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P-Waves: Definition & Velocity

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Instructor: Jeff Fennell

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

Primary (P) waves are compressional seismic waves that occur during an earthquake. This lesson will cover the properties of these waves and how they have helped us understand the inner workings of the earth.

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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  • 0:01 Definition of P-Waves
  • 0:41 Velocity
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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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