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pulled and pushed by forces deep within the planet. The Pacific plate is sliding northwest past North America at an average of about two inches a year, roughly the same rate at which fingernails grow. But movement along the fault usually occurs in bursts along most of the fault. The colder, more rigid rocks near the Earth's surface resist the plate motions. Eventually, enough strain develops along a segment off the fault to overcome the resistance. Then, in geological terms, that stretch of the fog breaks fails or ruptures, and a segment of the crust riding the Pacific plate surges north, creating an earthquake.