The transient occurrence of accelerated aseismic slip on crustal faults was discovered about two decades ago but its physical origin remains unclear or unconstrained. In this talk, we aim to understand slow slip on several major faults by combining fault zone modeling with multiple geophysical observations. In the first part, we study the deep fault zone conditions of the San Jacinto fault at Anza, where geodetic locking depth is anomalously shallower than the depths of active microseismicity. We suggest that spatially broad, heterogenous transitions in frictional properties at the base of the seismogenic zone, which promote accelerated slow slip, is required to reconcile the seismic and geodetic observations. The occurrence of slow slip events in the region is also supported by source inversions of transient strainmeter signals. In the second part, we focus on discerning the physical mechanisms of unsteady shallow fault creep, as documented by creepmeters and InSAR observations for the Superstition Hills fault (SHF) in southern California and the Ismetpasa segment of the North Anatolian fault (NAF) in Turkey. We find that conventional fault zone models, based on quasi-static rate-and-state friction, appear inadequate to explain the full range of observations from the recent seismic cycle at SHF and NAF. Processes that induce dynamic fault strengthening, such as dilatancy of fluid-saturated fault rocks, are likely active on the shallow faults and potentially critical for understanding surface faulting.