Investigating the Spatial Extent of a Barely Prehistoric Earthquake on the Bear River Normal Fault, Wyoming and Utah

To better constrain the length of a young prehistoric (significantly post-AD 1630) surface-rupturing earthquake recently discovered near the south end of the Bear River normal fault in Utah (Hecker and Schwartz, 2015), we excavated a trench on a strand of the fault 25 km to the north in Wyoming, where previous work had found clear evidence of two older late Holocene events (West, 1993). These two events, which have been identified to the south as well, were interpreted as comprising the entire history of this very young fault. The new trench across the 5-m-high scarp at the northern site exposed a 6-m-wide zone of faulting and two packages of colluvial-wedge deposits, each tabular and 0.5-1 m thick. The colluvial deposits, which bury Pleistocene alluvial deposits that in turn overlie Eocene bedrock, appear correlative with West's two-event stratigraphy. In the latest trench, however, both wedges are faulted, with strands extending to the ground surface, evidence of a third, younger event. The amount of displacement in the most recent event (MRE) in the trench is small (few 10s of cm at most) and distributed and has resulted in only minor colluviation. The event record is complicated by a shallow slope failure in the soil A-horizon on the scarp that we interpret as possibly occurring during the MRE. The slide formed a head scarp at a location underlain by MRE faulting and built a low bench at least 100 m long on the surface below the scarp. We sampled buried in-place soil below the slide for radiocarbon analysis, which should allow age comparison with the earthquake identified farther south. Ultra-high-resolution topography from balloon photography and terrestrial lidar enable detailed morphologic study of surface processes and deformation at the site.