SCEC Award Number 13125 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Three dimensional excavation of offset channels in the Carrizo Plain
Investigator(s)
Name Organization
Sinan Akciz University of California, Los Angeles Ramon Arrowsmith Arizona State University Lisa Grant Ludwig University of California, Irvine
Other Participants A graduate student from ASU
SCEC SURE intern
SCEC Priorities 2, 1, 4 SCEC Groups Geology, SoSAFE, WGCEP
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 Recent work using computer-based analytic tools and LiDAR topography suggested that offset during the 1857 Fort Tejon earthquake along the Carrizo section of the San Andreas fault was 5-6 m, significantly lower than the previously reported 8-10 m using traditional on-the-ground measurements. We are reevaluating surface displacement of the great 1857 Fort Tejon earthquake using 3D excavation of channel Sieh31, which has been interpreted to have been offset ~6m and ~9m by researchers using different methods. We selected channel Sieh31 as the best candidate to measure 1857 slip and investigate the potential for superposed tectonically offset older and deflected younger sections of a single channel. In preliminary 2013 work, 3 trenches exposed well-defined channels incised into stratified alluvial fan deposits. The excavations show that the original thalweg of the Sieh31 does not coincide with either the present day thalweg nor the median of the present-day channel margins, emphasizing the importance of measuring the offset with a future 3D excavation. Preliminary optically stimulated luminescence (OSL) dates from an earlier channel fill stratum indicate that incision of Sieh31 channel predates ~1735+-25 A.D. While additional data are needed for confirmation, if the channel records a single offset, its age implies that the penultimate earthquake occurred between ~1650 and ~1750 A.D. Preliminary results suggest different geomorphic and geologic measurements of tectonic displacement, and emphasize the importance of validation of surface offset reconstructions from remotely interpreted digital topographic data.
Intellectual Merit Our work at the Sieh31 site aims to answer several key questions about the rupture history of the San Andreas Fault in the Carrizo Plain: (1) What are the limitations of using offset incised stream channels in determining the magnitude of displacement from the most recent earthquake? (2) How do channel morphologies evolve after their initial incision? How representative are the present day channel margins (and even thalwegs) in making offset measurements. (3) What are the best geochronological methods/procedures in determining the incision age of young offset stream channels?
Broader Impacts Our SCEC SURE intern, Daniel Halford was trained in all aspects of paleoseismic trenching. He presented our initial results during the SCEC Annual Meeting in 2013. Three other UCLA undergraduate volunteers participated in the field work and one of them did majority of the IRSL sample preparations with the supervision of Prof. Ed Rhodes. A Master's student from Arizona State University has decided to pursue her thesis on the 4D evolution of channel Sieh31. Due to the delicacy and significance of the data being collected through such a disruptive investigation, we collaborated with our USGS colleagues to collect very high resolution digital topographic data (Mobile Laser Scanning) and Laser scanned all trench walls.
Exemplary Figure Figure 2(B). Summary figure of our 2013 excavation site. Basemap is a shaded relief map produced from mobile LiDAR data and displayed over a Google Earth satellite imagery. Red triangles mark the active portion of the SAF zone through the site. White lines show the three trenches opened in 2013. Green boxes mark the locations of buried channel thalweg with respect to the present day channel margins.(Mobile LiDAR data was collected by Ben Brooks (USGS)).
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