The preceding chapter describes the active-fault geology of the Pacific–North America plate margin from the Siberian international border to northern California, including Alaska, northwest Canada, and the Pacific Northwest as far south as the Mendocino Fracture Zone. The southern part of the North America Plate margin can be divided into two parts: the San Andreas fault system sensu lato and the Basin and Range Province of the western United States and México. This is the diffuse plate boundary south of subduction of the Juan de Fuca and Gorda plates, discussed in the preceding chapter, and north of subduction of the Rivera and Cocos plates off México and Central America (Figure 2.1), discussed in Chapter 4.
The development of active tectonics in this region accompanied the age of geological exploration of the western United States for mineral resources in the nineteenth century. This was led by the USGS and exploratory expeditions that were its predecessors. The scientific leader of that effort, both in the Basin and Range and the San Andreas system, was Grove Karl Gilbert, who studied the active Wasatch normal fault bounding the Utah Valley on the east, and the San Andreas fault north of San Francisco after the great San Francisco earthquake of 1906. Gilbert also visited the Lone Pine, California, exposures of the Owens Valley fault that was the source of a large earthquake in 1872, and that field work contributed to his understanding of faulting and hazard on the Wasatch fault close to Salt Lake City. Others contributed to the study of Basin and Range faulting, including José Aguilera of México, who studied the 1887 Sonora earthquake, but Gilbert is the individual who is most identified with the scientific effort.