Juan de Fuca Diagram (illustration)

Juan de Fuca Diagram. Part of the Century of Earthquakes poster. EARTHQUAKES, FAULTS, AND PLATES

As part of the investigation, H.F. Reid proposed the elastic rebound theory of earthquakes, where materials at distance on opposite sides of the fault move smoothly relative to each other, but friction on the fault “locks” each side and prevents it from slipping. Eventually the accumulated forces (stress) are more than the fault can endure, and the fault slips in an earthquake.

In the 1960s, geologists discovered that the Earth’s outer shell was made up of about 15 100-km thick tectonic plates that were smoothly moving by each other at speeds of a few inches (a few cm) per year (about the speed that fingernails grow). Earthquakes primarily occur at the boundaries where the plates converge, diverge, or slide past each other. At spreading centers both plates move away from the boundary, whereas at subduction zones the underthrusting subducted plate moves toward the boundary. At the third boundary type, transform faults (such as the San Andreas), relative plate motion is parallel to the boundary.

The San Francisco earthquake occurred on the San Andreas fault in northern California, part of the boundary along which the Pacific plate moves northward relative to the North American plate. Studies using the Global Positioning System satellites show that the two plates move by each other at a speed of about 4.5 cm/yr. Most parts of the San Andreas fault are “locked” most of the time, but slip several meters in a large earthquake every few hundred years. A simple calculation (earthquake slip/slip per year) suggests that such an earthquake with a 4-m slip should occur on average about 90 years. The real interval is not uniform, for reasons that are unclear, and longer, because some of the motion occurs on other faults.

Because plate boundaries extend for a total of more than 150,000 km, and some earthquakes occur in plate interiors, earthquakes occur frequently on Earth. An earthquake of magnitude 7 or greater occurs roughly every month, and an earthquake of magnitude 6 or greater occurs on average every three days. Earthquakes of a given magnitude occur about ten times less frequently than earthquakes one magnitude smaller.

Earthquakes are studied using global seismometer networks that produce data about their locations, times, sizes and the nature of faulting. Because earthquakes generally result from the motions of the plates, knowledge of the direction and amount of motion is important for understanding plate motions and the forces that cause them. Such studies are key to assessing the societal hazards posed by earthquakes.


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