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Cottonwood Holladay Journal

The potential dangers of developing on the Wasatch Fault

Sep 07, 2018 12:57PM ● Published by Jana Klopsch

Airborne data for mapping the Wasatch Fault zone at the mouth of Little Cottonwood Canyon. (Emily Kleber/ Utah Geological Survey)

By Cassie Goff | cassie@mycityjournals.com

A constant concern for recently elected Cottonwood Heights District 3 Councilwoman Tali Bruce is the city’s fault lines and other geological hazards.

During the work session city council meeting on July 17, Adam McKean and Steve Boman from Utah Geological Survey (UGS) presented their geotechnical analysis of the area.

UGS is a hazards program working to assist the Utah Division of Emergency Management. UGS creates geologic hazard maps and provides information on hazardous lands. They frequently make recommendations to regulatory agencies by providing maps and other information. They also create a variety of publications for anyone interested.

Boman began the presentation with some preliminary background information. “A fault study was completed in 1977; trenches identified sediment about 10,000 years old. We can estimate that Lake Bonneville existed around 12,000 to 30,000 years ago,” he said.

“An earthquake is generated when rocks rupture,” Boman continued. “The basin between the Wasatch Front and the Sierra Mountains is expanding so the rocks won’t be able to take it at some point and they will rupture.”

For Cottonwood Heights, the major hazard UGS presented (besides radon) is the Wasatch Fault. It is only one of the five central faults in Utah that are most active.

McKean is a geologic mapper who was tasked with mapping the entirety of the Wasatch Fault. The fault line lies right along Wasatch Boulevard. It also sits right between two tectonic structures.

McKean began mapping the fault line based on evidence from aerial photos and other similar resources. However, that didn’t give him the full picture. He needed to go out and visit the site to try to get a more accurate mapping, but he ran into a problem.

“In the cities, development was covering the geology we were trying to map,” said McKean.

Luckily, geologic mappers have many resources at their disposal. McKean used a tool called LiDAR to help him map the Wasatch Fault. LiDAR is a detection system that works on the principle of radar, but with light. It helps to provide high resolution topographic data.

Additionally, many developers will data dump information to the UGS. Many geologic mappers can use the information collected to continually update their maps.

By the end of McKean’s mapping journey, which may take anywhere from one year and beyond to complete, he had an accurate and complete map of the area. A complete geological map provides information about human disturbance, faulting, bed rocking, landslides, and other hazards.  

“If you have a fault zone, we expect that to reoccur,” McKean said. Through fault studies, UGS uses predetermined distances to estimate where the fault will occur again. For the Wasatch Fault, there is an 80-foot wide distance of anticipated reoccurrence.

The Cottonwood Heights city planning staff tries to accommodate for at least 25-feet variability. “We don’t care where the fault is, we care where it is not,” said City Engineer Brad Gilson.

“I am advocating tightening our city ordinances regarding earthquakes,” Bruce told McKean and Boman.

“We can be a resource to help guide the process,” Boman said. “Building codes are a balance of economics. Codes deal with structure collapse — wanting to make sure that people can get out. Not every building is built like a critical facility.”

“In Utah, we haven’t had a damaging earthquake since pioneer settlement,” Boman said. “There’s a 50 percent chance we get one in the next 50 years.”

For more information, visit UGS’ website at geology.utah.gov.  

 

 

 

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