A series of numerical experiments have been conducted to explore effects of strength heterogeneity and rupture propagation upon the average slip and magnitude distributions of earthquakes. Alternative rupture processes were represented by two possible sets of rules controlling rupture propagation. Ruptures were run on faults having strong patches that break to form subevents with various assumed stress drops. The subevents have a truncated power-law size distribution. The rules controlling rupture propagation in combination with the subevent stress drop have a strong effect upon the magnitude distribution. The most realistic rupture model favors ruptures whose length and width are comparable and produces an increase in b-value for events above magnitude 7.5. Magnitude distributions similar to observations of global seismicity are produced by models with subevents that have a stress drop near 100 MPa. The relationship between seismic moment and rupture area also agrees with observations of events not from subduction zones when the same model that fits the global magnitude distribution is adapted for a narrower fault width.