Final answer:
San Diego and Santa Barbara are migrating southward due to the movement of the Pacific Plate relative to the North American Plate along a transform boundary.
Step-by-step explanation:
San Diego and Santa Barbara in California are slowly migrating southward because they are located on the Pacific Plate, which is moving to the south relative to the North American Plate. This movement is occurring along a transform boundary, where the plates are sliding past each other horizontally. Transform boundaries are characterized by shear stress, which causes the crust to break and slide in opposite directions.