SHALLOW SUBSURFACE STRUCTURE OF THE WASATCH FAULT, NORTHERN UTAH, FROM AN INTEGRATED COMPRESSIONAL AND SHEAR WAVE SEISMIC REFLECTION PROFILE

Integrated compressional (P) and horizontally polarized shear (SH) wave seismic reflection and first-break tomographic profiles across the Wasatch fault zone in Utah reveal near-surface and shallow bedrock structures caused by geologically recent deformation along the Wasatch fault at the Colorado Plateau-Basin and Range transition. Combining information from the seismic surveys, geologic mapping, and terrain analysis provides a well-constrained cross-section of the upper ~500 m of the subsurface. Faults can be mapped from the surface, through shallow poorly consolidated deltaic sediments, and cutting through a rigid bedrock surface. The new seismic data were used to test hypotheses on changing fault orientation with depth, on the density of offsets within the fault zone, and on the utility of integrating separate acoustic strategies to provide information on a complex structural zone. The tomographic velocity profile provides a generalized picture of subsurface structure, including evidence of strong deformation near the main surface trace of the Wasatch fault. Although previous surface mapping has indicated only a few faults, the seismic-derived cross section shows a complex deformation zone with both synthetic and antithetic normal faults. Near the main surface trace of the Wasatch fault, the reflection profiles show offset strata near the surface and steeply inclined surfaces deeper. These deeper surfaces may be inclined strata or segments of the fault surface itself. The inclination of the faults decreases from almost vertical near the ground surface to shallower dips within bedrock strata. Our study demonstrates the need for a shallow geophysical survey (small hammer and plate SH-wave source, common-depth point interval equals ~0.76 m), a deeper penetrating survey (P-wave vibroseis source, common-depth point interval equals ~2.5 m), integrated with precise geologic surface mapping in order to constrain subsurface fault structure. Due to the extreme geologic complexity and steep dips of the ground surface and of buried structures, accurate seismic velocity information is essential. The new constraints on fault geometry may be used to refine estimates of vertical versus lateral tectonic movements and seismic hazard assessment along the Wasatch fault through the Provo urban area.