Testing Tectonic Geodesy

From UNC Tectonic Geomorphology

Gravel Hills fault cutting the southwest side of the Black Mountains basalt field, Mojave Desert, California.

Understanding continental deformation and its resultant seismic hazards requires linking of observations over several orders of magnitude of time. This project aims to understand how fault displacements from earthquakes relate to cumulative fault displacement over 100,000s to millions of years. The target of this study is the Mojave Desert portion of the Eastern California Shear Zone, where rapid geodetic rates of strain appear to disagree with long-term fault slip rates.

Primary (red) and secondary (blue) fault of the Pacific - North America plate boundary of California. Mojave Desert region shaded green.

LiDAR topography of fault scarp cutting Late Quaternary terrace surfaces, Lenwood Fault, Mojave Desert, California. Image shows area 600 meters in length. Pimple-like features are creosote bushes.

Probing the consistency of rates derived from geodetic, paleoseismic, and longer-term geologic deformation is a first step toward development of predictions of fault system behavior that are testable against the geologic record. The Mojave Desert portion of the Eastern California shear zone contains a very well-exposed system of active faults with an ongoing cluster of earthquake activity that is potentially correlated to an elevated rate of geodetic strain accumulation. Paleoseismic records indicate prior clustered earthquake activity across the fault system but at a time-averaged rate insufficient to balance the geodetic strain accumulation rate. Longer-term fault slip rates that average several earthquake cycles test the magnitude of geodetic-geologic rate discrepancy and its relationship to paleoseismicity. Youthful basaltic volcanism and geomorphic systems punctuated by climate-driven impulses of sedimentation record the 10⁴- to 10⁵-year time-scale of integrated fault slip in a transect of six active dextral faults in the central Mojave Desert. New tools of high resolution airborne LiDAR topography and cosmogenic dating of surficial deposits enable quantification of rates from the interaction of landscapes with fault slip. New results show that individual fault slip rates vary by an order of magnitude in the Mojave Desert ECSZ, and that three relatively fast-slipping faults take up the majority of geologic strain accumulation here. Preliminary slip rates, together with near fault deformation, may sum to the lower end of geodetic rates measured over the province.