X-ray computed microtomographic (XRCT) images of a fault core that slipped during the 1726 San Andreas faultzone earthquake

Uploaded are x-ray computed microtomographic (XRCT) images used to examine solid-fluid interactions within one of the near-surface fault cores that slipped during a circa (ca.) 1726 San Andreas Fault zone earthquake. The study site is 16 km northwest of Bombay Beach, California (33.45873, -115.8560), and our sample, collected at a depth of 1.2 m below sea level, is from a trench that exposes deposits of ancient Lake Cahuilla. The ca. 1726 earthquake occurred during a highstand of ancient Lake Cahuilla; our study site was ~55 m below the lake's surface at the time. Crustal deformation caused by the ca. 1726 earthquake has been documented for at least 85 km along the southernmost San Andreas fault zone, which has been used, alongside other observations, to constrain the earthquake's size to a magnitude 7.2 or larger with offsets on the order of ~3 m. Since the ca. 1726 earthquake, creep and triggered slip have occurred along the section of the fault we study, with estimates of ~3 mm/yr of motion over the last ~160 years.

We acquire XRCT images at the Advanced Light Source, Lawrence Berkeley National Lab, on beamline 8.3.2. Imaging uses a 50 mm LuAG scintillator, PCO Edge camera, and 1X Nikon lens. We image with white light x-rays, 13 ms exposure times, and 2625 projections through 180-degree continuous sample rotations. This produces 1280 two-dimensional image slices with voxels' linear dimensions of 3.24 microns. We reconstruct images and perform ring removal, center of rotation optimizations, and outlier removal using TomoPy. We name the sample FT_50_4_ZZZZ, where ZZZZ represents the image slice number; increasing numbers represent increasing distance into the outcrop.