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Faults are places in the Earth's crust where different tectonic plates meet and grind against each other. The tectonic plates continuously move extremely slowly, but at faults, there is great stress caused by this movement. This is due to the friction between the two tectonic plates, which are usually moving in opposite directions. When this stress is released and the tectonic plates "allow" each other to move a little past each other, the resulting movement is what we like to call and earthquake.
