Fault (geology)

• 1. Introduction

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  A fault is a crack in the rock beneath your feet where two slabs of the Earth have shifted past one another. Geologists define it as a planar fracture in a volume of rock across which significant displacement has occurred. The movement is driven by the slow grind of plate tectonic forces. Most of the world's earthquakes begin at one of these fractures, when rock that has been straining against rock finally lets go. Yet not every fault snaps. Some slip slowly and silently, year after year, through a process called aseismic creep. Why does one fault lock up and unleash a quake while another flows quietly? What decides whether the ground rises, drops, or slides sideways? And how do the people who build dams, tunnels, and hospitals learn to read these scars in the crust before they move again? The answers run from the language of old coal miners to the porphyry copper of northern Chile.

• 2. Locked And Loaded

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  Friction is what makes a fault dangerous. Because the constituent rocks are rigid and resist sliding, the two sides of a fault cannot always glide past each other, and sometimes all movement stops. The patches of higher friction where the fault becomes locked are called asperities. While the fault is locked, stress accumulates against it. When that stress exceeds the strength threshold of the rock, the fault ruptures and the stored strain energy escapes, partly as the seismic waves we feel as an earthquake. The depth in the crust changes how rock answers this stress. The ductile lower crust and mantle deform gradually by shearing, soaking up strain over time. The brittle upper crust does the opposite, reacting by fracture in an instantaneous release that drives motion along the fault. Even ductile rock can give way suddenly if the strain rate climbs too high. That contrast between slow shearing and sudden snapping sets the stage for how each side of a fault actually moves.

• 3. Walls And Wrenches

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  The vocabulary of faults was borrowed from people working underground. The two sides of a non-vertical fault are called the hanging wall and the footwall, terms that come from mining a tabular ore body. The miner stood with the footwall under his feet and the hanging wall above his head. That distinction matters, because it separates the two great families of fault motion. Slip is the relative movement of features on either side of the fault plane. Geologists break the separation into two components, captured in the phrase "Throw up and heave out": the throw is the vertical component and the heave is the horizontal one. To measure them precisely you must find a piercing point, a common intersection that once lined up on both sides of the break. In practice, only the slip direction can usually be found, leaving the heave and throw as approximations. Faults are classified by the angle the fault plane makes with the surface, called the dip, and by the direction of slip. Strike-slip faults run nearly vertical, with the ground sliding sideways and almost no vertical motion. Left-lateral motion makes a sinistral fault and right-lateral motion makes a dextral fault, each judged by an observer on the far side. When such a fault forms a plate boundary it becomes a transform fault, like the Dead Sea Transform in the Middle East or the Alpine Fault in New Zealand. These are called conservative boundaries, because lithosphere is neither created nor destroyed there.

• 4. Stretching And Squeezing

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  Dip-slip faults reveal whether the crust is being pulled apart or shoved together. They come in two kinds, normal and reverse, with the names inherited from English coal mining where normal faults are the most common. In a normal fault the hanging wall slides downward relative to the footwall, a signature of extension. Most normal faults dip at least 60 degrees, though some dip at less than 45. When the stress field flips between tension and compression over time, old faults can be reactivated with their movement inverted, so a normal fault may become a reverse fault and back again. Reverse faults record the opposite story, the hanging wall riding upward and the crust shortening under compression. A thrust fault shares that sense of motion but lies at a shallow angle, with the dip below 45 degrees. Thrust faults climb through rock by alternating flats, which follow weak bedding planes, and ramps, which cut upward through the layers. They build nappes and klippen across the great thrust belts. Subduction zones are a special class of thrust, the largest faults on Earth, and the source of the largest earthquakes. Extension carves its own landscapes. A block dropped between two normal faults that dip toward each other is a graben, while a block left standing between two faults dipping away from each other is a horst. A repeating sequence of grabens and horsts produces the characteristic basin and range topography.

• 5. Curves And Circles

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  A listric fault bends as it descends. It has a concave-upward shape, steep near the surface and flattening with depth, and a normal fault can evolve into one as its plane curves into the Earth. Where the hanging wall is absent, such as at a cliff, the footwall may slump and create multiple listric faults at once. Push the flattening further and the fault plane becomes nearly horizontal, with slip running along a decollement. These extensional decollements can grow enormous and become detachment faults, low-angle normal faults with regional significance. The curve creates a problem of empty space. As the hanging wall slides along a listric fault, a geometric gap opens between it and the footwall. Depending on the rock's behavior, the hanging wall may fold and sag into the gap as rollover folding, or break into imbrication fans and domino faulting to fill it. Faults also trace circles. Ring faults, also called caldera faults, form inside collapsed volcanic calderas and at bolide impact sites such as the Chesapeake Bay impact crater. They arise from overlapping normal faults that outline a ring, and the fractures they leave may be filled by ring dikes. Around a major fault, smaller ones take sides. Synthetic faults dip in the same direction as the main fault, while antithetic faults dip the opposite way, sometimes paired with rollover anticlines as in the Niger Delta structural style.

• 6. The Rock Between

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  Every fault has a measurable thickness, a band of crushed and altered rock that records where and how the faulting happened. The character of that rock depends on the level of the crust, the rock types involved, and any mineralising fluids present. Because a single fault may pass through many crustal levels, it can carry several distinct fault rocks along its surface, juxtaposed and overprinted by continued displacement. Cataclasite is one such rock, with angular clasts set in a finer matrix of similar composition and little or no planar fabric. A coarser version with more than 30 percent visible fragments is tectonic or fault breccia. Fault gouge is the clay-rich, fine-grained end, incohesive and holding fewer than 30 percent visible fragments. When it forms in clay-rich sedimentary layers that are sheared into the fault, it becomes clay smear. Deeper deformation leaves a different fingerprint. Mylonite is cohesive with a strong planar fabric, built by the tectonic reduction of grain size and dotted with rounded porphyroclasts. The most dramatic fault rock is pseudotachylyte, an ultrafine glassy material, usually black and flinty, that fills dilation fractures as thin veins. It likely forms only at seismic slip rates, which lets geologists use it as a fault rate indicator on faults that are no longer moving.

• 7. Reading The Hazard

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  California has banned new construction directly on or near faults that have moved within the Holocene Epoch, the last 11,700 years. Faults that also moved through the Pleistocene, stretching back 2.6 million years, may still draw scrutiny for critical structures like power plants, dams, hospitals, and schools. In geotechnical engineering a fault is a discontinuity that can dominate the strength and deformation of soil and rock for tunnels, foundations, and slopes. Judging a fault's activity is detective work in the dirt. Geologists examine soil features in shallow excavations and geomorphology in aerial photographs. Subsurface clues separate old soil from young: shears tied to carbonate nodules, eroded clay, and iron oxide mineralization point to age, while their absence points to youth. Radiocarbon dating of organic material buried beside or over a fault shear is often decisive in telling an active fault from an inactive one. From these relationships, paleoseismologists estimate the sizes of past earthquakes over several hundred years and sketch rough projections of what a fault might do next. The hanging wall can turn lethal underground, where its weight leads to severe stresses and rock bursts, as recorded at Frood Mine.

• 8. Veins Of Wealth

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  Many ore deposits sit on or alongside faults, and the reason is plumbing. The fractured rock of a fault zone opens pathways for magma to rise and for mineral-bearing fluids to circulate. Intersections of near-vertical faults are often the richest sites of all. Northern Chile's Domeyko Fault hosts a string of porphyry copper deposits, including Chuquicamata, Collahuasi, El Abra, El Salvador, La Escondida, and Potrerillos. Further south in Chile, the Los Bronces and El Teniente deposits each lie where two fault systems cross. Faults do not always act as open conduits to the surface. One proposal holds that deep-seated misoriented faults instead trap stagnating magma, giving it the time and conditions for igneous differentiation, until the differentiated magma bursts violently out of the trap toward shallower levels where porphyry copper forms. Water follows the same weaknesses. Because faults are zones of weakness, they let water reach the surrounding rock and intensify chemical weathering, which enlarges the weathered zone and makes room for groundwater. As a result, fault zones act as aquifers and help carry groundwater through the crust, turning the same fractures that shake the ground into channels for the water beneath it.