DETECTION OF A LANDSLIDE GLIDE PLANE USING SEISMIC REFLECTION METHODS AT SOUTH MOUNTAIN IN DRAPER, UTAH

Landslide hazards are a growing concern along the Wasatch Mountains in Utah as residential developments continue to expand to higher elevations. The South Mountain development on the bench along the northern flank of the Traverse Mountains near Draper, Utah (immediately south of greater Salt Lake City) has encountered a conflict from developing near or over an area mapped as a landslide. Radiocarbon age dating of undeformed sediments overlying the landslide area aged at 10,000 years before present, while sediments up slope were aged at about 4,000 before present. Common-depth-point high resolution seismic reflection profiles have been acquired during the Summer and Fall of 2004 with ~3-m group intervals, 28-Hz geophones, and an accelerated weight dropper (~45-kg hammer) P-wave source. Five 24-fold profiles were acquired throughout the development area in order to locate a possible landslide glide plane. The reflection data have been processed at Brigham Young University in order to produce a quality image of the landslide structure suitable for interpretation. A borehole was drilled next to one of the profiles to constrain the data to a known lithology. A clay layer at about 55-56 m depth was observed in the borehole and independently interpreted as a possible glide plane interval. This clay layer is directly correlated to a prominent, low-frequency reflection on two of the seismic profiles. The interpretation from this study will be useful in constraining slope stability models for the area. Our integrated geological and geophysical strategy allows for a relatively inexpensive method to investigate landslide hazards for sites under development. This project is a collaboration between Brigham Young University (BYU) and Intermountain GeoEnvironmental Services (IGES).