USING LIDAR DEMS FOR GEOMORPHIC ASSESSMENT OF LAKE BONNEVILLE WAVE-CUT TERRACES AND POST-BONNEVILLE DISPLACEMENT ALONG THE WASATCH FAULT

The Wasatch Mountains are a product of normal displacement across the Wasatch Fault (WF). During the Late Pleistocene the Mountains were the eastern shore of Lake Bonneville. At the Lake’s highstand and during periods of stability as it receded, wave-cut terraces were formed. Since that time, multiple surface rupturing earthquakes along the Salt Lake City segment of the WF have crisscrossed these terrace surfaces recording approximately 18,500 years of slip along the fault. In addition to discrete fault displacements, isostatic rebound has uplifted the terraces following the removal of the Lake’s mass. In 2008, LiDAR was acquired for the Salt Lake City region. This dataset presents the opportunity to estimate the elevations of these displaced terraces across the entire region, providing a high frequency sample of the relative displacements due to faulting along this segment. Understanding slip distribution over multiple earthquake cycles has the potential to improve our understanding of fault structure, segmentation and earthquake hazard.

Initially, we used a GIS approach to determine the elevations of the Bonneville terraces. Using preexisting geomorphic mapping we created 3-meter wide symmetrical buffer along ~100 meter sections of the mapped Bonneville polylines and extracted descriptive statistics for each of these areas based upon the 2008 LiDAR. This extraction suggested that variability in surface slip over the last 18.5 ka corresponds to mapped steps in the surface trace of the WF. However, the statistics also revealed that some of the sample polygons have large standard deviations in surface elevation (up to ± 12 meters). Geomorphic mapping using the LiDAR leads to two explanations for these deviations. Firstly, some of the polylines from pre-existing mapping are inaccurately positioned within the landscape. Secondly, significant geomorphic modification has occurred on these surfaces. This has led to inflation of the surface at the mapped wave-cut edge, but the amounts of inflation depend upon whether inflation is due to colluvium or fan deposition. This initial GIS method has been useful for identifying mapping inaccuracies and sites of geomorphic change. We are now expanding our analysis with a selective profile extraction method in order to interpolate the inter edge of these wave-cut terraces.