Geodesy can be used to infer long-term fault slip rates, assuming a model for crust and upper mantle rheology. We examine the sensitivity of fault slip rate estimates to assumed rheology for the Agua Blanca and San Miguel-Vallecitos faults in northern Baja California, Mexico, part of the Pacific--North America plate boundary zone. The Agua Blanca fault is seismically quiet, but offset alluvial fans indicate young activity. Current seismicity is confined to the nearby San Miguel-Vallecitos fault, a small offset fault better aligned with plate motion. GPS measurements between 1993 and 1998 suggest that both faults are active, with a combined slip rate of 4--8 mm yr-1 regardless of rheological model. However, slip rate estimates for the individual faults are sensitive to assumed rheology. Elastic half-space models yield 2--3 mm yr-1 for the Agua Blanca fault, and somewhat faster rates for the San Miguel-Vallecitos fault, 2--4 mm yr-1, with uncertainties of about 1 mm yr-1. Models incorporating viscoelastic rheology and seismic cycle effects suggest a faster slip rate for the Agua Blanca fault, 6 ¿ 1 mm yr-1, and a slower rate for the San Miguel-Vallecitos fault, 1 ¿ 1 mm yr-1, in better agreement with geological data, but these rates are sensitive to assumed rheology. Numerical simulations with a finite element model suggest that for similar rheological and friction conditions, slip on the San Miguel-Vallecitos fault should be favored due to better alignment with plate motion. Long-term faulting processes in the larger offset Agua Blanca fault may have lowered slip resistance, allowing accommodation of motion despite misalignment with plate motion.