Most of us have grown used to thinking that earthquakes have an epicentre at some fixed point beneath the surface. That is not at all true, as the event that set the Boxing Day 2004 tsunamis in motion as been shown to have been a lengthy rip that propagated from Sumatra NNE to the Nicobar Islands, over a period of about an hour.  Even quite small earthquakes are distributed and often migrate along a fault line.  Christine arson of the University of Colorado has captured what is effectively a movie of a magnitude 8.3 event off the island of Hokkaido, Japan, which can be viewed at spot.colorado.edu/~kristine/tokachi_rupture.gif. The data that she used comes from a network of  a thousand highly sensitive GPS receivers set up throughout Japan.  Instead of acceleration, measured by conventional seismometers, GPS records actual position in x, y, z coordinates.  That enable the actual motions to be imaged as in  the movie.