Analysis of force and torque balance on the Salton block, Southern California, yields estimates of time-averaged shear stress acting on the bounding faults and the base of the crust. Averaged over a depth of 30 km, the San Andreas and San Jacinto faults sustain time-averaged shear stress of ∼21–35 and 24–43 MPa, respectively. This implies that tectonic shear stress at seismogenic depths is greater than a typical 1–10 MPa earthquake stress drop and, with a corresponding effective friction coefficient of 0.10–0.21, lower than that predicted by laboratory experiments. Basal stress of 3–14 MPa also is required to drive the Salton block into the Transverse Ranges. Thus, the forces driving mountain building, basin formation, and the generation of earthquakes south of the Transverse Ranges in Southern California stem from stresses transmitted laterally across weak faults and from below.