THE APPLICATION OF OSL TO DATING PREHISTORIC SURFACE-FAULTING EARTHQUAKES ALONG THE WASATCH FRONT URBAN CORRIDOR: A CASE STUDY AT THE PENROSE DRIVE SITE, SALT LAKE CITY, UTAH

Quartz and/or potassium feldspar-bearing sediments that have been exposed to sufficient sunlight prior to burial can potentially yield accurate optically stimulated luminescence (OSL) ages. Eolian sediments are most suitable for OSL dating, but fluvial, lacustrine, and colluvial sediments can yield accurate ages if they are deposited over an appreciable amount of time prior to burial. Thus, OSL can be a very useful technique for dating deposits related to surface-faulting earthquakes, which generally contain subaerially transported sediment eroded from exposed fault ruptures. OSL is particularly useful in arid and semi-arid climates where carbon-bearing material may be sparse. Although sediments in fault-related deposits may only be transported short distances (a few meters), the advent of single–aliquot regeneration (SAR) and single-grain luminescence analyses can help resolve an inherited luminescence signal from those grains not fully exposed to sunlight. OSL dating of surface-faulting earthquakes has been applied worldwide, and has been used extensively in Utah and elsewhere in the Intermountain West.

During the last six years, the Utah Geological Survey and U.S. Geological Survey have collaborated in studies at eight sites along the Wasatch Front urban corridor to define the history of surface-faulting earthquakes and to refine knowledge about the area’s significant seismic hazard. Using both OSL and radiocarbon dating, these studies have revealed new details about the times of past large earthquakes and the interaction between fault segments, and have raised valuable questions about the application of OSL dating to fault investigations. For example, “when does the luminescence signal saturate in these sediments?”, “what is the best mineral to date?”, “what part of the fault-related deposits are best to date?”, and “how do radiocarbon and OSL results generally compare in these studies?” This presentation will address these questions by considering the results of OSL analyses of pre-Bonneville fan gravels, Lake Bonneville silts, and fault-scarp colluvium from two trenches at the Penrose Drive site in Salt Lake City, Utah.