I presume those who are interested saw the Horizon special last night, or the one last week on C4? It said it all there really. Earthquakes often come in groups because as MrM says, they are a sudden release of long built up forces. And once one goes in a big way, it releases the pressure on that region, but passes it on to another like tearing a piece of paper. Then of course there are the doomsday people who now predict a mass outbreak of quakes around the pacific rim. To a degree I suppose that could well happen due to the immense forces released in this massive magnitude 9 quake. But the biggest fear is the big one underneath Tokyo. According to the program last night, the likelihood has increased as the pressure has shifted. But in the end, who knows. All the scientists can do despite endless research and effort, is to predict the areas at risk. Not how big or when or which of those areas it will be.