FAULT TRACE MAPPING OF THE CENTRAL SAF

We are generating a fault trace and surficial geologic map of the creeping section of the Central San Andreas Fault, from San Juan Bautista to Parkfield using relatively new lidar topography. The aim is to utilize high-resolution lidar-derived DEMs to increase the spatial accuracy of mapped fault traces along with surficial and bedrock contacts. We will compare the surficial geologic and geomorphic mapping (representing surface and tectonic processes operating over millennial and longer time scales) to the decadal time scale topographic differencing (3D ICP) from Scott et al 2020 to better understand the temporal development of active fault traces across the fault zone of the creeping section.

We utilize the 2005 B4 and 2007 Earthscope lidar basemap datasets. We evaluate fault trace expression with geomorphic indicators to guide our mapping. We assign each trace a confidence ranking and categorize the trace as primary or secondary. Each fault trace is supported by geomorphic indicators with a total count of over 1,500 mapped indicators. Landscapes that preserve geomorphic indicators are likely to have higher confidence faults and more mapped secondary traces. The 3D differencing highlights the principal displacement zone and thus the primary trace over the majority of the creeping section, but does not show fine scale details.

From a large scale view, the primary trace in the 3D differencing generally agrees with the geomorphically mapped traces. Several sites along the creeping section are illustrative of the difference between the 3D differencing and geomorphic traces. At Mustang Ridge, strong geomorphic signals indicating activity over the past ~10 kyr show multiple sigmoidal fault traces, only one of which is active in the 3Ddifferencing. At Peach Tree Ranch, the wide fault zone is sensitive to geomorphic alteration. The 3D differencing has landslides and weaker signals that do not follow the geomorphically-defined fault traces. At Lewis Creek, large earth flows erase geomorphic indicators and add noise to the differencing. There is a strong contrast in movement, with horizontal tectonic displacements butted against vertically displaced landslid