FIGURE 3.1.-Sequential diagrams showing interactions between the North American, Farallon, and Pacific plates, assuming a constant relative motion of 6 cm/yr parallel to the San Andreas fault (modified from Atwater, 1970). Position of the North American plate in each time frame is relative to those of the Farallon and Pacific plates rather than to outlines of diagram. Lengthening interface between the North American and Pacific plates, shown in three upper diagrams, represents the San Andreas transform fault. Captions for each step indicate amount of time and lateral movement necessary for the North American plate to reach its present position relative to the Pacific plate. GS, Guaymas; MZ, Mazatlan; S, Seattle; SF, San Francisco.