PRELIMINARY RESULTS OF A GEOPHYSICAL AND TRENCHING INVESTIGATION OF SHALLOW DEFORMATION ALONG AN ANOMALOUS SEGMENT OF THE WASATCH FAULT ZONE

The Wasatch fault zone is a first-order tectonic boundary, separating the Colorado Plateau and the Basin and Range provinces. Although the fault zone follows a more or less north-south trend in northern Utah, it turns toward the east or west in places along topographic salients. In this study, we report on a seismic profile and a series of trenches that cross an anomalous approximately east-west oriented segment of the Wasatch fault zone located between the Provo and Salt Lake City fault segments. We integrate our observations with LIDAR and geologic map information. The series of trenches were dug to a depth of ~3 m, logged, and photographed. A vibroseis source was used for recording a common mid-point reflection profile with sweep frequencies of 20-160 Hz and 96 28-Hz geophones spaced ~3 m, providing a stacked section with an estimated maximum depth of ~400 m. A tomographic velocity model was derived from the first breaks and used to perform source and receiver static corrections, required due to the complex terrain and expected significant lateral velocity contrast. The migrated seismic profile reveals a ~200-m wide asymmetric graben in the hanging wall of the fault zone on which both synthetic and antithetic faults are interpreted where reflector offsets propagate upward toward the flanks of surface topographic depressions. These depressions correlate to zones of colluvial wedge development observed in adjacent trenches. Faults mapped from trenches and the seismic profile appear to correlate and also match up with topographic alignments derived from LIDAR gradient maps and regional geomorphology. Preliminary measurements of the fault parameters suggest apparent dips of 50-60 degrees that possibly become steeper with depth. Although the fault zone is likely broken up by numerous small faults, the long-wavelength structure in the hanging wall is fairly simple and dominated by two inward-facing ruptures. Our results indicate the feasibility of detecting and mapping fault zone deformation in an area of rugged terrain and complex near-surface geology using a relatively low-frequency vibroseis source. The integration of surface information, trenching, and seismic reflection provides constraints for assessing geologic hazards in an area of potential residential development.