GEOLOGIC HAZARDS

Utah Faults

Utah has experienced many earthquakes, large and small, because of its abundance of faults and fault zones. Some of the most active faults in Utah include the Wasatch fault along the Wasatch Front, the Hurricane fault in Southern Utah, and the Needles fault zone in Canyonlands National Park.

What is a fault and why is it a concern?

A fault is a break in the earth’s crust along which movement can take place causing an earthquake. In Utah, movement along faults is mostly vertical; mountain blocks (for example, the Wasatch Range) move up relative to the downward movement of valley blocks (for example, the Salt Lake Valley).

Faults with evidence of Holocene (about 10,000 years ago to present) movement are the main concern because they are most likely to generate future earthquakes. If the earthquake is large enough, surface fault rupture can occur.

With a large earthquake (about magnitude 6.5 and greater), the fault rupture can reach and displace the ground surface, forming a fault scarp (steep break in slope). The resulting fault scarp may be several inches to 20 feet in height, and up to about 40 miles in length, depending on the size of the earthquake.

An area hundreds of feet wide can be affected, called the zone of deformation, which occurs chiefly on the downthrown side of the main fault and encompasses multiple minor faults, cracks, local tilting, and grabens (downdropped blocks between faults). Buildings in the zone of deformation would be damaged, particularly those straddling the main fault.

Also, anything crossing the fault, such as transportation corridors, utilities, and other lifelines, both underground and above ground, can be damaged or broken. The ground can be dropped below the water table on the downdropped side, resulting in localized flooding.

Surface fault rupture can also cause tectonic subsidence, which is the broad, permanent tilting of the valley floor down toward the fault scarp. Tilting can cause flooding along lake and reservoir shorelines nearest the fault; along altered stream courses; and along canals, sewer lines, or other gravity-flow systems where slope gradients are lessened or reversed.

On the Holocene fault on which a magnitude 6.5 (approximate) or larger earthquake occurs. On average, these earthquakes may occur once every 120 years on various faults in the Wasatch Front region; once every 350 years somewhere along the central part of the Wasatch fault (between Brigham City and Nephi); once every 2,000 years at any specific locality along the central Wasatch fault; and once every 5,000 to 20,000 years or more on other Holocene faults in the state.

Faults can be avoided by setting homes back a safe distance. Special-study areas have been delineated along faults where geologic studies are recommended to assess the hazard, locate faults, and recommend setbacks. However, the use of special-study areas in land-use ordinances varies by county and city, as does the level of enforcement.

Therefore, buyers, particularly of older homes (pre-1985), should personally check available fault maps to see if the home is near a fault (within a few hundred feet) and, if so, may want a geological site investigation performed. For newer homes, buyers should check with the county or city to determine whether geologic studies were performed for the site or subdivision and, if so, look at a copy of the geologic report.

Survey Notes: Glad You Asked - Earthquake Early Warning System

Wasatch Front Earthquake Early Warning System

Earthquake early warning (EEW) systems work on the principle that an alert signal can be transmitted almost instantaneously, whereas seismic waves take longer to travel through the Earth’s crust. Earthquake early warning (EEW) systems work on the principle that an alert signal can be transmitted almost instantaneously, whereas seismic waves take longer to travel through the Earth’s crust. Sensors detect the first-arriving P wave and trigger the sending of an alert signal, which can give people and automated systems some time to take action before the arrival of stronger S waves and surface waves. This diagram illustrates a conceptual EEW system in the Wasatch Front urban corridor.

Exposing the Wasatch Fault

Past large earthquakes on the central, most active segments of the fault and how geologists interpret evidence of large, prehistoric earthquakes, with footage from the North Creek trench investigation.

North Creek Trench on Wasatch Fault

Time-lapse video of the excavation and investigation of the North Creek trench on the Nephi segment of the Wasatch fault zone.

Wasatch Fault Flyby

The Salt Lake City segment of the Wasatch fault and related geologic features.