These so-called "slow slip events" are common at subduction zone faults -- where an oceanic plate meets a continental plate and dives beneath it. They also occur on continents along strike-slip faults like California's San Andreas, where two plates move horizontally in opposite directions. Occurring close to the surface, in the upper 3-5 kilometers (km) of the fault, this slow, silent movement is referred to as "creep events."
http://www.sciencedaily.com/releases/2013/06/130603142313.htm