Geography Reference
In-Depth Information
During an earthquake, gazillions of tons of crust are moved. The amount of pressure required to do
that is incredible; and it does not accumulate in a day. Therefore, at a given location, on an active
plate boundary, earthquakes are not everyday ongoing events. Instead, it takes long periods of time
— years and decades — for tectonic force to build up enough pressure to move a mass of crust.
The mechanics are crudely similar to inflating a balloon until it bursts. As the volume of air increases
within, so does the pressure and tension along the surface of the balloon. Eventually, the pressure ex-
ceeds the balloon's capacity to contain it, and the balloon gives way . . . pop! Obviously, crust does
not inflate and pop, but pressure does build-up slowly, especially along plate boundaries. Tension in-
creases and keeps increasing over the years. And finally, perhaps after decades of pressure building,
the crust just can't take it any more. And so it suddenly gives. That is, it suddenly moves, releasing
the built-up pressure.
Because tectonic force exists everywhere, an earthquake can happen anywhere. But given what you
have read about plate tectonics, it should come as no surprise to see that the geography of earthquakes
(see Figure 6-6) largely coincides with the geography of plate boundaries (see Figure 6-2). Zones of
spreading sea floors are prime candidate locales. So, too, are areas where plates collide. And so, too,
is another possibility that has yet to be mentioned.
Figure 6-6: The
bolded lines
show the geo-
graphy of earth-
quakes.
Splitsville in California
Sometimes neighboring plates do not diverge or collide, but rather slide by each other. The linear
break in the rocks that marks the occurrence of this kind of movement is called a transform fault (see
Figure 6-5). California's San Andreas Fault, no doubt the most famous fault line in the United States,
is an example of a transform fault and is shown in Figure 6-7. The land on the western side of the
fault is part of the Pacific Plate and is slowly moving to the northwest. Meanwhile, the land on the
eastern side, which is part of the North American Plate, is slowly moving towards the southeast.
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