The earthquake happened on the San Andreas Fault, which is the major fracture of the Earth's crust. It is trending northwestward through southern and northern California, U.S., for 650 miles (1,050 km) and passing seaward in the vicinity of San Francisco. Movement along this transform fault is of the strike-slip type and is characterized by occasional large earthquakes originating near the surface along the path of the fault. The disastrous San Francisco quake of 1906 and the less serious earthquake of 1989 were both caused by movement along the fault. According to the theory of plate tectonics, the San Andreas results from the abutment of two major plates of the Earth's crust, the Northern Pacific and the North American. Along the fault, the Northern Pacific plate is sliding past the North American plate in a northerly direction, at a relative movement of about 1 cm (0.4 inch) per year over geologic time, though the rate of movement has been 4 to 6 cm (1

At almost precisely 5:12 a.m., local time, a foreshock occurred with sufficient force to be felt widely throughout the San Francisco Bay area. The great earthquake broke loose some 20 to 25 seconds later, with an epicenter near San Francisco. Violent shocks punctuated the strong shaking which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada. The highest Modified Mercalli Intensities (MMI's) of VII to IX paralleled the length of the rupture, extending as far as 80 kilometers inland from the fault trace. The earthquake was rupturing the northernmost 430 kilometers of the San Andreas Fault from northwest of San Juan Bautista to the triple junction at Cape Mendocino. It formed a 290 miles long crack on the ground. For comparison, the 1989 earthquake only had a rupture length of about 25 miles. There is also about 20 feet offset along the crack. The ground was moving at 2.7 km/s.

By studying quakes like the Great 1906 San Francisco Earthquake, scientists and engineers gain the knowledge and understanding necessary to assess the risk from future shocks and to reduce the vulnerability of buildings and other structures to damage in these inevitable and terrifying events. In this way they help to protect the lives and property of

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