Narrator: : On the surface of planet Earth, those inches add up. This is Science Today. When you look at the globe, you wouldn't think the continents are moving. But they are, constantly, in a process called plate tectonics. Geologist Mark Richards of the University of California, Berkeley says continental movement is slow but inexorable.

Richards: Average plate velocity is about 5 centimeters per year. So five centimeters is about two inches.

Narrator: : You wouldn't think that would add up to much, but over the course of centuries it has big consequences -- on California's San Andreas fault, for example.

Richards: Well, the San Andreas fault is actually moving at like 2 to 3 centimeters per year itself, and over a hundred years that's 2 to 3 meters. So if you have a hundred years between two earthquakes, you have 2 meters, 3 meters of slip all at once, that has consequences -- that's what a big earthquake is.

Narrator: : As recently as twenty years ago, plate movement hadn't yet been accepted as fact by all geologists. But Richards says it's hard to argue with today.

Richards: We have very precise satellite-based instruments called global positioning satellite receivers that can actually measure these on a year to year basis. And it works.

Narrator: : For Science Today, I'm Steve Tokar.