A high‐resolution seismic reflection and gravity survey of Quaternary deformation across the Wasatch Fault, Utah

Abstract

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°W at the surface to an estimated 45°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.