Late Holocene offsets from the Panamint Valley transtensional relay, CA, USA

Several historic earthquakes in the Eastern California Shear Zone (ECSZ) have involved complex and multi-fault ruptures. However, the physical conditions that promote or inhibit discontinuity-spanning coseismic ruptures are still poorly defined. These physical conditions tend to vary in space and time over multiple earthquake cycles, making it difficult to forecast the likelihood of coordinated or triggered ruptures between multiple fault systems. Using tectonogeomorphic mapping, we document paleoseismic evidence for late Holocene complex rupture in Panamint Valley, in the 10 km-wide transtensional relay zone between the Ash Hill and Panamint Valley faults. We quantify rupture kinematics using vertical, lateral, and total offsets from over 250+ piercing lines, measured from field mapping and backslipped reconstructions of newly generated, high-resolution (5 cm) structure from motion (SfM) digital surface models. Our measured right-lateral, vertical, and total offsets range from 0.24 - 2.65 m, 0.01 - 0.78 m, and 0.28 - 2.74 m, respectively. The strike-slip to dip-slip ratios for surface ruptures in the Panamint Valley transtensional relay (PVTR) range from 4:3 to pure strike-slip, with an average ratio of ~5:1. Measured total offsets of up to ~0.6-1.0 m of slip on single-event scarps in the PVTR support total surface rupture lengths between ~20 - 33 km, correlative to earthquake magnitudes of Mw ≈ 6.7 - 6.9.

Methods
We completed field and remote tectonogeomorphic mapping of faults within the Panamint Valley transtensional relay, at a resolution of 1:4000, using base maps consisting of 1) GeoEye aerial imagery, 2) newly developed 5 cm structure-from-motion (SfM) digital surface models (DSMs), and 3) slope and hillshade derivatives from National Center for Airborne Laser Mapping (NCALM) 0.5 m airborne lidar, collected from the EarthScope SoCal Lidar Project, accessed at OpenTopography (http://opentopography.org). Offset locations are provided in UTM (NAD 1983 UTM Zone 11N) and decimal degrees.

We resolved these smaller (~0.5 – 1 m) offsets in young Holocene surfaces with low < ~0.5 m geomorphic relief using our newly collected SfM DSMs, available on OpenTopography (LaPlante, 2024). We processed the photogrammetry using SfM analysis in Agisoft Metashape, after the methods described in Reitman et al. (2015), and the USGS UAS Data Post-Processing Guide (Over et al., 2021). The DSMs used for our offset measurements have a horizontal uncertainty of 0.5 cm (+1.7/-0.46) per meter of horizontal distance, and a vertical uncertainty of 0.6 - 10 cm. We generated curvature, slope, standard deviation, and topographic profile index (TPI; Jenness, 2006) derivatives of the DSMs to locate offset geomorphic piercing lines along mapped fault traces. We calculated slope and curvature grids based on nearest-neighbor approaches, while standard deviation grids were calculated over 0.05 - 7 m radii. For TPI analyses, we used fine-scale and coarse-scale annuli to identify offsets in short- and long-wavelength topography. The fine-scale annulus neighborhood consisted of an inner and outer radius of 1.5 m and 4.5 m, respectively, and permitted the identification of offset short-wavelength and small-amplitude bar and swale morphology. The coarse-scale annulus neighborhood consisted of an inner and outer radius of 15 m and 45 m, respectively, and highlighted ruptures that offset entire alluvial fan surfaces.

We measured lateral, vertical, and total offsets using piercing points defined by the intersection of bar edges and crests, swale thalwegs, and terraces risers with the traces of surface ruptures. Each piercing line was reconstructed using one of three methods: 1) field reconstructions of piercing points, 2) backslipped geomorphic features using the LaDiCaoz_v2 algorithm, and 3) Monte Carlo reconstructions of topographic profile regressions. In this dataset, each offset measurement is given a specific identifier where field measurements begin with “F”, backslipped measurements begin with “B” and reconstructions of topographic profiles begin with “T”. Right-lateral field measurements were taken by placing a measuring tape along the trace of the surface rupture and approximating a minimum, most likely, and maximum piercing line projection into the trace of the rupture, using several piercing lines along a single geomorphic feature (bar, swale, channel thalweg, terrace riser, etc). We measured statistically backslipped right-lateral, vertical, and total offsets using the LaDiCaoz_v2 algorithm, which cross-correlates topography on opposite sides of a fault using relief, width, and degree of symmetry (Zielke and Arrowsmith, 2012; Haddon et al., 2016). We measured vertical separation, including heave and throw, using a Monte Carlo method that measures the uncertainty of up-thrown and down-thrown surface regressions, and the location of the scarp midpoint, assuming fault dips of 70° – 90° (after the methods of Morell et al., 2017 and Thompson et al., 2002). We report the mean (µ) and standard deviation (σ) of lateral and vertical offsets, the ratio of vertical to lateral displacement, and the average total offset for each site, where applicable. All values are reported in meters (m).

Additionally, we assign a confidence value to each offset measurement, ranking the piercing line(s) and piercing point(s) on a scale from 1 (high confidence) to 3 (low confidence), based on several qualitative indicators to assess the preservation quality of each piercing point. These descriptors include assessing the obliquity of the piercing point projection intersection with a surface rupture, the sharpness of the piercing point edges, the distance of separation between the end of the piercing point and the fault trace, as well as the likelihood that modification, alteration, or bioturbation had affected the trace of the piercing point or the trace of the surface rupture.