PRELIMINARY ANALYSIS OF SEISMIC REFLECTION SURVEYS OVER POSSIBLE LANDSLIDE DEPOSITS IN THE TRAVERSE MOUNTAINS, WASATCH FRONT, UTAH

The Traverse Mountains are a complex ridge of eroding Tertiary volcanic rocks, alluvial fan sediments, and older bedrock, situated along the Wasatch Mountain Front, just south of Draper, Utah. Much of the area displays a hummocky topography characteristic of confirmed landslide deposits observed elsewhere in the Wasatch Mountains. The Traverse Mountain area, which has recently undergone concentrated residential development, has been the target of numerous geotechnical studies aimed at evaluating slope instability associated with landslide materials that might mantle parts of the ridge. In order to constrain the evaluation of portions of the ridge where further development is being considered, we conducted a program of seismic reflection surveys using a vibroseis source in which several kilometers of profiles were acquired over the northern, southern, and eastern portions of the ridge. The study is designed to evaluate the geometry and extent of possible landslide deposits and to detect geologically recent faulting. The seismic source was recorded into 120 channels using 28-Hz geophones spaced at 10-ft (~3.05 m) intervals. The profiles were oriented either in a dip or strike sense with respect to topography. Cross lines were surveyed where practicable so as to establish 3D control. The vibroseis source was used to create a 20 to 160 Hz sweep signal for 12 s followed by a recording time of 3 s. Three vibrations were usually stacked per station in order to suppress random noise. Processing of the data is challenging due to the rugged topography and due to the likelihood of strong lateral near-surface velocity variations. Derivation of a detailed shallow velocity model is therefore necessary in order to reduce distortion of the reflector image and to provide an alternate manner of detecting structural variations. Preliminary data processing has revealed reflectors that mimic to some degree the surface topography and may represent either possible landslide glide surfaces or a rigidity boundary not necessarily associated with a landslide. The geophysical results will be integrated with dedicated LIDAR imagery and detailed geologic mapping.