Why can't we predict the next big quake?

Why can't seismologists predict earthquakes? It's a question and a frustration that understandably springs to mind in the aftermath of a tragedy like the one that continues to unfold in Japan.

Source: http://www.cnn.com/2011/OPINION/03/21/hough.predict.earthquakes/index.html

navy times newspaper contra costa times newspaper marine times newspaper times newspaper online old times newspaper