A geological fault line represents a fracture in the Earth’s crust where blocks of rock move relative to one another, releasing accumulated stress and often resulting in seismic activity. The United States contains numerous active fault systems because it sits at the intersection of several tectonic plates, primarily the North American, Pacific, and Juan de Fuca plates. Interactions along these crustal boundaries create the most impactful major fault systems across the country.
The San Andreas Fault System
The most famous fault zone in the US is a right-lateral strike-slip system stretching over 800 miles. This structure marks the primary transform boundary where the Pacific Plate slides horizontally past the North American Plate. The fault runs through much of California, creating a significant seismic hazard for highly populated areas.
The San Andreas system is traditionally divided into three major segments, each with distinct behaviors and levels of risk. The northern segment ruptured spectacularly during the 1906 San Francisco earthquake, which had an estimated magnitude of 7.9. The central segment, conversely, is known as the “creeping section” because it moves slowly and continuously without building up enough strain for a massive event.
The southern segment, extending toward the Salton Sea, is considered “locked” and has not experienced a major rupture since the 1857 Fort Tejon earthquake. Scientists estimate this part of the fault has accumulated significant stress, making it capable of generating a great earthquake up to magnitude 8.3. The average annual slip rate along the entire fault is between 0.8 and 1.4 inches per year.
The Cascadia Subduction Zone
Located offshore from northern California up to Vancouver Island, the Cascadia Subduction Zone (CSZ) is a megathrust fault that operates differently from the San Andreas. Here, the oceanic Juan de Fuca Plate is diving beneath the lighter continental North American Plate. This geological process, called subduction, is responsible for creating the Cascade Mountain Range and its chain of volcanoes.
The boundary between the two plates is approximately 620 to 800 miles long. It is currently considered a “locked zone” in its shallow portions, meaning the plates are stuck together by friction. This causes immense strain to accumulate over time, which is expected to be released in a large-magnitude event, often referred to as a “great earthquake.”
Geological evidence indicates that the CSZ is capable of producing earthquakes of magnitude 9.0 or higher. The last known full-margin rupture occurred on January 26, 1700; the resulting tsunami was recorded in Japan. The average recurrence interval for these events is estimated to be approximately 500 years, though intervals vary widely.
A rupture of this magnitude would cause ground shaking lasting several minutes and trigger a massive tsunami that would reach the Pacific Northwest coast within 20 to 30 minutes. This tsunami could generate wave heights between 16 and 49 feet in some northern California coastal areas. The risk of a magnitude 9.0 earthquake occurring in the next 50 years is estimated to be between 7% and 15%.
The New Madrid Seismic Zone
The New Madrid Seismic Zone (NMSZ) is an intraplate fault system. This zone stretches across parts of Missouri, Arkansas, Tennessee, Kentucky, and Illinois. Its seismic activity is linked to a buried ancient geological feature known as the Reelfoot Rift, a failed rift valley formed hundreds of millions of years ago.
The NMSZ is historically significant for the series of three earthquakes that occurred between December 1811 and February 1812. These quakes are estimated to have reached magnitudes between 7.0 and 8.2, making them the most powerful recorded east of the Rocky Mountains. The seismic waves from these events were felt across a vast area, reportedly ringing church bells as far away as Boston.
The widespread impact of these historical earthquakes is partly due to the nature of the crust in the central and eastern US. The older, cooler, and more stable continental crust transmits seismic energy more efficiently than the fractured crust found in the western US. Consequently, an earthquake of the same magnitude in the NMSZ can cause damage over a much larger geographical area.
Other Regionally Significant Fault Zones
Beyond the three largest systems, several other fault zones pose significant localized hazards across the country. One such example is the Wasatch Fault, which runs for about 240 miles along the western edge of the Wasatch Mountains in Utah.
The Wasatch Fault is capable of producing earthquakes up to magnitude 7.5. It represents a major risk because approximately 80% of Utah’s population lives along its length, a region known as the Wasatch Front. This close proximity between an active fault and a dense urban area makes it the largest earthquake threat in the interior Western United States.
In Alaska, the Denali Fault System is a right-lateral strike-slip fault that arcs across the southern part of the state. This fault is active due to the ongoing subduction of the Pacific Plate beneath the North American Plate. The Denali Fault was the source of a powerful magnitude 7.9 earthquake in 2002, which ruptured for over 200 miles and caused damage to infrastructure, including the Trans-Alaska Pipeline System.