Narrator: This is Science Today. Geologists at the University of California, Riverside, are puzzling over an extraordinary event that happened in an underwater trench below the Fiji Islands: a massive, deep earthquake triggered a bigger earthquake seven minutes later in a place where earthquakes have never been reported before. Geology professor Harry Green says so far they can rule out the second quake being an aftershock.

Green: Aftershocks are very clearly defined as being close to the triggering earthquake-an earthquake goes off and of course the surface that moves is not infinite-it stops somewhere. And where it stops, it leaves strong stress concentrations around the borders, those areas break and produce earthquakes of their own. So aftershocks are always smaller than the triggering earthquake-usually at least one magnitude smaller-and they're close by the original earthquake, whereas the situation I'm talking about is happening a long ways away.

Narrator: For Science Today, I'm Larissa Branin.