Seismic Waves
The second type of deformation, dynamic motions, are essentially sound waves radiated from the earthquake as it ruptures.

The mechanical properties of the rocks that seismic waves travel through quickly organize the waves into two types. P-waves or primary waves (also known as compressional waves), travel fastest, at speeds between 1.5 and 8 kilometers per second in the Earth's crust. S-waves or secondary waves (also known as shear waves) travel slower, usually at 60% to 70% of the speed of P waves.

P waves shake the ground in the direction they are moving, while S waves shake perpendicularly or transverse to the direction of their movement.


Although wave speeds vary by a factor of ten or more in the Earth, the ratio between the average speeds of a P wave and of its following S wave is quite constant. This fact enables seismologists to simply time the delay between the arrival of the P wave and the arrival of the S wave to get a quick and reasonably accurate estimate of the distance of the earthquake from the observation station. (Just multiply the S-minus-P (S-P) time, in seconds, by the factor 8 km/s to get the approximate distance in kilometers.) That is the equation that seismologist use to find the time intervals between the earthquakes.

The dynamic, transient seismic waves from any substantial earthquake will move all around and entirely through the Earth. Given a sensitive enough detector, it is possible to record the seismic waves from even minor events occurring anywhere in the world at any other location on the globe. The force of the 1906 earthquake was measured from a remote spot 9100 miles away (from another country).

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