An earthquake occurs when two blocks of the earth suddenly slip past one another, releasing built up energy from plate tectonics, regional stress regimes, and induced from fluid injection or underground mining activities. The surface between these two blocks is called a fault or fault plane. When these blocks move, they produce seismic waves that are transmitted outward through the rock in all directions, producing ground shaking and secondary effects. Utah is situated away from the primary tectonic plate boundaries where a significant portion of the world’s earthquakes typically occur. Instead, it lies within the western portion of the North American plate. Nevertheless, seismic activity in Utah is ultimately influenced by its connection with the Pacific plate, primarily along the plate’s edge along the west coast of the United States.

Earthquakes are unique multi-hazard events with the potential to cause excessive damage and loss of life. Earthquake secondary effects often include surface fault rupture (generally≥ magnitude [M] 6.5), liquefaction and lateral spreading can be triggered as low as approximately 0.1 g.4 Landslides, rock fall (generally ≥ M 4), tectonic subsidence, seiches and tsunamis. Effects from ground shaking may include building and infrastructure damage, fires, building, dam and canal failure, hazardous material releases, and non-structural building damage such as toppled cabinets, bookcases, and other furniture or equipment that was not restrained, falling ceiling tiles, lights, and other ceiling mounted items, and movement of unrestrained furniture, equipment, and other building interior items.

Utah straddles the physiographic region boundary between the extending Basin and Range Province to the west and the relatively stable Rocky Mountains and the Colorado Plateau to the east. This boundary coincides with an area of earthquake activity called the Intermountain Seismic Belt (ISB). The ISB is a zone of significant earthquake activity up to 120 miles wide extending in north-south direction 800 miles from Canada to northern Arizona and eastern Nevada. Large, damage-causing earthquakes in Utah are likely to occur in the ISB that generally extends through the center of the state, essentially following Interstate 15, where there are many active faults capable of producing earthquakes. Unfortunately, this location is also where over 85% of Utah’s population lives along the Wasatch Front, and also includes the rapidly urbanizing St. George and Cedar City areas.

Moderate-to-large earthquakes, generally M 6 and greater, can cause substantial damage to buildings, roads, bridges, and utilities often leading to injuries and fatalities. Background earthquakes are defined as those events less than M 6.75 ± 0.25 that cannot be associated with a known fault. A classic example of a background earthquake within the Wasatch Front region is the 1975 M 6.0 Pocatello Valley, Idaho, earthquake. Utah’s only historical surface fault rupturing earthquake is the 1934 M 6.6 Hansel Valley earthquake that also caused two fatalities.