SCEC Award Number 13147 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Examining the cause of significant ground deformation between 780 and 1031 A.D. at the Dry Lake Valley Paleoseismic Site: Do large earthquakes rupture the creeping section of the San Andreas Fault?
Investigator(s)
Name Organization
Nathan Toke Utah Valley University J Ramon Arrowsmith Arizona State University
Other Participants J. Barrett Salisbury (Ph.D. Student at ASU)
Tsurue Sato (M.S. student at ASU)
2 Undergraduate Students from UVU
SCEC Priorities 2, 1, 4 SCEC Groups Geology, WGCEP, SoSAFE
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 Ultra-high resolution fault zone mapping and trench investigation of the Dry Lake Valley site in San Benito County, CA presents a ~ 5000 year record of fault deformation due to creep. From 2012-2014 central California was historically dry. Nearby creep meter data suggests that the San Andreas Fault (SAF) experienced 3-4 cm of aseismic creep during this draught. In the absence of significant exogenic soil disturbance the creep manifest itself as left-stepping, en-echelon, opening-mode, ground cracks with cm-scale right lateral offsets which were documented at this site with a structure from motion approach. During this period nine trenches, including 15 separate fault zone exposures, were documented using traditional paleoseismic methods. These trenches revealed pervasive fabrics of deformation consistent with surface slip due to fault creep. This site preserves a fault zone stratigraphic record extending more than 5000 years into the past with compelling evidence of a long history of aseismic strain and no unequivocal evidence of large magnitude surface rupturing earthquakes. This study corroborates previous work suggesting that that creep manifests itself as a network of stepping ground cracks with minor to modest offset and opening on each crack. These structures heal through infilling by fluvial overland flow and possibly eolian deposition, followed by expansion and contraction of soils due to seasonal temperature and moisture patterns. Importantly, this study shows that during long periods of statically stiff soil conditions these cracks can organize into a pattern mimicking the rupture of a moderate magnitude earthquake.
Intellectual Merit This project demonstrates that the creeping section of the San Andreas Fault has experienced significant amounts of creep over the Holocene. We have documented how this type of deformation is recognizable in a paleoseismic trench and how it is manifest at the surface in dry soils. The project has demonstrated that creep can create modest surface rupture that mimics surface deformation seen in a moderate magnitude event.
Broader Impacts This SCEC project benefited the educational experience of one graduate student from Arizona State University and three undergraduates from Utah Valley University. All of these undergraduates are now in or on their way to graduate school in either active tectonics or geomorphology. This project furthered our knowledge about seismic hazards in central California. Undoubtedly the results will influence future efforts to characterize that hazard and henceforth impact society. The project also had a modest impact of the local economy near King City, CA.
Exemplary Figure Figure 7. Figure 7. During 2012-2014 the central California Coast Ranges were historically dry. We returned to the DLV site(Figures 1-2) in September 2014 and observed that the stiff and dry soils presented visual evidence of right-lateral shear along the SAF trace . We documented the ground cracking evidence with Structure from Motion to produce orthophotography (A), digital elevation models (B), and perspective photomosiacs (C). Surface cracking was observed at
other sites along the creeping section including the DeRose Winery (D). This pattern of overlapping oblique-to-the-SAF crack sets composed of multiple left-stepping, opening-mode, sub-meter-scale cracks accomodating overall right-lateral displacement is consistent with observations from our 2012 and 2013 fault trench exposures (Figures 3-6).

Credits: N.Toke and M. Bunds