NEW CONSTRAINTS ON THE TIMING OF PREHISTORIC EARTHQUAKES ON THE NORTHERNMOST PART OF THE NEPHI SEGMENT OF THE WASATCH FAULT, UTAH

The Wasatch fault, which runs right through the heavily populated Wasatch Front urban corridor, appears to be divided into eleven segments. The area along the Nephi segment of the Wasatch fault is undergoing rapid urbanization, so an understanding of the earthquake history of the Nephi segment is crucial. Previous studies conducted on the southern strand of the Nephi segment constrained the timing of the three most recent earthquakes to 150-400 yr BP, 1,100 -1,400 yr BP, and 1,600-2,550 yr BP (Crone and others, 2008). A previous study on the northern strand of the Nephi segment in found evidence of an earthquake at about 500 years ago, consistent with the younger event identified on the southern strand of the segment (DuRoss and others, 2008).

We excavated a trench across a well-defined scarp on the the northern strand of the Nephi segment during the summer of 2007, north of the site that was excavated by DuRoss and others (2008). Our trench exposed evidence of two large prehistoric earthquakes. The most recent of these quakes produced about three meters of vertical displacement of the ground surface. The trench also exposed organic-rich soil layers that were offset by the earthquakes. Radiocarbon dating of organic material indicates that the two earthquakes occurred at about 2,500 years ago and 3,500 yr BP. The earthquake at about 2,500 years ago is consistent with the event at 1,600-2,550 yr BP that had previously been identified on the southern strand of the Nephi segment by Crone and others (2008). The earthquake at about 3,500 years ago was not apparently identified by previous studies of the Nephi segment. The lack of evidence in our trench for earthquakes at 150-400 and 1,100 -1,400 years ago, and the lack of evidence in DuRoss and others' (2008) trench for any earthquakes older than about 500 years, highlights the difficulty in establishing a full paleoseismic record, even for faults with relatively simple geometry and well-defined scarps.