This work investigates the physical mechanisms underlying aseismic deformation transients in subduction zones, their relation to deep non-volcanic tremors and nearby seismicity, and implications to the slip budget throughout seismic cycles. We simulate subduction earthquake sequences by applying the single-state-variable "ageing" rate and state friction law to 2D and 3D thrust fault modeling, with temperature and hence depth-dependent frictional properties. Our results show that aseismic deforma tion transients are a natural outcome of the rate and state processes, observed in laboratory fault-sliding experiments. Transients can arise spontaneously for certain effective normal stress [Special characters omitted.] variations with depth. Velocity-weakening to strengthening stability transitional properties are suggested to be an ingredient allowing transients near the end of the seismogenic zone. When fluid pressure is near-lithostatic around and down-dip from that transition, the system exhibits self-sustained short-period aseismic oscillations, with lab values of friction parameters. A typical northern Cascadia 14-month recurrence interval is predicted at low [Special characters omitted.] of 2 to 3 MPa.

Evidence of such high fluid pressure conditions is independently provided by the occurrence of non-volcanic tremors as apparent responses to extremely small stress changes, and by petrological constraints on the regions of metamorphic dehydration in shallow-dipping subduction zones. Transients can also be triggered by interseismic stress perturbations, due to extensional earthquakes in the descending slab, fluid pressure changes or other sources. Properties of the triggered transients depend on the time, location and magnitude of the perturbations. A systematic seismicity catalog study in Guerrero, Mexico, shows that three large transients in 1998, 2001-2002 and 2006 are all spatial-temporally correlated with high seismic rates. The initiation of the transients coincides with a cluster of extensional earthquakes far inland from the trench, and may be followed by thrust earthquakes near the trench, or bracketed by both. This suggests transients may act as a mechanism of stress communication between distant seismicity clusters in shallow subduction zones. We also investigate the system stability with a two-state-variable interpretation, which better describes the high-temperature friction behaviors revealed by Blanpied et al. [ 1998]. Activation of aseismic oscillations in the down-dip velocity-strengthening region may provide another contributing mechanism to the occurrence of transients.