The Wasatch Fault Zone (WFZ) is an active, approximately 240-mile-long fault system stretching along the western base of the Wasatch Range in Utah. It runs directly through the state’s most densely populated corridor, known as the Wasatch Front, representing a significant seismic hazard. The Wasatch Fault is classified as a normal fault, resulting from large-scale tectonic forces actively pulling the Earth’s crust apart in the western United States.
Defining the Normal Fault
A normal fault is a type of dip-slip fault characterized by movement that is primarily vertical along the fault plane. It involves two blocks of crust: the hanging wall (above the fault) and the footwall (beneath the fault). Tensional forces cause the hanging wall to move downward relative to the footwall. This movement creates a dramatic offset, which forms the steep western face of the Wasatch Mountains. The mountain block acts as the footwall, moving upward relative to the valley block (the hanging wall) that subsides to form the valley floor.
Geological Mechanism Driving Extension
The Wasatch Fault is a normal fault due to the ongoing extension of the Earth’s crust in the western United States. This process defines the Basin and Range Province, which includes western Utah, and the WFZ marks its eastern boundary. Tensional forces have subjected the North American plate to slow, east-west pulling, fracturing the brittle upper crust into a series of parallel normal faults. The cumulative extension has resulted in alternating long, north-south-trending mountain ranges and valleys. The Wasatch Range is one such mountain block, uplifted as the crust on its western side drops down along the fault.
Physical Segmentation of the Fault Zone
The Wasatch Fault Zone is not a single continuous crack but rather a series of distinct, linked segments. The entire fault zone is approximately 240 miles long and is divided into ten structural segments. Five segments in the central part of the fault are considered the most active in the Holocene epoch. These central segments average 25 to 30 miles in length, and each is capable of generating a major earthquake independently. Segmentation is a fundamental concept in seismic hazard analysis because the maximum potential magnitude is dictated by the length of the specific segment that ruptures.
The five active central segments, listed north to south, are:
- Brigham City
- Weber
- Salt Lake City
- Provo
- Nephi
Assessing the Seismic Risk
The Wasatch Fault Zone represents the largest earthquake risk in the western United States, as it underlies the densely populated Wasatch Front. Paleoseismic studies show that the central segments have an average recurrence interval for a major surface-rupturing earthquake (magnitude 6.5 to 7.5) of 1,700 to 3,000 years for any specific segment. Considering the entire central fault zone, a major surface-rupturing earthquake has occurred about every 395 years over the past 6,000 years. Experts estimate a magnitude 6.0 or greater earthquake could occur on the WFZ within the next 50 years. A major event would cause surface rupture, offsetting the ground vertically by 6 to 20 feet, which poses a direct danger to infrastructure. The populated valley floor is also composed of soft lake sediments from ancient Lake Bonneville, increasing the potential for liquefaction and amplified ground shaking.