A geological fault line is a fracture, or zone of fractures, between two blocks of rock within the Earth’s crust. These fractures are planes where the rock blocks have moved relative to one another. When accumulated stress along these planes is suddenly released, it generates seismic waves that travel through the Earth, which we experience as an earthquake.
The Underlying Tectonic Mechanism
Faults are created by the constant, slow motion of the Earth’s rigid outer layer, the lithosphere, which is broken into large tectonic plates. The interaction of these plates at their boundaries is the fundamental driver of almost all fault formation. As plates move, they generate three primary types of stress in the crust: compression, tension, and shearing.
Compression, where rock is squeezed together, occurs at convergent boundaries where two plates collide. Tension, a pulling-apart stress, defines divergent boundaries where plates move away from each other. Shearing stress involves forces moving parallel but in opposite horizontal directions, characteristic of transform boundaries where plates slide past one another. These forces cause the rock to deform, and when its strength is exceeded, it breaks and forms a fault.
Classifying Fault Types
Geologists classify faults based on the direction of movement along the fault plane, the flat surface separating the two rock blocks. The blocks are described in terms of a footwall (the block below the fault plane) and a hanging wall (the block above it).
A normal fault forms under tensional stress, causing the hanging wall to move downward relative to the footwall, effectively lengthening the crust. Conversely, a reverse fault forms under compressional stress, causing the hanging wall to move upward and shortening the crust. A specialized type of reverse fault, a thrust fault, has a fault plane that dips at a shallow angle. Strike-slip faults, which occur under shearing stress, involve blocks moving horizontally past each other with little vertical movement.
Major Global Fault Zones
The majority of the world’s most active fault lines are concentrated along the borders of tectonic plates, forming vast, seismically active belts. The Pacific Ring of Fire is the most extensive of these zones, forming a 40,000-kilometer horseshoe shape around the Pacific basin. This zone is dominated by convergent boundaries, specifically subduction zones, where oceanic plates are forced beneath continental plates. This subduction process creates powerful earthquakes and is responsible for approximately 75% of the world’s active volcanoes and 90% of its largest earthquakes.
The second most seismically active region is the Alpine-Himalayan Orogenic Belt, which stretches for over 15,000 kilometers from Java through the Himalayas and the Mediterranean. This belt results from the collision of the African, Arabian, and Indian plates with the Eurasian plate, a massive, ongoing continental convergence. The immense compressional forces in this zone create significant faulting and uplift, forming the highest mountain ranges on Earth. The Mid-Atlantic Ridge, a major divergent boundary, is characterized by a central rift valley where the North American, Eurasian, African, and South American plates are moving away from each other, but the earthquakes here are smaller than those at convergent and transform boundaries.
Notable Regional Examples
The global fault zones contain specific, named fault systems that significantly impact human populations. The San Andreas Fault system in California is a famous example of a transform boundary, where the Pacific Plate slides horizontally past the North American Plate. This fault is a right-lateral strike-slip fault, meaning an observer sees the opposite block moving to their right. The San Andreas extends for approximately 1,200 kilometers, slicing through California from the Mendocino Triple Junction in the north to the Salton Sea in the south.
Another prominent strike-slip fault is the North Anatolian Fault (NAF) in Turkey, which is geologically analogous to the San Andreas Fault. The NAF is approximately 1,500 kilometers long and marks the boundary where the Anatolian plate is being forced westward against the Eurasian plate. This fault poses a seismic hazard to Istanbul, as the section beneath the Sea of Marmara has not ruptured in a major earthquake since 1766.
A different system, the New Madrid Seismic Zone (NMSZ), is notable because it is an intraplate fault located far from a plate boundary in the central United States. This series of buried faults stretches about 240 kilometers through Missouri, Arkansas, Tennessee, and Kentucky, marking a weak spot in the crust known as the Reelfoot Rift. Earthquakes in the NMSZ, such as the powerful 1811–1812 series, can affect a much wider area than similar-magnitude events on the West Coast due to the underlying geology.