Question: Do you think that all the little earthquakes that Japan always have really been a lead up to the massive earthquake/ tsunami that has just happened?

David Pyle answered on 25 Mar 2011:

Good question! Yes, and no. The movement of the plates is a continuous process, but as the plates slide past each other, they tend to get stuck (because of friction), and so they only move in little jumps, each of which causes an earthquake. So all the little earthquakes would have been happening along the plate boundary which was stuck. The problem in many places is the small earthquakes don’t allow the plates to move as fast as they need to, so that stresses continue to grow. Eventually, it will take a very large earthquake to release all of that stress in a single event. So the small earthquakes aren’t really ‘leading up’ to the main event – they just happen all of the time; but if there had been more of them, perhaps the stresses required for the large earthquakes wouldn’t have grown so large.