FIRST REFERENCES TO THE SAN ANDREAS FAULT [c1, p4]

The San Andreas fault first came into prominence only after it was fully understood by geologists as the cause of the great San Francisco earthquake of 1906. The name had been first used only 9 years previously by A.C. Lawson (1895) for a small segment of the fault on the San Francisco peninsula, where he reported that "a remarkably straight fault *** has conditioned the San Andreas and Crystal Springs Valley" (p. 439). Lawson applied the name "San Andreas fault" almost incidentally in a discussion of "subsequent streams that flowed in the valleys." He suggested vertical displacement on the fault but apparently was not convinced of that, inasmuch as he failed to show the fault or, with one exception, displaced strata on several cross sections in his report (for example, pl. 7). Clearly, neither the amount of displacement on the fault nor its great regional and tectonic significance was appreciated at the time of Lawson's work.

In one of the first reports about the 1906 earthquake, G.K. Gilbert (1907) accurately described the fault and its characteristic displacement, but he did not use the name "San Andreas fault." In his field notes for April 28, 1906, just 10 days after the great earthquake, Gilbert described 20 ft. of right-lateral displacement of a road where it crosses the fault at the head of Tomales Bay. He had been following the surface rupture for several days, and on April 26 he recorded in his notes that along the west side of Bolinas Bay "some of the cracks were clearly secondary; others may have been primary." By the time the final report of the State Earthquake Investigation Commission was published (Lawson, 1908), the name "San Andreas" had been adopted, and its characteristics and role in causing the earthquake were clear. That report, which contains a remarkably extensive and accurate account, constitutes a major milestone in our understanding of the San Andreas fault and of strike-slip faults as a class.