SHALLOW SUBSURFACE STRUCTURE OF THE WASATCH FAULT ZONE FROM SEISMIC REFLECTION PROFILES

As part of a joint effort between the U. S. Geological Survey and Brigham Young University, seismic reflection and first-break tomographic profiles have been generated across two different segments (Provo and Provo-Salt Lake transition) of the Wasatch fault zone in Utah. The results reveal near-surface and shallow bedrock structures caused by geologically recent deformation, including features not previously detected by surface mapping. Combining information from the seismic surveys and independent geological data provides well-constrained cross-sections of the upper 500 m of the subsurface. Faults can be mapped from the surface, through shallow, poorly consolidated sediments, and cutting through a rigid bedrock surface. The new seismic data can be used to test various hypotheses, such as how fault orientation changes with depth and how subsidiary faults are distributed within the fault zone. Although previous surface mapping has indicated only a few faults in our study areas, the seismic sections show a complex deformation zone with both synthetic and antithetic normal faults and localized grabens. Our results indicate the feasibility of mapping fault zones in rugged terrain and complex near-surface geology using low-frequency vibroseis. Further, the integration of geologic mapping and two types of seismic profiles can extend surface observations in areas where structural deformation is obscured by poorly stratified or otherwise unmappable deposits.