THE TAHOE-SIERRA FRONTAL FAULT ZONE VISUALIZED AND CHARACTERIZED WITH AIRBORNE LIDAR TOPOGRAPHY

We use high-resolution bare-earth airborne LiDAR topography to identify, map, characterize, and visualize tectonic geomorphology in densely vegetated mountainous terrain west of Lake Tahoe, California. The bare-earth LiDAR topography reveals active normal faults that displace late-Quaternary alluvium, colluvium, and glacial moraines along 30 km of linear right-stepping range front that comprises the Tahoe-Sierra frontal fault zone (TSFFZ). The LiDAR topography reveals tectonic geomorphic features such as: linear scarps in unconsolidated alluvium, colluvium, and glacial moraines; triangular facets in unconsolidated glacial till; linear side-slope troughs aligned with scarps in moraine crests; antithetic scarps; hanging-wall grabens; and back-titled moraine crests, all of which are not visible in aerial photography due to the dense vegetation.

We have developed new techniques that utilize the three-dimensional (3D) LiDAR data to mathematically constrain tectonic net slip displacements of moraine crests and vertical slip of faulted alluvium and colluvium, yielding numerically robust 3D displacement models. Recalculated terrestrial cosmogenic nuclide surface-exposure ages for Tioga (20.7 ± 2.1 ka) and Tahoe (68.8 ± 7.4 ka) age moraines at Meeks Bay, California, are used to establish minimum and maximum limiting ages for the faulted late-Pleistocene moraines along the TSFFZ. The limiting ages are coupled with the 3D LiDAR displacement models to constrain the vertical slip rate at twenty locations along the TSFFZ. The right-stepping en echelon range-front segments of the TSFFZ have progressively greater slip rates along strike to the northwest commensurate with greater extension and structural relief in the northern part of the Lake Tahoe basin (LTB). At three locations along the Mt. Tallac segment, the models constrain a vertical slip rate of 0.3 ± 0.1 mm/yr. At Emerald Bay, near the southern end of the Rubicon Peak segment, the modeled rate is 0.6 ± 0.1 mm/yr. Further north along the Rubicon Peak segment is the maximum vertical slip rate of 1.5 ± 0.5 mm/yr. Based on a range of surface-rupture lengths and depths to the base of the seismogenic zone, we estimate the range of potential earthquake moment magnitudes (M) for the TSFFZ to be between 6.3 and 7.1, which represent a substantial seismic hazard to the LTB.