Normal faults are caused by tensional stress, also called extensional stress. This is the force that pulls the Earth’s crust apart in opposite directions. When rock is stretched beyond its breaking point, it fractures along an angled plane, and one block of rock slides downward relative to the other. Normal faults are among the most common fault types on Earth and are found wherever tectonic forces are actively stretching the crust.
How Tensional Stress Creates a Normal Fault
The Earth’s crust is under constant stress from the movement of tectonic plates. Three types of stress act on rock: compression (squeezing together), tension (pulling apart), and shear (sliding sideways). Each produces a different style of faulting. Tensional stress is the one responsible for normal faults.
When two sections of crust are pulled in opposite directions, the rock between them eventually fractures at an angle, typically between 45 and 60 degrees from horizontal. The fracture creates two blocks. The block sitting above the angled fault plane is called the hanging wall, and the block below it is the footwall. In a normal fault, the hanging wall drops downward relative to the footwall. Gravity helps drive this motion once the rock has fractured, which is why normal faults are sometimes called gravity faults.
This downward slip distinguishes normal faults from reverse faults, where compression pushes the hanging wall upward instead. The movement in a normal fault is vertical (called dip-slip), meaning the blocks move up and down along the fault’s slope rather than sliding horizontally past each other.
Where Normal Faults Form
Normal faults dominate in tectonic settings where the crust is being pulled apart. The most important of these are divergent plate boundaries, where two tectonic plates move away from each other. Mid-ocean ridges, the underwater mountain chains running through every ocean basin, are zones of large normal faults where new crust is continuously created as plates separate.
On land, continental rifts are the primary setting for normal faulting. The East African Rift System is the best active example. There, the African plate is splitting into two smaller plates, and the stretching has created a chain of deep valleys, volcanoes, and long lakes running thousands of kilometers from Ethiopia to Mozambique. In the Lake Turkana region of Kenya, north-south striking normal faults bound the rift basin and are actively extending the crust at a rate of roughly 3.5 to 5.8 millimeters per year. That sounds tiny, but over millions of years it’s enough to tear a continent apart.
The Basin and Range Province of the western United States is another classic example. Stretching across Nevada, Utah, and parts of surrounding states, this region has been pulled apart so extensively that estimates of total crustal extension reach 39 percent (about 188 kilometers of stretching) across the northern portion near 40°N latitude. In the southern Basin and Range around 36°N, extension estimates run even higher, in the range of 80 to 100 percent. The result is a distinctive landscape of parallel mountain ranges separated by flat valleys, repeated across hundreds of kilometers.
Horsts, Grabens, and the Landscape They Create
When tensional stress produces multiple parallel normal faults, the landscape develops a pattern of raised and sunken blocks. A graben is a block of crust that has dropped down between two normal faults, forming a valley or basin. A horst is the block that remains elevated between two faults, forming a ridge or plateau. The USGS describes a graben as a “down-dropped block of the earth’s crust resulting from extension, or pulling, of the crust.”
This alternating pattern of horsts and grabens is exactly what you see in the Basin and Range: each mountain range is a horst, and each valley between them is a graben. The same pattern shapes the East African Rift, where rift valleys like the one holding Lake Turkana sit in grabens bounded by steep border faults. Lake Turkana itself is about 250 kilometers long and 30 kilometers wide, with the rift floor sinking deep enough to give the lake a maximum depth of 120 meters. That basin exists because normal faults on either side have allowed the central block to drop.
On a smaller scale, normal faults also create features like escarpments (steep cliff faces where one block has dropped relative to another) and half-grabens, where only one side of a valley is bounded by a major fault.
Earthquakes on Normal Faults
Normal faults produce earthquakes when accumulated tensional stress is suddenly released as the hanging wall slips downward. These earthquakes tend to be shallower than those on other fault types, because the crust in extensional zones is often thinner and hotter than average. The seismic signature of a normal fault earthquake is distinctive: seismologists identify it through focal mechanism diagrams (sometimes called “beach balls”) that show the characteristic extensional pattern of motion.
Normal fault earthquakes can be damaging but generally don’t reach the extreme magnitudes seen on the world’s largest subduction zones, which involve compression. The largest normal fault earthquakes typically fall in the magnitude 6 to 7 range, though exceptions exist. In the Basin and Range, the Owens Valley area along the Sierra Nevada front is one well-known zone of normal faulting seismicity. The 2009 L’Aquila earthquake in central Italy, a magnitude 6.3 event that killed over 300 people, occurred on a normal fault in a zone of crustal extension.
Why Tension, Not Compression
The key distinction worth remembering is that the type of stress determines the type of fault. Tensional stress pulls rock apart and creates normal faults where the hanging wall drops down. Compressional stress pushes rock together and creates reverse (or thrust) faults where the hanging wall is shoved upward. Shear stress slides rock horizontally and creates strike-slip faults. If you’re looking at a fault and the upper block has moved downward, you’re looking at the signature of extensional forces at work.
This relationship between stress and faulting style is consistent everywhere on Earth, from the ocean floor to continental interiors. Wherever the crust is being stretched, whether by diverging plates, rising mantle plumes, or gravitational collapse of thickened crust, normal faults are the structural result.