Runnerstrom,
E.E.(1), Grant, L.B.(1), Arrowsmith, J R.(2), Rhodes, D.D.(3), & Stone,
E.M.(2)
(1)Dept. of
Environmental Analysis and Design, University of CA, Irvine, 92697-7070
(2)Dept. of
Geological Sciences, Arizona State University, Tempe, 85287-1404
(3)Dept. of Geology
and Geography, Georgia Southern University, Statesboro, 30460-8149
Displacement across the Cholame segment of the San Andreas fault between
1855 and 1893 from cadastral surveys
Oral presentation by
Runnerstrom at Seismological Society of America Annual Conference 2001
in San Francisco, California.
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 3-6
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.