SCEC Award Number 16201 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Short- and Long-Term Slip Histories along the South-Central San Andreas Fault: Completing OSL and C14 Geochronology of the Phelan Creeks and Van Matre Ranch sites, Carrizo Plain, CA
Investigator(s)
Name Organization
Ramon Arrowsmith Arizona State University
Other Participants J. Barrett Salisbury, Ph.D. Candidate
School of Earth and Space Exploration
Arizona State University. We are in close communication with our collaborators Tom Rockwell, Sinan Akciz, Lisa Grant-Ludwig with whom we have worked on these topics
SCEC Priorities 1a, 2a, 4b SCEC Groups EFP, Geology, SoSAFE
Report Due Date 03/15/2017 Date Report Submitted 03/14/2017
Project Abstract
 At Van Matre Ranch in the Carrizo Plai,n we demonstrated (with sub-surface excavation and high-resolution geochronology) that existing interpretations of single- and multiple-earthquake offset features need to be reconsidered. Geomorphic offset measurements made there are of landforms that are not actually offset from the originally inferred sources, or if the reconstruction is correct, the total offset is not from only a single earthquake. Therefore, slip at a point in successive earthquakes is variable, and that at least for the Carrizo Plain, it is inaccurate to assume a perfect correspondence between the earthquakes preserved in subsurface sediments and with the earthquakes preserved in the geomorphic record. We calculated a new centennial-scale slip rate (31.6 +5.9/-4.3 mm/yr) for the region and refined the estimate of slip in the most recent event. While the new short-term slip rate is consistent with existing studies in the Carrizo Plain, our estimate of slip in the most recent event (3.8 m) is smaller than that of existing studies (>5 m). At Phelan Creeks we dated several stages of channel reconstruction to show that the millennial-scale slip rate of the San Andreas fault in the Carrizo Plain is relatively uniform through the middle and late Holocene (Exemplary Figure).
Intellectual Merit We show several ways that remote analyses and sub-surface investigations of fault offset landforms, when combined with new analytical techniques (e.g., structure-from-motion topography, pIR-IRSL age analyses), can advance our understanding of active fault behavior even in previously well-studied areas. Understanding the frequency of small-scale landform formation versus landform displacement is essential to correct interpretation of small and large offsets alike.
Broader Impacts This collaborative, largely field-based investigation required several field campaigns and help from many participants. We employed 5 undergraduate student researchers (2 from ASU, Sutton (F) and Fischman (F); 1 from SDSU, Marquez (F); and 2 from UCI, Marin (M) and Olsen (F)), a USGS undergraduate intern (Midttun (M)), 2 graduate students from ASU (Salisbury (M) and Williams (F)), and four professional investigators (Arrowsmith, Rockwell, Akciz, and Grant Ludwig). The undergraduates learned basics of 3-D trench excavation and logging, balloon aerial photography, and structure from motion (SfM) generation of high-resolution models of trench exposures and surface topography. Students helped with post-field data assimilation and some attended the 2015 & 2016 SCEC annual meetings.
Exemplary Figure Figure 2 (Exemplary Figure) - Age vs. Offset plot for the Carrizo Plain for the last 15,000 years. Box dimensions represent age and offset uncertainties. Black boxes represent age vs. offset plots generated by these projects. The red line represents the slip rate calculated by Sieh and Jahns (1984) for the last 3,700 years from the small (130 m) offset at Wallace Creek. Magenta lines represent uncertainties. The dashed black line is the slip rate (36 mm/yr, ~475 m offset) calculated at the 13,250 yr timescale in the same study. The dashed black line also represents the geodetic rate of 36 mm/yr (Schmalzle et al., 2006). Figure is from Salisbury, 2016.