San Andreas fault: History of concepts MASON L. HILL 1401,7 E. Summit Drive, Wbitticr, California 90602 ABSTRACT strike-slip on the San Andreas (Noble, 1926); (3) the proposal of more than 560 km of cumulative right-slip on the fault (Hill and The long and active San Andreas fault was revealed by the San Dibblee, 1953); and (4) the introduction of the San Andreas as a Francisco earthquake of 1906. Strike-slip movement on a major transform fault separating major crustal plates (Wilson, 1965). crustal fracture was first established by that event. The elastic re- Justification for this history is to show how concepts about the San bound theory was developed in an analysis of this earthquake. It Andreas fault have changed, and to indicate that future studies will was proposed in 1926 that cumulative horizontal movement on the result in new interpretations. My authorship may be justified by San Andreas amounted to several miles, but such a great displace- knowing some of the geology and many of the geologists involved ment was generally agreed to be unreasonable. In 1953, new evi- in this history. History writing, like much in science, is in large dence of cross-fault stratigraphic correlations of Pleistocene to Cre- measure subjective. Thus, it will be obvious to the reader that I taceous rocks was presented which seemed to require tens to hun- have "axes to grind." Specifically, I object to (1) confusion between dreds of miles of strike-slip displacement. Controversy and fault separation and fault slip; (2) the long-held assumption that additional studies ensued, resulting in general acceptance of such Franciscan strata were deposited on granitic rocks; (3) resistance to movements by 1968. Since the 1965 proposal that the San Andreas miles of strike-slip on the fault; and (4) the present tendency to ac- is a transform fault, within a plate-tectonics mechanism, reser- cept a simplistic role for the San Andreas in plate tectonics. Em- vations about great horizontal movements of the Earth's crust have phasis is placed on the roles of new data and new insights in the been essentially eliminated. The single most important factor in de- evolution, or revolutions, in concepts about the San Andreas. Sev- laying acceptance of miles of strike-slip on the San Andreas has eral controversies are recorded to show how they have stimulated been the long-continued confusion between fault separation and new studies and new interpretations. However, the overriding fault slip. Lawson, Noble, Taliaferro, Hill and Dibblee, Wilson, incentives to intensified studies of the San Andreas have been (1) and a few others played the more leading roles in interpretations of the San Francisco earthquake; (2) the proposals of tens to hundreds the fault. Post-earthquake studies by Gilbert again confirmed his of miles of cumulative strike-slip, (3) the recent interpretation of reputation as a great geologist. The San Francisco earthquake was the San Andreas as a transform fault; and (4) the currently inten- the chief contributor to knowledge about the San Andreas, but now sified research on earthquake prediction. there are more questions than ever regarding the nature, geologic This history of work on the San Andreas cites the studies and in- history, and significance of-this important crustal structure. The volvement of many persons of renown. Some of them are recog- present consensus about the role of the fault in local and global nized in brief biographic sketches, alphabetically arranged in Ap- tectonics surely will be modified by revolutionary new conceptual pendix 1, and identified by asterisks in the text. models. THROUGH THE 1906 EARTHQUAKE INTRODUCTION It appears that Lawson" was the first to recognize, map, and This historical account of recognition, mapping, and interpreta- refer in print to a segment of the San Andreas fault. (Refer to Fig. 1 tions of the San Andreas fault provides background for present for location of San Andreas fault and a few of the places mentioned concepts and problems relating to this important structure. The San in the text. A good geographic-geologic reference for some readers Andreas is an important crustal fracture because it (1) is at least is the 1:750,000 Geologic Map of California by Jennings, 1977.) 960 km long; (2) produces great earthquakes; (3) is characterized His description, probably based on field work beginning in 1890 or by strike-slip displacement; (4) has a generally agreed upon 1 891, appeared in the first volume of University of California, Bul- cumulative right-slip of ~320 km; and (5) is widely accepted as a letin, Department of Geology, in "The post-Pliocene diastrophism transform fault which separates the North American and Pacific of the coast of Southern California" (Lawson, 1893). He said (p. lithospheric plates. 149): The history recounted here begins with the initial mapping of segments of the fault, follows with highlights of continuing studies The line of demarkation between the Pliocene and Mesozoic terraines is a and interpretations, and concludes with an analysis of this history nearly straight [northwest trending] line coincident with the projection of and its possible bearing on future studies of the San Andreas. The the axis of San Andreas Valley to Mussell Rock. selection of accounts considered to be significant, and their role in this history, are surely colored by personal bias and are, in some He also explained (p. 150) the uplift and presumed erosion down measure, self-serving. The story is separated into five eras by (1) the into Mesozoic rocks southwest of the fault by "orogenic upthrust" State Investigation Commission's report on the 1906 earthquake against the Pliocene Merced series to the northeast, with sub- (Lawson, 1908; Reid, 1910); (2) the suggestion of 38 km of sequent epeirogenic uplift of both the Pliocene and Mesozoic ter- Geological Society of America Bulletin, Part 1, v. 92, p. 112-131, 1 fig., March 1981. 112 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/3/112/3419054/i0016-7606-92-3-112.pdf by guest on 28 September 2021 SAN ANDREAS FAULT: HISTORY OF CONCEPTS 2 ranes. His section (p. 146) indicates the position of the fault with- fault separation (geometry) and fault slip (kinematics) has been ac- out showing offset. Lawson did not name the fault or give it any commodated by a dual classification of faults (Hill, 1959). importance except in the context of the paper's title. Even after the Branner seems to have been the second geologist to have evidence of strike-slip at the time of the earthquake, Lawson and mapped a segment of the San Andreas fault, although according to others continued to infer that the offsets revealed in geologic sec- Jordan (1907), Branner was first. However, since Branner did not tions resulted from dip-slip movements. This confusion between come to Stanford until the fall of 1892, I doubt that his mapping preceded Lawson's. Lawson (1895) apparently first introduced the name, "San An- dreas fault," in the 15th Annual Report of the U.S. Geological Sur- OREGON vey. His 77-page account, Sketch of the Geology of the San Fran- cisco Peninsula, was professed to be (p. 406): the first time the method of systematic mapping has been applied to the study of the Coast Ranges, and the results have strengthened the writer's conviction that this is the only effective method of arriving at clear concep- tions of their general geology. His colored map clearly shows the San Andreas lineament extend- ing southeastward from Mussel Rock through Lake San Andreas and Crystal Springs Lake. In describing the structure of the San Francisco Peninsula, Lawson discussed the fault-bounded San ^SHELTER COVE Bruno and Montara Mountains. Describing the geology within the Montara Mountain block, he wrote (p. 468): The deformation of the Merced series [isj along a line of its present contact with the Franciscan terrane, i.e., along a line coincident with the San An- dreas fault. LPT. ARENA 9\ W 1906 BREAK AND EPICENTER k PT. REYES \CALAVERAS FAULT \ kSAN\FRANCISCO PENINSULA \ ft \ i SAN JUAN BAUTISTA o o dì CHOLAME \ N 7.P Q, 1857 BREAK AND EPICENTER S \ \ TEJON PASS FT TEJON ANTELOPE VALLEY ANSVERSE SAN CAJON PASS GABRI MOUNTAINS RANGES GORGON IO PASS 100 200 300 400 km * * Figure 1. Index map, San Andreas fault, California. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/3/112/3419054/i0016-7606-92-3-112.pdf by guest on 28 September 2021 114 M. L. HILL This casual use of the name "San Andreas" for the fault is the only Osmont (1904) appears to have been the next geologist to map one in this report. Clearly it was not considered an important struc- and describe segments of the San Andreas fault before the 1906 ture. However, its name is appropriate and Lake San Andreas can earthquake. His two long geological traverses from the Pacific be accepted as the fault's "type locality." Ocean to the Sacramento Valley crossed the San Andreas in Marin It appears that the third geologist to describe, although not map, County, where it was called the Baulinas-Tomales fault. He refer- a segment of the San Andreas fault was Fairbanks5' (1894), who red to the fault (p. 79) as follows: said (p. 495): At Bodego Head (section AB) and Point Reyes Peninsula (section CD) the pre-Franciscan granitics (diorite) have probably been brought to light by a The effects produced by a great earthquake, probably that of 1872, can be great fault along the line of Bodega and Tómales Bay, seemingly the north- traced a number of miles along the stage road from Gorman Station to An- west extension of the San Bruno fault ..