A mathematical model for historical horizontal crustal deformation is presented for the southern California region that extends northward from Mexico for approximately 100 km to 33.5¿N latitude. Model parameters are estimated from tringulation, trilateration, and astronomic azimuth data observed during the past century. Also geologica and seismic data indirectly participate in specifying the model. The model relates crustal deformation to the episodic movement experienced during large earthquakes (M≥6), secular slip rates on geologic faults, and secular strain rates over large geographic areas. The episodic movement is modeled in accordance with the theory of dislocation in an elastic halfspace. For the secular motion the modeled region is partitioned into a mosaic of districts that are allowed to individually translate, rotate, and homogeneously deform as a linear function of time. The derived strain pattern supports the hypothesis that the modeled region spans the Pacific-North American plate boundary. Secular shear strain is maximum in the NW direction parallel to the major faults, and shear strain rates decrease with distance from a NW-SE trending strip. To the region's north this strip is 40 km wide and is bounded on the west by the San Jacinto fault zone and on the east by the southern extension of the San Andreas fault. To the south this strip converges in width onto the Imperial fault. The secular velocity for a point near the city of San Diego relative to a point at the region's northeast corner is 4.3¿3.1 cm/yr in the direction N 17¿W¿5¿. This rate provides a rough estimate for the relative velocity between the Pacific and North American plates.