Building codes and safety videos provide only so much protection.

So governments in some earthquake-prone countries, including Japan and Mexico, institute early warning systems to alert the public to expect potentially hazardous shaking. (Read up on some earthquake safety tips.)

This week, California followed suit, adopting a new law that requires the Office of Emergency Services to institute a public early warning system for earthquakes.

But how can you issue a warning for a phenomenon we can't even predict? (Related: "Scientists Seek Foolproof Signal to Predict Earthquakes.")

We spoke to seismologist Richard Allen, director of the Seismological Laboratory at the University of California, Berkeley, to understand how an earthquake early warning system works and what one might look like for