Abstract

Changes since 1855 in reported section-line lengths and positions of survey monuments that span the San Andreas fault (SAF) were used to measure displacement interpreted to be from the 1857 Fort Tejon earthquake in south-central California. In 1855-1856 James E. Freeman established township and range lines across the SAF between Rancho Cholame and the northern Carrizo Plain. At least 26 1-mile sections lines spanned the SAF in the area between present-day California Highways 46 and 58. Each section line was marked by monuments at the midpoint and endpoints. Section lines across portions of the SAF were resurveyed in 1893 by J. M. Gore. We projected changes in line length onto the fault zone to measure displacement. The measurements indicate right lateral displacement of 16.2 ± 6.0 m across the fault zone. The resulting tectonic displacement exceeds the maximum reported geomorphic offsets (∼6.7 m) attributed to the 1857 earthquake along the Cholame segment. Although we recognize great uncertainties in the accuracy of our small, historical data set, we tentatively conclude that total displacement in the 1857 earthquake along the SAF over this ∼2 km wide aperture was significantly greater than the 3- to 6-m slip previously reported for the Cholame segment from narrower aperture geomorphic and trenching studies. These differences may be compatible if slip along the fault increases down dip rapidly from ∼3-6 m at the surface to ∼20 m within several hundred meters of the surface. Our inference of high slip along this portion of the Cholame segment in 1857 is at odds with most rupture models of the central SAF and suggests that geomorphic offsets may not represent total displacement across the fault zone.

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