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 region is 25 percent in 50 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 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 is 16.5% 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.

Related News Release
Major Wasatch Front Earthquake More Likely Than Originally Thought (January 1996)