Prof. Ron Harris Ph.D, BYU
Wasatch Fault Presentation given Nov. 9, 2005

Large Earthquakes on the Wasatch Fault

Average Repeat Time
     The combined average repeat time for large earthquakes (magnitude greater than 7) on any of the five central segments (Brigham City, Weber, Salt Lake City, Provo, and Nephi segments) of the Wasatch fault zone is 350 years. The average repeat time on any single segment ranges from about 1,200 to 2,600 years. The time since the last earthquakes on the five central segments ranges from 620 to 2,120 years.

Earthquake Probability
     Based on the historical earthquake record and assuming earthquakes are random, the probability of a large earthquake

• somewhere in the Wasatch Front area is 16 percent in 50 years and 30 percent in 100 years.

     Based on geologic studies and assuming earthquakes are random, the probability of a large earthquake

• on the central segments of the Wasatch fault alone is 13 percent in 50 years and 25 percent in 100 years.

     Assuming that large earthquakes occur regularly and not randomly, the probability of a large earthquake

• on the Weber, Provo, or Nephi segment of the Wasatch fault is only 1-7 percent in 100 years, because of the short times since the last earthquakes on these segments.
• on the Brigham City or Salt Lake City segment is much greater because the time since the last earthquake is equal to or greater than the average repeat time. On the Salt Lake City segment, the probability may be as high as 57 percent in 100 years.

Reference
McCalpin, J.P., and Nishenko, S.P., 1996, Holocene paleoseismicity, temporal clustering, and probabilities of future large (M>7) earthquakes on the Wasatch fault zone, Utah: Journal of Geophysical Research, February, 1996.