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.

Combining seismic reflection with multifold wide‐aperture profiling: An effective strategy for high‐resolution shallow imaging of active faults

“…The strongest events, around 80 ms and 50–70 m distance, arch before truncating against the northernmost fault (C). This feature could indicate colluvial wedges deposited behind a fault [ Stephenson et al , 1993].…”

Tomographic imaging of late Quaternary faulting, Oquirrh Mountains, Utah

“…Seismic tomography inverts first‐arrival travel times to generate subsurface velocity distributions and has been used to successfully image colluvial wedges generated by surface‐rupturing faults [ Morey and Schuster , 1999; Sheley et al , 2003]. Traditional land‐based seismic reflection surveys have determined fault locations but have not clearly resolved colluvial wedges [ Stephenson et al , 1993; Morey and Schuster , 1999]. The near‐surface environment promotes scattering, strong surface waves, and static shifts making it difficult to process seismic data for subsurface reflections.…”

Outcrops and well logs as a practicum for calibrating the accuracy of traveltime tomograms

“…Seismic reflection methods, on the other hand, map acousticimpedance contrasts across layer boundaries. Seismic reflection surveying has been used in several paleoseismology applications (e.g., Stephenson et al, 1993;McCalpin, 2009), although velocity tomograms may be obtained from the same set of seismic data and are better suited for imaging colluvial-wedge deposits (e.g., Morey and Schuster, 1999).…”

“…High-resolution seismic surveys are useful in site characterization, especially prior to excavation. However, the results are commonly limited by resolution and complex stratigraphy adjacent to the faults, where vertically stacked colluvial wedges contain crucial information concerning the number and magnitudes of earthquake events represented in the subsurface deposits (e.g., Stephenson et al, 1993;Morey and Schuster, 1999;McCalpin, 2009).…”