(Will appear in the October 2008 issue of Community News)
Earthquakes don't happen at the surface of the Earth. They originate inside the Earth, often at very great depths, for instance where two of the immense slabs of rock called tectonic plates suddenly slide against each other, or one slips a bit further under another. The place on the Earth's surface directly above the deep point where an earthquake originates is called its epicenter, from the Greek word epi, which means upon. It's not necessarily the point on the surface where the effects of the earthquake are most strongly felt; it's just the point vertically above the real center of the quake. You'll often hear epicenter misused to mean a more intense sort of center. Years ago, I heard a preacher refer to Boulder as the epicenter of various kinds of behavior he disapproved of. That would have been clever if he'd been implying that the behavior was demonic and its real center was somewhere inside the Earth, but I'm sure he was just misusing the word and trying to impress upon us that Boulder was, like, you know, really, really the center of that bad stuff, man.
I'm collecting all of these at: http://www.dvorkin.com/davidsdefs.html