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. For example, the time of the most recent (MRE) and penultimate (PE) faulting events of the Nephi segment south of Provo were still uncertain as recently as 2005: two previous studies were unable to accurately determine the time of the MRE, and the age of the PE could only be bracketed to have occurred between about 1,600 and 3,300 years ago.

Details of the faulting history along this section of the fault needed to be investigated promptly because the increasing pressure on public land and development of private land that the fault crosses. The Utah Geological Survey (UGS) recognized that research to clarify the history of large earthquakes on the Nephi segment of the Wasatch fault zone was a very high priority (William Lund, UGS, written commun., 2004). To address this need, the USGS and the UGS (in collaboration with URS Corporation) conducted companion and cooperative studies to further investigate the paleoseismicity of the Nephi segment in 2005. The work is being funded by the National Earthquake Hazards Reduction Program through the USGS. The USGS excavated 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 Utah Geological Survey investigated the Nephi segment at a site near the town of Santaquin, Utah, about 20 km north of the Willow Creek site (DuRoss and others, 2008).

The Willow Creek site is due east of Mona, Utah, where the U.S. Forest Service 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 6-8 meter high scarp (Fig. 3) on 3,000-year-old and older alluvial-fan deposits. Trenches excavated across the scarp at the Willow Creek North (WCN) and Willow Creek South (WCS) sites allowed us to better define the times of the last three surface-faulting events on this segment and to estimate the amount of vertical offset during each earthquake.

See detailed maps of the trenches, the age data, and a more complete description of the Willow Creek study

Our data indicate that the most recent event on this part of the Nephi segment occurred about 150-400 years ago, shortly before the historical settlement of Utah.  The previous two earthquakes occurred about 1,100-1,400 and 1,600-2,550 years ago, respectively.  (http://www.seismosoc.org/meetings/showabstract.php?recid=38131).  Our study has refined the chronology of recent earthquakes on this part of the fault and indicates that the recurrence interval between the last three events is about 1,000 years.  This is considerably shorter than previously thought.

The results of this research has lead to a better understanding of the seismic hazard posed by this part of the Wasatch fault zone and allows 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.

The evidence that the most recent large earthquake that ruptured the fault to the surface occurred only a few hundred years ago is important because it may indicate that the likelihood of movement in the near future (that is, 30 years) could be relatively low. Ground shaking caused by a large (M 6.5 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.

References

DuRoss, CB., McDonald, G.N., and Lund, W.R., 2008, Paleoseismic investigation of the northern strand of the Nephi segment of the Wasatch fault zone at Santaquin, Utah: Utah Geological Survey Special Study 124,  Paleoseismology of Utah, v. 17, 30 p., 1 pl., ISBN 1-55791-789-2.