicenter down to two places (A & B)
  • Three locations can pinpoint the epicenter of an earthquake (A, B, & C)

epicenter

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Video Transcript

Secondary Waves Defined

Secondary waves, or S-waves, are seismic waves produced by an earthquake. 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.

S-waves are lateral waves that move side to side as a sine wave perpendicular to the direction of the wave. They are the second seismic wave to be felt or recorded during an earthquake, after the conveniently named primary wave. If you have ever experienced an earthquake, the intensity of the earthquake grows after the first initial jolt. When compared to other seismic waves, S-waves are the most destructive of all seismic waves.

S-waves are more destructive than P-waves. The lateral movement of the wave produces a rolling effect along the surface that can cause damage to all types of structures.

During the 1989 Loma Prieta earthquake in San Francisco, S-wave were especially destructive. Parts of San Francisco were built on wet, sandy soils. When the S-wave passed through these soils, a process called liquefaction occurred that shook the soils so much it essentially acted as a liquid.

Velocity

S-waves travel slower than the primary waves. We can use the time difference between the arrival of the P-wave and S-wave to determine how far the epicenter is located from the location recording.

The farther away a location is from the epicenter of an earthquake, the greater the arrival time difference is between the of the P & S waves.

  • One location can determine its distance to an epicenter (A)
  • Two locations can narrow the epicenter down to two places (A & B)
  • Three locations can pinpoint the epicenter of an earthquake (A, B, & C)

epicenter

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Frequently Asked Questions

What is the meaning of S waves?

An S wave is a shear wave that cause particles to vibrate perpendicular to the direction the wave is propagating. It is sinusoidal

Do S waves travel through liquid?

S waves cannot travel through liquid. Since the rigidity of a liquid is zero, the velocity of an S wave is also zero. S waves can only travel through solid media.

What is the difference between P waves and S waves?

P waves are the first waves to arrive after an earthquake. They move faster than S waves and are compressional waves. They move in a back and forth motion in the direction the wave propagates. S waves arrive second and are transverse or shear waves. They move side to side perpendicular to the direction the wave travels.

How do P waves and S waves travel through the Earth?

P and S waves are mechanical waves, which means they travel through the earth by moving energy through the rock from particle to particle. They require a medium to travel. P waves can also move through the liquid outer core, but S waves cannot.

Are P waves and S waves transverse?

P waves are not transverse. They are lateral (or longitudinal). They vibrate parallel to the direction they propagate. S waves are transverse. They propagate perpendicular to the direction in which they travel.

What are examples of S waves?

S waves are transverse waves. If you take a slinky with one or both ends fixed and move it side to side, the waves that propagate along it are similar to S waves.

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