Douglas Metcalf and Dr. Matthew Bekker, Geography Department

Geologists use various dating techniques to determine the ages of geologic phenomena. Dendrochronology- the analysis of annual growth-rings in trees- is one of these methods (Stoffel and Bollschwieler, 2008). Unlike other dating procedures, it can determine the exact years in which various events occurred. Such events include earthquakes, which produce distinct responses in tree-ring growth patterns (Jacoby, 1997).

Dendrochronologic research has been done by my mentor on normal faults in the western United States. Plate boundaries have been researched extensively. Away from these, dendrochronologic earthquake studies are rare, including those by Bekker (2004) and Sheppard (1995). This ongoing project is a continuation of my mentor’s research. We have obtained preliminary results for research at a site in Utah, where such a study has never been done. Additional research has potential to continue until my graduation, three years from now. The ideal goal of our project is to pinpoint the occurrence of an unknown earthquake (preferably in Utah), by studying trees growing near fault scarps.

Finding tree-ring evidence for a previous earthquake requires trees old enough to have experienced the event. The closer to the fault these trees are, the more likely it is that they were damaged and have a response to that damage recorded in their rings. One major drawback for us was that Utah’s oldest trees usually grow at high elevations, while most of its active faults lie much lower, near the base of the Wasatch Front.

In our search for a tree-sampling site we sought help from a geologist at the Utah Geological Survey in Salt Lake City. He answered many of our questions and helped us find Google Earth imagery (USGS, 2006) showing active faults. This was more available and easy to examine, so it replaced our original GIS (Geographic Information Systems) data. After over five months of extensive searching, we found a location just south of Santequin, near the Mona Reservoir, on the southern portion of the Nephi segment of the Wasatch Fault. Imagery of this area showed many large coniferous trees growing near fault lines. According to Machette et al (2009), the Wasatch fault’s most recent major earthquake occurred here between 150 and 400 years ago. While not entirely unknown, the wide time range for this event made further investigation of the area worthwhile.

We found two potential sampling sites here and thoroughly traversed one of them on our first visit. After examining the trees and taking two samples, we concluded that none of them were sufficiently old to provide useful data, which was frustrating. On our next visit, we sampled several trees at the second site. This site showed much greater potential, and we named it the NFS (Nephi Fault Scarp) site. Two more day-long visits were made between July and September. 14 samples have been collected thus far.

These “core samples” (about the width of a pencil) were taken from the center of the trees’ trunks with a handheld mechanical drill. (This process is harmless to trees.) All samples are currently being dried, mounted, and sanded to maximize the clarity of the rings, which will be analyzed in detail with a binocular microscope. Any earthquake responses found will then be documented. The NFS samples will be compared with each other and with a set of control specimens not affected by the earthquake. This will solidify dating accuracy (Stokes and Smiley, 1968), and ensure that any responses found are not due to more widespread causes, like fires, disease or climatic variations.

Preliminary results indicate that only one of the 14 trees sampled (about 180 years old) reaches into the time range for the earthquake, though this tree is smaller and appears younger than the others sampled. Despite the younger ages of most trees at the site, this anomaly indicates that there may be additional older trees growing nearby. Even if no evidence for the earthquake is found, a sufficient number of such trees may narrow the time range for it. If trees show a response to this event, however, the year of its occurrence will be calculated and published. This precise date will provide clarity to the seismic history of the Wasatch Fault, and may aid geologists in predicting when or where the next major earthquake in Utah could occur. That information could be useful in preparing at-risk populations for the earthquakes prior to its occurrence. Such knowledge may aid future research as well as emergency planning efforts in other regions.

If this site does not provide the year of this earthquake, our research will continue at another location, possibly outside of Utah. Such areas include Borah Peak, Idaho, where an earthquake struck in 1983, or eastern Colorado, where one occurred in 1882. A study at Borah Peak would further establish dendrochronology as an earthquake dating method. It is uncertain which fault produced Colorado’s largest known (1882) earthquake, and sampling trees on several faults there may resolve this uncertainty. The final results of our research will be published as a co-authored article in a peer-reviewed journal. I am grateful for this opportunity, am learning much and hope to benefit others through it.

References

• Bekker, M.F., 2004, Spatial variation in the response of tree rings to normal faulting during the Hebgen Lake Earthquake, Southwestern Montana, USA: Dendrochronologia, v. 22, p. 53-59.
• Sheppard, P.R., 1995, Tree-ring responses to the 1978 earthquake in Stephens Pass, northeastern California: Geology, v. 23, no. 2, p. 109-112.
• U.S. Geological Survey (USGS), 2006, Quaternary fault and fold database for the United States, accessed Oct 7, 2009, from USGS web site: http//earthquakes.usgs.gov/regional/qfaults/.
• Machette, M., Crone, A., Personius, S., Lidke, D, DuRoss, C., Lund, W., McDonald, E., and Olig, S., 2009, Refining the Chronology of Quaternary Movement on the Nephi Segment of the Wasatch Fault Zone, accessed Oct 7, 2009, from USGS website: http://earthquake.usgs.gov/regional/imw/imw_nephi/.
• Jacoby, G.C. 1997. Application of tree ring analysis to paleoseismology, Review of Geophysics, v. 35, p. 109-124.
• Stoffel, M., and Bollschwieler, M., 2008, Tree-ring analysis in natural hazards research—an overview: Natural Hazards and Earth System Sciences, v. 8, p. 187-202.
• Stokes, M.A. and Smiley, T.L., 1968, Introduction to tree-ring dating, Chicago: University of Chicago Press.