Preliminary Holocene history of fault slip for the Mojave section of the San Andreas fault

Tracy Compton, Eric S. Cowgill, Katherine M. Scharer, Ryan D. Gold, Rolf Westerteiger, Tony Bernardin, & Louise H. Kellogg

Published December 2012, SCEC Contribution #1787

The Mojave section of the San Andreas fault (MSAF) shows an apparent discrepancy between slip rates where geodetic rates are systematically slower relative to geologic rates. Resolving this discrepancy is important for determining whether or not the MSAF exhibits temporal changes in slip, advancing the understanding of the mechanical behavior of fault systems, and improving seismic-hazard assessment for the MSAF. Paleoseismic data along the MSAF suggest temporal variations in strain release over the last 2 kyr, but more studies are needed to extend the slip history back in time. Here we address the problem of the apparent slip rate discrepancy and possible temporal variations in strain release by employing Monte Carlo analysis of previously reported displacement-time data to investigate the extent to which these data constrain the Holocene slip history. We evaluated 42 previously reported piercing lines for possible inclusion in our analysis, 15 of which were unused because they are either duplicate reports or poorly documented. The remaining 27 data points reveal that slip rates are nonexistent for 5 offset distances (19-27m, 33-42m, 45-63m, 65-129m, and 131-300m) and for 3 time periods from 10-3.9 kyr, 3.9-2.8 kyr, and 2.8-1.4 kyr BP. Results of this analysis suggest slip rate along the MSAF varied between 0 and 4.5 kyr BP, with 5 possible phases of strain release, 3 of which are faster than the average of ~30 mm/yr. The oldest fast phase was from 4.5-2.9 kyr with an average slip rate of 61 mm/yr. The next fast phase, with an average rate of 81 mm/yr, was from 1.5-1.1 kyr. The youngest fast phase resulted in a rate of 36 mm/yr between 0.4 kyr and the 1857 event. Slower phases of slip occurred from 2.9-1.5 kyr, with an average rate of 12 mm/yr, and from 1.1-0.4 kyr, with a slip rate of 20 mm/yr. These slip history findings are considered preliminary because they are based on a limited dataset that contain data gaps. To aide in our search for additional potentially dateable offsets preserved along the MSAF, we used available airborne LiDAR and virtual-reality visualization software Crusta. Crusta is a virtual globe application that supports real-time interactive visualization of large (>60GB) LiDAR digital elevation models and merges the mapping functionality of ArcMap with the terrain visualization capabilities of Google Earth. On a 100km section of the MSAF we found 60 offsets ranging from 20 to 297m, many of which show promise as potential slip-rate sites. We determined offset uncertainties using the slicer tool, which is a recently developed function in Crusta that allows the user to assign a fault plane by dropping nodes along a linear fault trace and interactively moving one fault block relative to the other in any slip direction. This functionality allows the user to incrementally restore fault slip until topographic features on opposite sides of the fault are aligned. This tool has been helpful in identifying, evaluating, and reconstructing possible landform offsets. We plan to continue remote analysis using additional visualization tools such as a fully immersive CAVE environment and we will conduct fieldwork to determine ages of numerous identified offsets to build a more robust dataset for further slip history analysis.

Citation
Compton, T., Cowgill, E. S., Scharer, K. M., Gold, R. D., Westerteiger, R., Bernardin, T., & Kellogg, L. H. (2012, 12). Preliminary Holocene history of fault slip for the Mojave section of the San Andreas fault. Oral Presentation at AGU Fall Meeting 2012.