Earthquake, aftershock. What's the difference?

Matthew:

I'm confused.What's the difference between an earthquake and an aftershock? Sometimes we have an earthquake only to be told it's an aftershock of an earthquake two years ago. How can they tell?

-- Shaken, the net

Hey, you forgot the foreshocks. Don't leave them out. More confusion! For us civilians with crockery falling on our heads, the difference is meaningless. But this is the kind of thing that keeps seismologists off the street and out of trouble. So here goes. Say we have a 4.5 (Richter) quake at a particular point on a particular fault. Earthquake they call it. Time goes by, la-di-dah. Nothing. Nothing. Then there's a 5.4 quake at the same point on the same fault. Now our old 4.5 is reclassified a foreshock to the 5.4 earthquake because it's smaller than the most recent one. Time goes by, la-di-dah. Then there's a 3.0, same place, same fault. That is an aftershock to the 5.4 biggie because it's smaller than the one preceding it. So classifying earthquakes depends on where it happens, how big it is, and how it fits with the history of quakes from that location. Sometimes that involves relabeling an event. Got it? Hope so.