We present an analysis of the forces acting on the Sierra Nevada block in the western United States. Vertically integrated loads acting on the block are quantified through numerical modeling of the state of stress in the interior of the block constrained by stress observations. The Sierra Nevada block is located adjacent to the Pacific–North American plate boundary allowing determination of (1) the vertical distribution of shear stress at the plate boundary and the strength of the San Andreas fault system, (2) the horizontal distribution of shear stress coupling the Pacific and North American plates, and (3) the magnitude of stresses transmitted between rigid crustal blocks. We find that the San Andreas fault system supports a relatively large vertically integrated shear stress of ∼2.4 ± 0.4 TN/m to the northwest of San Francisco and only ∼0.3 TN/m to the southeast. The majority of this load is likely supported in the frictional portion of the crust, and therefore, the San Andreas fault system is both frictionally “strong” and “weak.” Shear stress coupling the Pacific and North American plates diminishes away from the plate boundary from ∼1.5 TN/m on the western margin of the Sierra Nevada block to ∼1.1 TN/m on the eastern side. A south directed load of ∼2.4 TN/m on the northern end of the block balances this reduction in shear stress across the Sierra Nevada block and indicates that the Sierra Nevada block is compressing the rigid Cascade fore arc from the south, contributing to the forces causing N–S compression of the Pacific Northwest.