Earthquakes – National Geographic

Pulled and pushed by forces deep within the planet, the Pacific plate is sliding northwest past North America at an average of about 2 inches a year – roughly the same rate as 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 of the fault to overcome the resistance. Then, in geologic terms, that stretch of the fault “breaks,” “fails,” or “ruptures” and segments of the crust riding the Pacific plate surge north, creating an earthquake. In the magnitude 7.7 San Francisco earthquake of 1906, which killed more than 3,000 people, a 270-mile-long segment of the San Andreas from south of San Juan Bautista to Cape Mendocino surged northward as much as 21 feet in a few seconds. Half a century earlier in 1857, during a similar but little known 7.8 quake, much of coastal , southern California drifted north.