The Formation, Preservation, and Documentation of Holocene Deformation from Aseismic Surface Creep using Paleoseismology and Structure from Motion at the Dry Lake Valley Site on the Central San Andreas Fault

Nathan A. Toke, James B. Salisbury, Michael Bunds, Ramon R. Arrowsmith, Ephram C. Matheson, Lawrence T. Kellum, Nicole M. Abueg, Tsurue Sato, Daniel Horns, James Anderson, Joseph K. Carlson, & Jeff Selck

In Preparation 2015, SCEC Contribution #2101

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.

Citation
Toke, N. A., Salisbury, J. B., Bunds, M., Arrowsmith, R. R., Matheson, E. C., Kellum, L. T., Abueg, N. M., Sato, T., Horns, D., Anderson, J., Carlson, J. K., & Selck, J. (2015). The Formation, Preservation, and Documentation of Holocene Deformation from Aseismic Surface Creep using Paleoseismology and Structure from Motion at the Dry Lake Valley Site on the Central San Andreas Fault. Geosphere, (in preparation).