Plate tectonics became widely accepted in the 1960s after geologists worked out many of the details of plate motion. The major plates of the earth's crust are always moving, and the direction is indicated by arrows. Molten rock below them acts much like a conveyor belt. Ridges (notice, fore example, the Mid-Atlantic Ridge) are areas in which this molten rock wells up to the surface, creating new crust and pushing outward, away from the ridge. At the other end of the plate, crust is often destroyed by grinding into or under another plate. Or it may override the other plate. Plate boundaries tend, therefore to be unstable. Most, but not all, major earthquakes occur along plate boundaries. Sometimes, however, the stresses of plate motion can cause faults in the interior of a plate to rupture.

Speaking of faults (Japanese: dansō 断層), they are fractures or fracture zones in rocks. These discontinuities might be exist far below the surface, or, as in the two photos here, they can extend all the way to the earth's surface. The fault in the photo to the right is the famous San Andreas Fault in California.

Stress causes faults in the first place, and many faults remain under stress as the continental plates move and grind their way across the globe (at a speed of, say, several centimeters a year). When this stress builds up to the point where it is stronger than the friction and other forces holding things in place, the fault ruptures. The rupture throws off several types of seismic waves that move through the earth's crust (and everywhere else) at varying speeds, eventually reaching the surface. When the waves approach the surface, they cause ground motion. It is the ground motion that causes all the problems for people and their centers of civilization.

Ground motion can vary widely in response to many variables. The magnitude of an earthquake (i.e., the amount of energy given off by the ruptured fault) is not necessarily a good predictor of the extent of ground motion, nor is proximity to the epicenter (the point on the surface above the place where the fault starts its rupture). Of course, these variables are important elements, but the type of soil base, for example, also makes a big difference. The nature of the soil base was the most important factor in determining the extent of damage in the 1855 Ansei Edo Earthquake.

(All images here are from Bruce Bolt, Earthquakes [New York: Freeeman and Company, 1997].)

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