A HIGH-RESOLUTION SEISMIC-REFLECTION AND GRAVITY SURVEY OF QUATERNARY DEFORMATION ACROSS THE WASATCH FAULT, UTAH

High-resolution reflection seismic and detailed gravity data were acquired across an exposure of the Wasatch fault, Utah, near a trench excavated for dating of Quaternary fault displacement. The seismic data across the Quaternary Wasatch fault are interpreted to show three subparallel surfaces dipping from 70-degrees-W at the surface to an estimated 45-degrees-W at 40 m, displaced unconsolidated sediments, and colluvial material abutting the main fault. Detailed gravity data were of assistance in mapping the sediment-bedrock interface from the rangefront across the Wasatch Fault Zone. A combined high-resolution seismic and gravity interpretation reveals that bedrock is 80 m deeper in the hanging wall than in the footwall, suggesting that displacement has occurred on several en echelon blocks in the near-surface of the Wasatch fault. Movement on the most recent Wasatch fault trace began prior to deposition of Pleistocene lacustrine sediments. The twelvefold, high-resolution reflection seismic data were recorded and processed to achieve vertical resolution of approximately 1 m and horizontal resolution (Fresnel zone radii) of as good as 5 m on the deepest reflecting interfaces at 40 m. Reasonably high frequencies and good bandwidth (about 80-300 Hz) in the stacked seismic data permitted a direct comparison to the trenched cross section. Good correlation between the trench stratigraphy and structure and the seismic data suggests high-resolution seismic data can be a valuable tool for analyzing near-surface faulting in unconsolidated sediment and for locating potential trench sites.