Strike-slip faults

Strike-slip faults separate portions of the earth's crust that have moved horizontally past each other. They can be thousands of kilometers in length, with offsets of hundreds of kilometers across them. Many of the most devastating earthquakes occur along strike-slip faults.

Characteristics of Strike-Slip Faults

A fault is a surface within the earth's crust across which displacement has occurred. A surface can be curved, flat, tilted, vertical, or horizontal. A curved surface may be subdivided into smaller pieces, each of which can be considered to be a plane. At any location, therefore, a fault can be thought of as a planar element. The orientation of this element is referred to by two angles: its “strike” and its “dip.” The strike is the orientation of a horizontal line on the plane, given as a compass direction. The dip is the angle at which the plane tilts down into the ground.

Stipulating that a fault occurs within the earth's crust implies something about the scale of the process. Although some may consider fractures on a centimeter or meter scale to be faults, in general faults are expected to be tens of meters or kilometers in size, and some can be hundreds of kilometers long. Faults seldom consist of a single, clean fracture, so the term “fault zone” is used when referring to the region of complex deformation associated with the fault plane.

Displacement across the fault, or “slip,” refers to the amount and direction of relative motion of the blocks of rock on opposite sides of the fault. If this motion is entirely horizontal and neither block has moved up or down, the slip will be in the direction of the strike on the fault surface; such faults are called “strike-slip” faults. Conversely, if the motion is in the direction of the slope on the fault plane, it is called a “dip-slip” fault. There are two kinds of dip-slip faults. When the motion is such that the block above the fault plane slides down the fault, it is a “normal” fault. When the motion is such that the block above the fault plane slides up the fault, it is a “reverse” fault.

For many reasons, some mechanical and some geological, most faults on earth are one of these three types. Less common are faults with significant components of both dip-slip and strike-slip motions, which are called “oblique-slip” faults. In addition, a surface may be a strike-slip fault over most of its length, but portions may be dip-slip or oblique-slip. If the relative motion between the blocks of rock on either side is predominately horizontal, it is considered to be a strike-slip fault.

Types of Strike-Slip Faults

Because their motion is restricted to being horizontal, only two kinds of strike-slip faults are possible. These are defined on the basis of how the block of rock that is across the fault from the observer appears to move. It is not necessary to know which block actually moved; only their relative motion is important.

Consider two buses parked next to each other with their drivers' sides adjacent and facing in opposite directions. If either bus were to move forward, passengers in one bus would see the other bus appear to move forward. They might not even be certain whether it was their bus moving or the other one. To passengers looking out the side windows, the view would be of a bus moving to the left, past the windows. This relative motion could be called “left-lateral” or “sinistral.” Similarly, if either bus were to move in reverse, passengers in either bus, looking out the side windows, would see the other one move to their right. The relative motion between the buses could be called “right-lateral” or “dextral.” Such relative motion has nothing to do with the absolute motions (movement across the ground) of either bus. If one bus were to move in reverse and the other bus were to move forward somewhat more rapidly, relative motion would be left-lateral or sinistral, even though both buses were moving across the ground in the same direction.

Geologists can study features that are offset across a strike-slip fault to determine the sense of relative motion and then name the fault either right-lateral or left-lateral. As an example, if a fence is offset across an east-west-trending fault such that the fence north of the fault has moved horizontally to the east (or the fence south of the fault has moved horizontally to the west), then it is a right-lateral strike-slip fault.

This concept was successfully applied for decades but needed modification when seafloor spreading was discovered. Offsets along mid-ocean ridges, where new ocean crust is being manufactured, appear to have a sense of relative motion that is opposite the sense of motion observed along the strike-slip faults connecting them. This can be demonstrated by considering twelve fast-growing trees planted in pots. Imagine that they are all in two parallel north-south lines initially. Next, imagine that the six at the north end of the lines (three in each line) are moved a few meters to the east. This produces an offset in the lines with a right-lateral sense. Finally, imagine that all of the trees are knocked down: Those on the western lines fall to the west, while those on the eastern lines fall to the east. Temporarily ignoring some tenets of biology, imagine that all the trees continue to grow taller, but because they are lying down, this growth is horizontal. Along the offset, trees to the north will be growing to the west, and trees to the south will be growing to the east. This motion is left-lateral, opposite the offset in the pots. Mid-ocean ridge segments are thus not good indicators of the sense of motion on the strike-slip faults connecting them because new crust is created along them. The term “transform” fault refers to this situation and to similar manifestations of strike-slip faults.

Recognizing Strike-Slip Faults

Two approaches are used to find and delineate strike-slip faults: identifying features that have been juxtaposed or offset by the fault, and detecting features in the landscape that are known to have been produced by the faulting process. If an area has experienced fault displacements after human development, obvious offsets of anthropogenic structures are generally easy to find, measure, and date. Highways, railroads, fence lines, pipelines, and buildings have been studied over the years to decipher recent displacement histories on a great number of faults. Other, less obvious features include surveys of real estate boundaries, such as town and village borders. Although less tangible, these surveys are usually done with high precision and provide good estimates of regional deformation associated with faulting.

Faulting is episodic by nature, and movement on a particular fault may recur on a time scale of centuries. Such lengthy recurrence intervals are too long to be reflected in offsets of most cultural features, so topographic characteristics are also examined. Offsets in the courses of rivers and streams, interruptions in hills and valleys, and other topographic changes can reveal relative displacements along a fault. Unlike anthropogenic structures, such alterations of a natural topography can be difficult to date with great precision. By extending the time scale back thousands of years, however, they often provide important data.

Interruptions in the deposition in lakes and swamps near strike-slip faults can reveal considerable detail about the timing and intensity of former earthquakes. In an area called Pallet Creek, along the San Andreas Fault in California, evidence for eight earthquakes has been dated using radiocarbon techniques. The earliest occurred at about 750 c.e., and the most recent in 1857. Unfortunately, the slip associated with each of these earthquakes is not easily ascertained from the sedimentary strata.

During an earthquake, a fault may slip several meters in a matter of seconds, but, averaged out over millions of years, most faults have displacement rates on the order of a few centimeters per year. Over the course of 1 million years or so, the displacement across most faults will be on the order of a few tens of kilometers. The San Andreas Fault is thought to have been in existence for about 29 million years. Hundreds of kilometers of displacement are possible over this vast period of time. Topographic features persist for thousands, perhaps even hundreds of thousands of years, but few could be expected to exist after millions of years. Here, the juxtaposition of rock types and the records left in the magnetic minerals of the ocean floor can be used to delimit the time of inception and the subsequent movement history of a fault.

The faulting process itself produces clues, distinct from offsets or juxtapositions, that can reveal the presence of a fault. In general, the broken rock in the vicinity of the fault will weather and erode more easily than sound rock some distance away. This results in the formation of long, linear valleys along fault lines.

By grinding up the rock in the immediate vicinity of the fault, faulting often modifies the groundwater system. Sometimes rock that was impermeable has its permeability increased by the new fractures produced. Other times, a permeable rock is rendered less permeable because the conduits that permitted water to flow through it are disrupted by the fracturing process. These changes may result in the formation of springs or lakes called “sag ponds” directly above the fault.

Finally, some faults are revealed by the earthquakes that occur on them. By studying the seismographic data obtained from an earthquake, geophysicists can determine the location of the earthquake and the direction in which the blocks of rock moved, even if the fault involved lies kilometers beneath the surface.

Occurrence

The surface of the earth is made up of twelve or so tectonic plates that are roughly 100 kilometers thick and persist with little deformation within them for hundreds of millions of years. Plates diverge from each other along ridges (generally beneath the oceans but occasionally running through a continent, such as the African rift valley). Connecting ridge segments, which may be separated by hundreds of kilometers, are strike-slip faults. Plates converge, with one plate moving beneath the other, along subduction zones. The transform fault is a special class of strike-slip faults, where such faults form a plate boundary. When, as is often the case, the direction of convergence is not perpendicular to the subduction zone (oblique convergence), a strike-slip fault may develop to accommodate the horizontal component of relative motion. Sometimes the horizontal stresses produced by plate convergence are sufficient to extrude a wedge-shaped piece of a plate to the side, which results in strike-slip faults. If plates move past each other without diverging or converging, the motion is horizontal and is accomplished along strike-slip faults.

The majority of strike-slip faults connect ends of ridge segments. In the ocean floor, such ridge segments are often called offset spreading centers (OSCs), and the reason for their existence is not well understood. Active faulting occurs along the transform faults between ridge segments, resulting in an age difference across a line that extends the fault beyond the offset region. Because the ocean floor cools, contracts, and sinks as it gets older, this age difference is often expressed by significant topographic relief, with cliffs stretching out for hundreds of kilometers from the transform fault as an oceanic fracture zone.

Oblique convergence is frequently accommodated by a partitioning of relative motion between a dip-slip subduction zone and a strike-slip fault parallel to the subduction zone. Some examples of strike-slip faults that are parallel to subduction zones are the Great Sumatran fault on the island of Sumatra and the Denali fault in Alaska. In 1995, the Hyogoken-Nanbu earthquake near Kobe, Japan, occurred on one of these faults, killing more than 5,000 people and causing more than $200 billion of damage.

Horizontal stresses perpendicular to the trend of a subduction zone can also produce strike-slip faults. Sometimes called “watermelon seed” or “horizontal extrusion” tectonics, this process occurs when a segment of a plate is wedged off to the side by plate convergence. The motion of this plate segment is similar to that of a watermelon seed when it is squeezed between the thumb and forefinger. Mechanical engineers say that this type of deformation is caused by a “rigid indenter,” and the faults produced may be called “indent-linked” strike-slip faults.

The subcontinent of India has acted as a rigid indenter as it has pushed northward into the Eurasian plate. Subduction produced the HimalayaMountains and the Tibetan plateau; at the same time, huge, wedge-shaped pieces consisting of most of Southeast Asia have been moved off to the side. As much as one-half of the convergence has been accommodated by this eastward extrusion along the Altyn Tagh, Haiyuan, and related strike-slip fault systems, resulting in many devastating earthquakes. The North Anatolian fault in Turkey is another indent-linked strike-slip fault. In this case, much of Turkey is being extruded to the west as the Arabian and Eurasian plates converge. Movement on this fault brought Turkey 1.2 meters closer to Europe during a devastating 1999 earthquake.

If the relative motion between plates has little convergence or divergence, it may be taken up almost entirely along strike-slip faults. In California, motion between the North American plate and the Pacific plate occurs largely across the San Andreas fault, for example. Although it is presently a variety of transform fault that connects ridge segments in the Gulf of California to the Mendocino triple junction (where it meets a trench and another strike-slip fault), the San Andreas fault has such a complex history that it is often best to consider it as a plate boundary where most of the relative motion has a right-lateral strike-slip sense.

Formation and Secondary Features

There are often long periods of time between episodes of motion on strike-slip faults. During these periods, soil and other unconsolidated sediments can accumulate over the fault region. When motion again occurs on the fault, the offset in these new layers at the surface can be seen to develop in a complex but systematic way. Similar processes have been observed in laboratory models involving clay cakes being offset above moving plates.

Initially, many small offsets develop above the fault in a parallel, offset geometry resembling the slats on a venetian blind. These are called Riedel shears, conjugate Riedel shears, or P shears, depending on their angular relationship to the underlying fault. Complex technical issues are involved in their formation, but of particular interest is the fact that many minor faults form initially, and only later do the principal displacement shears develop. These are the strike-slip faults across which most of the movement occurs. Study of strike-slip faults in bedrock often reveal the complexities introduced by the early shears.

The complicated geometry of strike-slip faults means that motion across some parts of them will not occur in a simple, strike-slip sense. In some places, the fault surface will bend, or be offset, resulting in either extension or compression. Imagine an east-west-trending right-lateral strike-slip fault with an offset across which the eastern side has been offset to the south relative to the western side. In the vicinity of the offset, the crust will be extended. Sometimes called “transtension,” this stretching may result in normal faults bounding a down-dropped block of crust, producing a “pull-apart” basin. The Salton Sea in Southern California and the Dead Sea in Israel are examples of these features.

Using the same geometry, but this time considering the offset to be of a left-lateral strike-slip fault, the vicinity of the offset would be in compression. This “transpression” might be expected to produce buckling and mountain ranges. The Transverse Ranges of California, occurring near the “big bend” of the San Andreas fault, probably owe their existence, in part, to these compressive stresses.

Significance

Many of the horizontal movements on the surface of the earth occur along strike-slip faults. If a complete set of slip data for all of the strike-slip faults on the planet could be constructed, it would reveal most of what is known about tectonics. Most strike-slip faults extend to the surface, providing exposures where they can be studied in detail. Knowledge of the geometry, offset history, and earthquake recurrence intervals on strike-slip faults is therefore more developed, and based on better data, than similar knowledge for normal or reverse faults.

Because they often cut through the continental crust, strike-slip faults are likely to traverse populated regions. Their effects on topography may even encourage development of the most earthquake-prone areas. Disastrous earthquakes are common on strike-slip faults today, as they have been in the past, and will certainly be in the future. Learning more about these faults and the earthquakes that occur along them is likely to help in predicting those earthquakes and in mitigating their negative consequences.

Principal Terms

dip: a measure of slope; the angle between a plane and the horizontal, measured in the vertical plane perpendicular to the strike of the plane

fault: a fracture in the earth's crust across which there has been measurable movement

plate tectonics: a theory that holds that the surface of the earth is divided into about twelve rigid plates that move relative to one another, producing earthquakes, volcanoes, mountain belts, and trenches

slip: the relative motion across the surface of a fault

strike: the orientation of a horizontal line on a plane; it is measured using compass directions and represents the angle between the horizontal line on the plane and a horizontal line in the north direction

Bibliography

Cunningham, W. D., and Mann, P. Tectonics of Strike-Slip Restraining and Releasing Bends, Special Publication no. 290. Geological Society of London, 2008. Highly technical, but thorough look at strike-slip tectonics. It explores the mechanics and distribution of bends.

Davidson, Jon P., Walter E. Reed, and Paul M. Davis. Exploring Earth: An Introduction to Physical Geology. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 2001. Chapter 10, “The Conservative Boundary: Transform Plate Margins,” provides an easily understood treatment of strike-slip faulting that covers the common transform types. However, it does not deal with indent-linked strike-slip faults. Profusely illustrated with colored maps and diagrams, it is suitable for high school readers.

Davis, George H., and Stephen J. Reynolds. Structural Geology of Rocks and Regions. 2d ed. New York: John Wiley & Sons, 1996. Provides a thorough, comprehensive treatment of faults and faulting, as well as a great deal of information about the strength of rock, the accumulation of strain, and other related aspects of geology. More technical than the other references cited.

Fossen, Haakon. Structural Geology. New York: Cambridge University Press. 2010. This text is well written and easy to understand. An excellent text for geology students or resource for geologists. Provides many links between structural geology theory and application. Photos and illustrations add great value to the text. Contains a glossary, references, an appendix of photo captions, and indexing.

Fowler, Christine Mary Rutherford. The Solid Earth: An Introduction to Global Geophysics. 2d ed. Cambridge: Cambridge University Press, 2004. This book provides an outstanding treatment of how offsets between plates can be determined, excellent descriptions of the detailed structure of oceanic transform faults, and a useful discussion of the extrusion tectonics associated with the formation of the Himalaya Mountains. Although not suited for high school readers, its treatment of these topics requires no mathematics beyond algebra and trigonometry.

Wyld, Sandra J., and James E. Wright. “New Evidence for Cretaceous Strike-Slip Faulting in the United States Cordillera and Implications for Terrane-Displacement, Deformation Patterns, and Plutonism.” American Journal of Science 301 (2001): 150-181. Discusses the relationship between strike-slip faulting and terrane displacement. Highly technical.

Yeats, Robert S., Kerry Sieh, and Clarence R. Allen. The Geology of Earthquakes. New York: Oxford University Press, 1997. A thorough and detailed exploration of all aspects of earthquakes, with emphasis on the geological evidence used to study them, this book is an excellent resource. Although some of the treatment may be too detailed for a beginner or casual reader, these areas can be skimmed over easily. Concepts are explained well, and great care is taken to keep terminology concise and understandable. Profusely illustrated with black-and-white maps and diagrams, it also has a very useful index.