Tomographic imaging of late Quaternary faulting, Oquirrh Mountains, Utah

Seismic tomography can be used to image colluvial material in the subsurface by inverting first arrival travel times for velocity. Colluvial material deposited at the base of a fault‐scarp free face often appears as a low‐velocity zone (LVZ) on a tomogram because it is generally less compacted and cemented than the surrounding alluvium. A tomogram generated from a forward model of a synthetic velocity structure successfully images two LVZs stacked in the hanging wall of a normal fault. The Mercur fan, Oquirrh Mountains, Utah, provides an opportunity to look for stacked LVZs in a distributed fault zone. Three tomographic images across fault scarps on an intermediate age alluvial fan can be used to identify two stacked low‐velocity zones. Interpretation of a fourth tomogram is less conclusive. These two low‐velocity zones are interpreted as colluvial packages separated by higher‐velocity alluvial material and suggest that tectonic activity is interspersed with pulses of fan building.