Petra....I think it would be hard to call whether some signal means a quake is imminent....but continuous GPS and other measures might, for example, show a major creep even somewhere down the subduction zone that might spur local officials to make sure that they have warning systems and education in place for at least a tsunami...even if they can't tell the timing well enough to get people out of their buildings. But, the real payoff might be for future great subduction quakes...for example, in the Pacific northwest...
Chris