Fault Lines

A fault line is a crack in Earth's crust along which rocks on either side have moved past each other. Faults can be tiny (a few centimetres long) or enormous (running for thousands of kilometres). Most earthquakes happen along faults, when the rocks on either side suddenly slip past each other after years or centuries of being locked in place by friction. There are three main types of fault depending on how the rocks move. Some faults are famous for causing massive earthquakes; others have slowly built mountains, rift valleys and ocean basins over millions of years.

  • Main types3Normal, reverse (thrust), strike-slip
  • Famous faultSan Andreas1,200 km long, in California
  • Longest fault on landEast African RiftApprox. 6,400 km
  • Movement typeSudden slips during earthquakesPlus slow creep between events
  • Speed of slipUp to several m/sDuring the worst earthquakes
  • Most active regionPacific Ring of FireBoundaries between many plates

The three types of fault

  • Normal faults: form when the crust is being pulled apart. One block of rock slides downward relative to the other. Common at divergent plate boundaries. The Great Rift Valley in Africa is full of normal faults.
  • Reverse faults (or thrust faults): form when the crust is being pushed together. One block slides up and over the other. Common at convergent plate boundaries, especially where continents collide. The faults at the foot of the Himalayas are reverse faults.
  • Strike-slip faults: form when rocks slide horizontally past each other. The San Andreas Fault is the most famous example.

Famous faults around the world

  • San Andreas Fault, California: a strike-slip fault, 1,200 km long, between the Pacific and North American plates. The Pacific side moves north past the American side by approx. 5 cm per year.
  • North Anatolian Fault, Turkey: a strike-slip fault, about 1,500 km long, responsible for many devastating Turkish earthquakes, including the 1999 Izmit earthquake.
  • East African Rift: a giant normal fault system, 6,400 km long, where the African Plate is slowly splitting into two. In tens of millions of years, eastern Africa may break away to form a new continent.
  • Alpine Fault, New Zealand: a strike-slip fault running 850 km up the South Island.
  • Cascadia Subduction Zone, off the US Pacific Northwest: a thrust fault where the Juan de Fuca Plate dives under North America. Capable of producing the largest possible earthquakes.
  • Wasatch Fault, Utah: a normal fault that has been quiet recently but is overdue a big earthquake.

How faults build (or destroy) landscapes

Over millions of years, fault movements shape the landscape.

  • Mountains: reverse faults push rock layers up, building mountain ranges. The Rocky Mountains and the Alps were built this way.
  • Rift valleys: normal faults pull rock apart, creating long flat-bottomed valleys with steep sides. The East African Rift, the Rhine Rift Valley in Europe, and Death Valley in the United States are all rift valleys.
  • Offset landmarks: strike-slip faults gradually shift the landscape sideways. Rivers may bend sharply, fences may be offset by several metres after a single earthquake.
  • Earthquake lakes: sudden faulting can dam rivers, creating new lakes.
Fact The 1906 San Francisco earthquake caused parts of the San Andreas Fault to slip by up to 6 metres in a single moment. Fences, roads and pipes that crossed the fault were suddenly offset by that distance. The earthquake released as much energy as approximately 6 million tonnes of TNT and killed an estimated 3,000 people, mostly in the fires that followed.

Locked faults and "fault creep"

Some faults release their pent-up energy in sudden earthquakes every few decades or centuries. Other faults instead release stress slowly and continuously, by a process called fault creep: rocks slide past each other a few millimetres at a time, without big earthquakes.

Both behaviours can happen on different parts of the same fault. The central section of the San Andreas Fault, for example, creeps steadily without large earthquakes, while the northern and southern sections stay locked and release their energy in occasional huge earthquakes.

Did you know? Some faults move so steadily and predictably that they are used as natural laboratories. Buildings and fences built across creeping sections of the San Andreas Fault gradually deform, with one side slowly sliding past the other. In some old farms in central California, walls built straight a century ago now have noticeable kinks where the fault has carried part of them away.
Deeper dive: the East African Rift, where a continent is splitting

One of the most remarkable fault systems in the world is the East African Rift, a network of normal faults running from the Red Sea down through Ethiopia, Kenya, Tanzania and Mozambique. The rift is approximately 6,400 km long, and it marks the line along which the African Plate is slowly splitting in two.

The rift began forming approximately 25 million years ago, when convection currents in the mantle beneath East Africa started pulling the surface apart. As the crust stretched, normal faults developed along its length, and huge blocks of rock dropped down to form long, flat-bottomed valleys with steep walls. Lava poured up through the cracks, building volcanoes like Mount Kilimanjaro and Mount Kenya along the rift.

At the moment, the rift is widening at only a few millimetres per year. But over the next 5 to 10 million years, the gap will grow into a narrow sea, and eventually a full ocean. The eastern part of Africa (a slice including Somalia and parts of Kenya, Tanzania and Ethiopia) will break away to become a new continent, separated from the rest of Africa by an ocean.

The rift is also famous as the cradle of human evolution. Many of the most important early human fossils (including Lucy, Ardi, and Homo habilis) have been found in volcanic and lake sediments deposited along the rift over the last few million years. The rift's constantly changing landscape, with its mixture of forests, lakes, savannas and mountains, may have been one of the key environments in which our ancestors evolved.

For more, see the San Andreas Fault and tectonic plates.