Refining the Chronology of Quaternary Movement on the Nephi Segment of the Wasatch Fault Zone

USGS: Michael Machette, Anthony Crone, Stephen Personius and David Lidke.
UGS: Chris DuRoss, William Lund, Eric McDonald
URS Corp.: Susan Olig

Although the Wasatch fault zone in Utah is probably the most-studied normal fault in the world, important information about its recent behavior is still missing for some parts of the fault. In particular, the time of the most recent (MRE) and penultimate (PE) faulting events of the Nephi segment south of Provo is still in dispute: two previous studies have been unable to accurately determine the time of the MRE, and the age of the PE can only bracketed between about 1,600 and 3,300 years ago.

Obviously, these discrepancies in the fault history need to be solved and soon, owing to increasing pressures on public and private lands that the fault crosses. In a recent review of earthquake hazard priorities for Utah, the Utah Geological Survey (UGS) declared that further research on the Nephi segments of the Wasatch fault zone is their highest priority (William Lund, UGS, written commun., 2004). To address this need, the USGS and the UGS (in collaboration with URS Corporation) are planning a cooperative effort to further investigate the paleoseismicity of the Nephi segment in May 2005. The work is being funded by the National Earthquake Hazards Reduction Program through the USGS. For our part, the USGS will excavate one or two exploratory trenches across the fault zone near the mouth of Willow Creek (Fig. 1) to investigate the timing, amount of displacement, and recurrence of major (magnitude 6.5+) earthquakes in the Nephi region. The study site is due east of Mona, Utah, where the U.S. Forest Service has granted access to federal land and permission to conduct the study. In this area, the fault forms a prominent scarp at the base of the Wasatch Mountains, which rise abruptly to Mount Nebo (Fig. 2). At this site, movement on the fault zone has created a prominent 6 to 7 meter high scarp (Fig. 3) on 5,000-6,000 years old alluvial deposits, and just to the north, recent movements have formed a 7 to 8 meter high scarp on locally derived fan deposits. A trenching study of these two scarps may allow us to better define the times of the last 3-4 surface faulting events on this segment, and to estimate the amount of vertical offset during each earthquake.

The results of this research lead to a better understanding of the seismic hazard posed by this part of the Wasatch fault zone and will allow State and local officials to make informed decisions about steps and policies to help reduce the hazard. Thus, this information of prehistoric earthquakes will directly benefit to the public in Juab and Utah counties. The timing and displacement information generated from such a study will feed directly into the national, state and local probablistic seismic-hazards assessments, which are key sources of information in establishing and applying national building codes. Currently, there is considerable ambiguity about the time of that last major (M>6.5) earthquake on the Nephi segment and no clear consensus about the recurrence intervals (repeat times) for older such earthquakes. If the return time is about 1,500 years and the last major earthquake was 1,200 years ago (one possible option based on the current data), then the likelihood of future movement could be relatively high. Conversely, if the last major earthquake occurred only 300-400 years ago (another option based on the current data), then the likelihood of movement in the near future (i.e., 30 years) could be relatively low. Ground shaking caused by a M6.5 or larger earthquake on the Nephi segment of the Wasatch fault zone would have a major impact on Juab and Utah Valleys, the latter of which contains roughly 200,000 people and associated infrastructure. Thus, knowledge of the Nephi segments recent history of movements directly affects social and economic decisions that impact a large number of people in Utah.