One type of quake we have here, and it is hard to detect, is a really huge release of energy over a very long time. Months, I think. Apparently, some of our local faults will have a month or so of movement that releases a lot of pent-up energy but doesn't really shake anything. As I recall, geologists were not sure if these actually relieved stress on the subduction zone (lessening the chance of the "big one") or were just another quake in nature's toolbox.