The Physics
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Speed of Sound through the Earth

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Spaulding, Namowitz, Heath. Earth Science. Heath, 1994. [Figure 15.9] 8–13 km/s
"Earthquakes." Microsoft Encarta. CD ROM. 1996. "P waves are the fastest seismic waves; they travel in strong rock at about 6 to 7 km (about 4 mi) per second." 6–7 km/s
What Happens When an Earthquake Occurs? Recent Earthquakes in California and Nevada. USGS. "The fastest waves, the P-wave, travels outward at a speed of about 3 to 5 miles/second." 5–7 km/s
Magill's Survey of Science. Earth Science Series, Vol. I. Magill-Salem, 1984. "P-wave: a type of seismic wave generated at the focus of an earthquake traveling 6 -8 km/s, with a push and pull vibratory motion parallel to the direction of propagation; ''P"stands for primary, as P waves are the first and fastest to arrive at a seismic station." 6–8 km/s
Parent's Guide - P. Earthquake ABC. USGS. "Local rock type and the depth of the earthquake cause slight variations, but the number of seconds between the P and S wave times 5 is approximately the distance in miles to the earthquake. (Remember that some of that distance may be down into the Earth.)" 8 km/s

An earthquake is the sudden movement of rocks along a fault, which causes vibrations. These vibrations transmit energy through the earth in the form of waves. Earthquakes produce three different types of waves.

P waves or body waves are the first waves produced by an earthquake. The "P"stands for "primary" or "pressure". These waves are the smallest yet fastest waves generated by the earthquake.

The waves begin at the rupturing fault, the beginning point of the earthquake. P waves are compression waves. They move just like sound waves move through air. P waves are the only waves that are able to move through fluids and rock. This ability allows the waves to travel through the earth's core and outer crust.

P waves are the first waves to arrive at a seismic station. A P wave generated at the focus of an earthquake can travel at a speed of between 6 to 8 kilometers per second. They have a push-pull vibratory motion that is parallel to the direction of "propagation"making them longitudinal waves.

By knowing the speed at which P waves travel through the earth and by the precise times that they arrive at several seismic stations, the distances and directions of an earthquake can be calculated. The velocities of the P waves in the upper crust of the earth are about 7 kilometers per second.

Many animals can hear and feel P waves but humans can not. This is why it is thought that animals can sense when an earthquake is coming. The animals can feel the beginning of the earthquake that the less sensitive humans do not feel.

Pamela Spiegel -- 2001