April 28.1997

The Nephi segment contains evidence of what may be the most recent earthquake activity along the fault. Radiocarbon dating points to a surface-faulting event, or movement strong enough to rupture the surface, that is younger than 1,200 years. But the condition of the cliff-like slope, or scarp, that characterizes the fault, particularly the lack of vegetation, indicates that the most recent activity may have been just 300 to 500 years ago. Evidence gathered from trenches dug across the scarp shows that the next two prior events occurred 3,000 to 4,500 years ago.

The 1:50,000-scale map was completed through a cooperative project funded by UGS and the United States Geological Survey National Earthquake Hazards Reduction Program. The principal investigator was Kimm M. Harty, deputy director of UGS. The map is a scientific guide to the zone and is intended to provide basic geological data that scientists can then use to more fully assess the seismic history and hazards of the fault.

The Wasatch Fault is recognized as the most active fault zone in Utah, and the most likely source of major earthquakes to affect the populous Wasatch Front. The five segments that have now been mapped to the 1:50,000 scale, extending from Brigham City to Nephi, show evidence of activity that is more frequent than the still-unmapped segments at the extreme northern and southern ends of the fault.