A 3D, static fracture mechanics model of earthquake rupture that incorporates cohesive end zones (cezs), or zones of increased frictional strength, is tested to determine whether it helps to understand the observed scaling behavior of average slip with rupture dimensions for shallow (<20 km), continental, interplate strike-slip earthquakes. Our new compilation of average source parameters suggests that (1) average slip increases with aspect ratio (along-strike length/down-dip width), although in decreasing proportions for progressively larger aspect ratio ruptures, and (2) a gradual scaling change exists at an aspect ratio of ∼6. These general trends match the functional form predicted by the cez model. Despite these general trends, significant scatter in average slip is apparent among similarly sized ruptures. We test the hypothesis that the cezs represent strength heterogeneities along the rupture surface that result from velocity-strengthening frictional behavior; and that this heterogeneity in frictional behavior along the fault is the primary reason for the failure of a universal (constant stress drop) scaling law. cez lengths are measured from slip and stress drop distributions determined from published inversions of geophysical data for the 1984 Morgan Hill, 1992 Landers, 1999 Hector Mine, and 1999 İzmit earthquakes, and range from ∼15 to 40% of the rupture segment lengths. These lengths are an order of magnitude larger than that inferred from characteristics of high-frequency seismic radiation (i.e., fmax). These data indicate that the ratio of average coseismic slip to rupture length decreases in the presence of large cezs. Measured cez lengths, rupture dimensions, and average slip are used to calculate average resolved shear-driving stresses and cez shear-yield strengths on the order of ∼10–30 MPa. In our new model of earthquake rupture, stress drop is predicted to be a small fraction of fault strength and thus supports a partial stress drop model of earthquake rupture for strike-slip interplate events.