Nature News has a good article by Naomi Lubick on recent efforts to set up seismic arrays in Japan and northwestern U.S along the Cascadia subduction zone around the Olympic Peninsula Seattle area to monitor swarms of tiny earthquakes or tremors.
So far these swarms have not been connected to larger earthquakes and cannot be called precursors.. but that's what seismologists are hoping.. that ultimately we may be able to use the pattern of tremors to understand changes in stresses along major faults, giving us a better handle on the timing and location of bigger quakes.
The swarms in Japan seem to be spatially restricted along bands and one speculation is that the subduction of chains of seamounts may be triggering these swarms.
The earthquake swarms along the Cascadia subduction zone in the Olympic
Penninsula seem to follow a time line..appearing every 12-14 months.
This regularity is an angle I am sure astrologers are going to be having a field day with ..keep a lookout for claims that these tremor swarms follow planetary cycles (possible for tiny disturbances?) and that you can "predict" big earthquakes by the pattern of these tiny ones.