To investigate the possible physical mechanisms of recently observed aseismic slip events in the Cascadia, Japan and Mexico subduction zones, we apply a Dieterich-Ruina rate and state friction law to a three dimensional model of a shallow subduction fault. That is loaded by imposed steady plate slip rate far downdip along the thrust interface. Friction properties are temperature and hence depth-dependent, such that sliding is stable at depths below about 30 km. The system is perturbed into a significantly nonuniform slip mode along strike by introducing small along-strike variations in the constitutive parameters a and (a - b). In addition to large heterogeneous earthquake slip at seismogenic depths, and associated postseismic transients, we found that slip events which have clearly aseismic slip rates emerge spontaneously around the downdip end of the seismogenic zone. Both transients which start well after a seismic event, and those which are triggered by other transients, are observed from the simulations. The slip velocity, depth range, and, sometimes, along-strike migration speed of simulated transients are similar to the observations from natural subduction zones. Unstable-stable transitional friction properties near the downdip end of the seismogenic zone are suggested to be an ingredient allowing such transients. Simulated transients can weaken the locking intensity of the updip seismogenic zone, while enhancing that of the transition zone. Spatial-temporal correlation of aseismic transients and nearby seismicity in the Guerrero, Mexico, area suggests that transients may signal a period of increased probability for nucleating their high-frequency counterparts, as damaging subduction thrust events.