Episodic Tremor and Slip (ETS), involving transient deformations accompanied by emergent, low-frequency tremor occurs in subduction zones around the world. ETS events increase the shear stress on locked megathrusts and may potentially trigger damaging earthquakes. Despite the clear association of tremor and slip the physical relationship between them is unresolved. Tremor appears to result from slip on small asperities on the plate interface due to either creep on the surrounding fault, or stress increases ahead of the propagating slow-slip front. Previous studies of migrating slow slip events have not had sufficient spatial and temporal resolution to differentiate between these two models. To address this, we invert GPS data from the August 2009 ETS event in central Cascadia for the space-time evolution of fault slip-rate. We find a correlation in both space and time between tremor epicenters and the independently determined position of high fault slip-rate. This supports the first hypothesis that tremor asperities are loaded directly by slow slip, rather than by stress increases ahead of the slip front, and provides new insights into the mechanics of ETS.