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Strain accumulates in the earth along faults from the movement of tectonic plates.
When the rocks break the strain accumulated is suddenly released. The energy goes into heat and making seismic waves. Hundreds of small earthquakes happen every day, most of which go unfelt. A magnitude 8 earthquake usually occurs somewhere on the earth once every year. The largest earthquake in Nevada was the 1915 Pleasant Valley earthquake, which had a magnitude of 7.3.