Major intracontinental strike-slip faults tend to mark boundaries between lithospheric blocks of contrasting mechanical properties along much of their length. Both crustal and mantle heterogeneities can form such boundaries, but the role of crustal versus mantle strength contrasts for localizing strain sufficiently to generate major faults remains unclear. Using the crustal velocity field observed through the Global Positioning System (GPS) in the epicentral area of the M7.2 2010 El Mayor-Cucapah earthquake, Baja California, we find that transient deformation observed after the event is anomalously small in areas of relatively high seismic velocity in the shallow upper mantle (∼50 km depth). This pattern is best explained with a laterally heterogeneous viscoelastic structure that mimics the seismic structure. The mantle of the Southern Colorado River Desert (SCRD) and Peninsular Ranges (PR), which bound the fault system to its east and west, respectively, have anomalously high viscosity and seismic velocity. We hypothesize that compared with the rest of the San Andreas fault (SAF) system to its north, the strike-slip fault system in northern Baja California is narrow because of the presence of the PR and SCRD high-viscosity regions which bound it.