Major ruptures along transform faults such as the San Andreas Fault System (SAFS) are driven by stress that has accumulated in the upper locked portion of the crust. While the present-day stress accumulation rate on any given fault segment of the SAFS is fairly well resolved by current geodetic measurements, the total accumulated stress depends on the rupture history of the fault over the past few thousand years, which is only poorly constrained by paleoseismic data. Here we show, using a simple 3-D kinematic model of the earthquake cycle, that stress accumulation rate is highly dependent on the fault locking depth. Model stress accumulation rates vary between 0.5 and 7 MPa per century and are inversely proportional to earthquake recurrence intervals (500-20 yrs). Assuming complete slip release during major ruptures, we also simulate accumulated stress at crustal depths for both past and present-day conditions. These more speculative results indicate that the southern San Andreas, which has not ruptured in a major earthquake in over 300 years, is currently approaching a historically critical stress level.