Terrestrial Laser Scanner analysis of diffusion degradation models in morphological dating of fault scarp surfaces

Abstract:

Morphological dating of fault scarps has the potential to provide a fast and cost-effective means of determining the spatial and temporal pattern of paleo-seismic events over broad regions. Scarp-morphology dating algorithms are based on diffusion degradation relations but have not been adequately calibrated for sensitivity or repeatability. Using Terrestrial Laser Scanner (TLS) images to characterize geomorphic surfaces at centimeter scale, we characterize fault scarps of known and differing ages ( 20 to 13,500 years) to evaluate the applicability of morphology-degradation models in estimating ages of fault scarps. We acquired TLS images of five Quaternary fault scarp surfaces formed in unconsolidated sediments in different parts of the western Basin and Range in western Nevada and southeastern California. Diffusion models assess the initial and final configuration of a scarp to yield the product of a diffusivity constant (K) and the scarp age (T). With known T we can solve for K and vice versa. Our sensitivity analysis of the diffusion models demonstrate that small variations in the measurable parameters such as maximum slope angle (θ), far field slope angle (α), and surface offset (d) can produce large variations in the calculated age of scarp formation. Even a 0.5° uncertainty in the value of θ or α will cause 10 to 20 percent variability in K, and a 25 cm uncertainty in d will make a 20 percent difference in calculated K. This uncertainty in K value will yield ± 3000 year variation in the estimated scarp age. Using 10 cm resolution digital elevation models of fault scarps acquired with TLS, we best fit profiles and digitally measure the geometric parameters. The ages of the scarps are known via historical records or through age brackets establish in trenching studies. Using the known ages and the modeled morphology, we compute K for the various scarps formed in similarly material and environments. Estimates of K range from 1.57 m²/kyr to 13.74 m²/kyr and clearly are not a constant. Using a constant K for the region of 1.1 m²/kyr estimated in previous studies, the calculated ages deviated from known ages by 40 to 1000 percent. Although the general observation of scarp morphology degradation through time holds, the reliability of dating the fault scarp using diffusion modeling is highly questionable.