CHARACTERIZATION OF SEGMENTATION AND LONG TERM SLIP RATES OF WASATCH FRONT FAULT SYSTEMS, UTAH

The segmentation model of the Wasatch Fault Zone [WFZ] in north-central Utah has been central to understanding normal fault systems around the world. In a recent paleoseismology study, late Holocene paleoseismic data allows for partial-segment, spillover, and multisegment ruptures along the central segments of the WFZ. These findings call into question the classic segmentation model and complicate seismic hazard analyses. This study tests the segmentation model by examining displacement profiles of Lake Bonneville highstand shorelines along the Brigham City, Weber, Salt Lake City, and Provo segments. Displacement of these late Pleistocene shoreline features indicates whether classic segment boundaries act as barriers to rupture over longer timescales.

Methods for extracting shoreline elevations incorporate and adapt previous studies of Lake Bonneville shoreline deformation, where surface projections of the wave cut platform and sea cliff were used to define an elevation “datum”. This datum represents the elevation of the shoreline at its formation. Transects of Bonneville and Provo shoreline highstands are extracted from +/- 20 cm vertical accuracy lidar (2013-2014) sponsored by the state of Utah. Along the Salt Lake City and the Provo segments, urban development has encroached on or completely erased these shoreline features. In order to produce a complete displacement profile along each segment, digital elevation models of historical aerial photographs (1935-1959) are built in Agisoft Photoscan. These models are referenced to the lidar and have the same vertical accuracy.

Displacement profiles along the Brigham City segment do not exhibit the “half-ellipse” model of normal fault segment displacement, rather two half-ellipses are evident in the shoreline displacement profiles. These half-ellipses overlap at a jog in the fault near Box Elder Canyon, indicating that the Brigham City segment likely includes a complex subsegment boundary. This type of displacement pattern could also indicate the linkage of two older fault segments, supporting the idea that there can be complex interactions between segments.