Fault Lines: Where Earthquakes Happen and Why

What Is a Fault?

A fault is a fracture or zone of fractures in the Earth's crust along which rocks on either side have moved relative to each other. Earthquakes occur when stress accumulated along a fault is suddenly released — the locked portion of the fault slips, sending seismic waves propagating outward in all directions. The location on the fault where slip initiates is called the hypocenter (or focus); the point directly above it on the Earth's surface is the epicenter.

Not all faults are actively dangerous. A fault must be "active" — meaning it has experienced displacement in the past 10,000 to 11,000 years — to pose a significant seismic hazard. Identifying which faults are active, and estimating when they last ruptured and how often they recur, is one of the central tasks of earthquake hazard assessment.

Types of Faults

Faults are classified by the direction and type of movement along them:

• Strike-slip faults: The two sides move horizontally past each other. The San Andreas Fault is the world's most famous strike-slip fault. The North Anatolian Fault in Turkey is another major example. Strike-slip earthquakes can be extremely destructive due to surface rupture and horizontal ground displacement.
• Normal faults: The hanging wall (the block above the fault plane) moves down relative to the footwall. Associated with extensional tectonic environments — where the crust is being pulled apart. Common in the Basin and Range Province in the western United States.
• Reverse faults (thrust faults): The hanging wall moves up relative to the footwall. Associated with compressional environments — where the crust is being pushed together. The 1994 Northridge earthquake (Los Angeles) and the 2011 Christchurch earthquake (New Zealand) were both reverse fault events. Thrust faults generate some of the world's most powerful earthquakes.
• Subduction zone faults: Where one tectonic plate dives beneath another. These generate the world's largest earthquakes: magnitude 8-9+ megathrust events. The Cascadia Subduction Zone (Pacific Northwest), the Japan Trench (Tohoku 2011), and the Nazca/South American interface (Chile 1960) are critical examples.

Major US Fault Systems

San Andreas Fault System (California)

The San Andreas Fault runs approximately 800 miles through California, marking the boundary between the Pacific and North American tectonic plates. It last ruptured in its northern section in 1906 (San Francisco earthquake, ~7.9) and in its central section in 1989 (Loma Prieta, 6.9). The southern section — from Palm Springs to the Salton Sea — has not had a major rupture in approximately 300 years, accumulating elastic strain. Scientists estimate a significant probability of a "Big One" (magnitude 7.8+) on the southern San Andreas in the coming decades.

Cascadia Subduction Zone (Pacific Northwest)

This fault system extends from northern California to British Columbia and is capable of generating magnitude 8.0-9.0+ megathrust earthquakes. The last great Cascadia earthquake occurred in January 1700 (estimated 9.0), which was documented from Japanese tsunami records and Native oral histories. The recurrence interval is approximately 200-500 years, making the next event potentially overdue. The Pacific Northwest has built significantly since 1700 with insufficient attention to this hazard.

Wasatch Fault (Utah)

Running through Utah's most populated corridor (Salt Lake City to Provo), the Wasatch Fault is a major normal fault capable of magnitude 7.0+ earthquakes. Unlike California, Utah has had relative seismic quiet in the historical period — but geological evidence shows large prehistoric ruptures. The urban development along the Wasatch Front has grown dramatically with the fault largely absent from public consciousness.

New Madrid Seismic Zone (Central United States)

The most powerful earthquakes in continental US history occurred in 1811-1812 near New Madrid, Missouri — three events estimated at magnitude 7.2-8.1. The fault zone lies in an intraplate setting far from plate boundaries. Much of the central US infrastructure was built without seismic design standards; if a repeat event occurred today, damage estimates are catastrophic. The seismic hazard is real but recurrence intervals are poorly constrained.

How Faults Are Monitored

The USGS and state geological surveys operate networks of seismometers that continuously record ground motion, providing real-time location and magnitude estimates for earthquakes. GPS networks measure millimeter-scale ground deformation, tracking how stress accumulates on faults over years. LiDAR (airborne laser scanning) maps fault traces through vegetation cover. Paleoseismology — digging trenches across fault zones to examine sediment layers and offset features — allows scientists to estimate the timing and magnitude of prehistoric earthquakes going back thousands of years.