Evaluation of Wasatch fault segmentation and slip rates using Lake Bonneville shorelines

Abstract

ABSTRACTAnalysis of Lake Bonneville shorelines using lidar digital elevation data challenges accepted models of Wasatch fault deformation since the late Pleistocene. While footwall deformation of the Weber segment of the Wasatch fault is consistent with back‐rotation of the footwall block and greatest displacement rate toward the center of the segment, shorelines along the footwall of the Salt Lake City segment decrease in elevation toward the interior and are highest at the segment boundaries, an opposite pattern of footwall deformation than predicted for boundaries arresting or strongly inhibiting displacement during earthquakes. The spatial pattern of footwall rebound implies that some of the proposed persistent fault segment boundaries do not stop earthquake ruptures that originate on adjacent fault segments, nor constrain ruptures initiated within the Salt Lake City segment. Net vertical fault displacement at the boundary between the Salt Lake and Provo segments is 16–20 m over the past 16.3–18.5 ka, corresponding to a vertical displacement rate of 0.8–1.2 mm/yr, a net fault slip rate of 2.0–2.8 mm/yr, and horizontal extension rate of 1.8–2.6 mm/yr on the 25° west‐southwest dipping fault that forms the southern Salt Lake City segment boundary. Shoreline analysis suggests isostatic rebound caused by a drop in lake level was concentrated during a relatively short (~2000 year) time period following the Bonneville flood at ~16 ka. Lidar‐derived topography in conjunction with robust geomorphic datums improves our ability to map deformation associated with lithospheric flexure and faulting while demonstrating the limitation of lacustrine shorelines in this type of analysis.