September 27, 2005

On Friday, October 14, 2005, from 1:30 to 2:30 p.m., you can visit a trench excavated across the Wasatch fault and learn about earthquake history along the Nephi portion of the fault.

This summer geologists excavated trenches on the Nephi segment of the Wasatch fault zone in the Santaquin and Nephi areas to better understand how often large (magnitude ~7) earthquakes have occurred here over the past 5000 years, how large they were, and how regularly they occurred.

This information will help scientists evaluate the likelihood of the next large earthquake in the area, and calculate the severity of earthquake ground shaking that should be expected in the future. It will also help determine whether Provo or Nephi was the site of the most recent large earthquake on the Wasatch fault.

The Utah Geological Survey (UGS) excavated trenches near Santaquin on the Nephi segment of the Wasatch fault zone. And earlier this summer, the U.S. Geological Survey excavated trenches near Mona, also on the Nephi segment.

The Wasatch fault zone is Utah’s longest and most hazardous fault, extending about 220 miles from southern Idaho to central Utah, along the base of the Wasatch Range. The fault is divided into ten segments, each thought to be capable of generating large-magnitude earthquakes. These new fault trenches are along a southern part of the fault, where the history of prehistoric earthquakes is poorly understood.

For those interested in visiting the trench site, a nontechnical field review is scheduled for Friday, October 14, 2005, from 1:30 to 2:30 p.m. During this informal review, UGS geologists will present preliminary results of the Santaquin trench study.

Be prepared for hot, cold, and/or inclement weather. Bring water, sun protection, and a hard hat if you have one; hard hats must be worn by those entering the trench. A limited number of hard hats will be available during the review. High-clearance or 4-wheel-drive vehicles are not required to reach the site.

Please meet at the trench site. To reach the site coming from the north take Santaquin exit 244 from I-15. At the stop sign, turn left (east) and continue under the interstate and north on a frontage road for 0.4 miles. Turn right (northeast) into a residential subdivision (1st entrance) and continue ~0.25 miles southeast to a dirt parking area. Please park here and walk along a dirt Forest Service road east (uphill) 0.1 mi to a Forest Service sign; take the left fork downhill 0.1 miles to the trench site. The Forest Service road to the site is passable with a high-clearance vehicle; however, parking is limited – please park near the subdivision unless you have special needs.

This project is partially funded by a grant from the U.S. Geological Survey National Earthquake Hazards Reduction Program. For additional information contact Chris DuRoss (801-537-3348; christopherduross@utah.gov) or William Lund (435-865-9041; billlund@utah.gov) at the Utah Geological Survey, 1594 W. North Temple, #3110, Salt Lake City, UT 84116.