But the lack of observations means at most little is happening in the minutes to years prior to big quakes.

This week in Nature, there was an interesting summary of a potential decade-scale precursor:

Earthquakes: A movement in four parts?

Joanne Bourgeois

Abstract
From time to time, over millennia, northwestern North America has experienced huge earthquakes. These events may be preceded by tell-tale subsidence, but the evidence is devilishly difficult to decipher.

Over the past year, two groups of micropalaeontologists1, 2 have presented evidence that coastal land subsides not only during a great earthquake but also in the decade or so before such events. Their study areas are the Alaska and Cascadia subduction zones in northwestern North America. A subduction zone is where one tectonic plate dives beneath another, and in Alaska and Cascadia tectonic shifts have given rise to earthquakes of moment magnitude 9 or more (for comparison, the Sumatra–Andaman earthquake that led to the tsunami of 26 December 2004 was moment magnitude 9.2). The observations and interpretations of pre-earthquake subsidence are complex and subtle. But if this subsidence does indeed occur, the implication is that warning signs are detectable for some time before one of these huge earthquakes occurs.