EVIDENCE OF EARTHQUAKE-INDUCED SURGING IN PLEISTOCENE LAKE BONNEVILLE CHALLENGES A WIDELY ACCEPTED 130-YEAR-OLD THEORY AND OPENS QUESTIONS RELATED TO THE BONNEVILLE FLOOD

An axiom in geology for the last 130-years has been that the depression along the Wasatch Fault at the mouths of Big and Little Cottonwood Canyons in Utah, and which cuts the glacial moraines of Little Cottonwood and Bells Canyons, is a graben (subsiding block) in the Wasatch Fault. This was first presented by G. K. Gilbert in 1890 and has been immortalized in scientific studies of the area since.

The prominent “graben” in this area are instead the fissures at the top of shifts of large areas of glacial moraine and deltaic materials which occurred during a major earthquake when Pleistocene Lake Bonneville was near its high point. Earthquake-induced waves (surging, tsunami, seiche) precipitated the shift of these masses sliding on underlying transgression lakebed sediment. The underlying sediment was in a state of liquefaction due to the combination of seismic sheer forces and saturation from the high lake level.

Over 27 square kilometers shifted under earthquake-induced surging in Lake Bonneville, beyond just shallow, shoreline effects. The scope of effect identified may assist in predicting the surge hazards associated with other large inland bodies of water on major fault lines.

The model holds implications regarding the Bonneville Flood and evidence of early human habitation in the area.