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Rocks on both sides of an existing fault are deformed by tectonic forces. Frictional resistance holding the rocks together is overcome. Slippage at the weakest point (the focus) occurs. Vibrations (earthquakes) occur at the deformed rock springs back to its original shape. Earthquakes most often occur along existing faults whenever the frictional forces on the (elastic rebound) fault surfaces are overcome. The longer it has been before a rock snaps back, the more powerful the earthquake. The spot where the first energy release occurs is called the focus. Energy released radiates in all directions from its source. Two types based on mode of travel. They propangate using shear strength (only through solids) Sensitive instruments around the world record the event. Seismographs are instruments that measure and record the vibration of seismic waves. Usually at least 3 seismographs - one records n-s motions, one records e-w motions and one records vertical motions.

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