Fault Trace Mapping of the Central San Andreas Fault from Lidar-derived Topography

Joseph H. Powell, Chelsea P. Scott, Ramon Arrowsmith, & Stephen DeLong

Submitted September 10, 2023, SCEC Contribution #13198, 2023 SCEC Annual Meeting Poster #079

We created a fault trace and surficial geologic map of the creeping section of the Central San Andreas Fault from San Juan Bautista to Parkfield, California. This mapping has been completed at a 1:10,000 scale to refine existing mapping efforts using data not previously available. We utilized the 2005 B4, 2007 Earthscope, and 2018 USGS 3DEP high-resolution lidar-derived DEMs along with satellite imagery to increase the detail and spatial accuracy of our mapped fault traces comparatively to the USGS Qfaults database.

This fault trace mapping uses geomorphic indicators as a guide. Geomorphic indicators are landforms along the fault zone likely formed by fault slip such as scarps and offset drainages. We evaluated fault trace expression and assigned each trace as primary (main displacement) or secondary (distributed) with a confidence ranking. Each trace is supported by geomorphic indicators with a count of over 1,500 total mapped indicators.

We conducted surficial geologic mapping to support the fault trace placement. We assume that the features we have mapped have developed from long-term surface and tectonic processes over thousands to tens of thousands of years. The traces are compared to active shear zone strain rate maps identified through decadal time-scale 3D topographic differencing of repeat airborne laser scans. The comparison shows that the principal displacement zone is consistent with the primary fault traces we mapped over the majority of the creeping section, but does not represent sub 1:24,000 details due to the coarseness of the 3D differencing results as well as the difference in duration of straining.

There are instances of variation between the 3D differencing and our mapping results: At Paicines Ranch (~110 km northwest of Parkfield), strong geomorphic signals indicate activity over the past ~10 kyr and show multiple sigmoidal fault traces, only one of which is creeping in the 3D differencing. At Peach Tree Ranch (~100 km northwest of Parkfield), the wide fault zone is sensitive to alteration. There, the 3D differencing includes landslides and displays weaker active fault indication signals that are likely due to geomorphic processes. At Lewis Creek (~40 km northwest of Parkfield), large earth flows erase geomorphic indicators and add noise to the differencing. This site displays a notable disparity in movement, with horizontal tectonic displacements juxtaposed against vertically displaced landslides.

Key Words
Geomorphology, San Andreas Fault, Mapping, Geology, Surface Geology, Quaternary

Powell, J. H., Scott, C. P., Arrowsmith, R., & DeLong, S. (2023, 09). Fault Trace Mapping of the Central San Andreas Fault from Lidar-derived Topography. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology