CONSTRAINING THE HISTORY OF LARGE (MAGNITUDE-7) EARTHQUAKES ON 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. Although there have been no large historic earthquakes on the Wasatch fault, geologic studies indicate that each segment is capable of producing earthquakes of about magnitude-7. Two segments closest to the Provo/Orem metropolitan area are the Provo and Nephi segments. Recent studies have indicated that the northern part of the Nephi segment may rupture at the same time as the Provo segment. If this is true, then the Provo segment may be capable of producing earthquakes larger than had previously been thought. We excavated a trench across the northern part of the Nephi segment of the Wasatch fault in order to better-constrain the history of earthquakes on the segment. The trench exposed evidence of two large prehistoric earthquakes. Each of these quakes produced about five to seven feet of displacement of the ground surface. The trench also exposed organic-rich soil layers that were offset by the earthquakes. We are in the process of determining the ages of these soil layers, which will allow us to constrain the ages of the two earthquakes. Once we constrain the ages of the two earthquakes on the northern part of the Nephi segment, we will be able to assess if those represent earthquakes that ruptures both the Nephi and Provo segments.