Investigate the causes and common locations of earthquakes

In this section, you will learn what causes earthquakes and why. You will also learn the locations of common earthquakes.

The Nature of Earthquakes

Seismology

Causes of Earthquakes

The following video explains the cause of earthquakes.

https://youtube.com/watch?v=VSgB1IWr6O4%3Fenablejsapi%3D1

Overview of Elastic Rebound Theory

In an earthquake, the initial point where the rocks rupture in the crust is called the focus. The epicenter is the point on the land surface that is directly above the focus. In about 75% of earthquakes, the focus is in the top 10 to 15 kilometers (6 to 9 miles) of the crust. Shallow earthquakes cause the most damage because the focus is near where people live. However, it is the epicenter of an earthquake that is reported by scientists and the media (figure 1).

Figure 1. In the vertical cross section of crust, there are two features labeled—the focus and the epicenter, which is directly above the focus.

 

Watch this animation summarizing elastic rebound theory.

Figure 2. Fault types

Tectonic earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. The sides of a fault move past each other smoothly and aseismically only if there are no irregularities or asperities along the fault surface that increase the frictional resistance. Most fault surfaces do have such asperities and this leads to a form of stick-slip behavior. Once the fault has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy.

This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake’s total energy is radiated as seismic energy. Most of the earthquake’s energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth’s available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth’s deep interior.

Earthquake Fault Types

There are three main types of fault, all of which may cause an interplate earthquake: normal, reverse (thrust) and strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip.

Reverse faults, particularly those along convergent plate boundaries are associated with the most powerful earthquakes, megathrust earthquakes, including almost all of those of magnitude 8 or more. Strike-slip faults, particularly continental transforms, can produce major earthquakes up to about magnitude 8. Earthquakes associated with normal faults are generally less than magnitude 7. For every unit increase in magnitude, there is a roughly thirtyfold increase in the energy released. For instance, an earthquake of magnitude 6.0 releases approximately 30 times more energy than a 5.0 magnitude earthquake and a 7.0 magnitude earthquake releases 900 times (30 × 30) more energy than a 5.0 magnitude of earthquake. An 8.6 magnitude earthquake releases the same amount of energy as 10,000 atomic bombs like those used in World War II.

Figure 3. Aerial photo of the San Andreas Fault in the Carrizo Plain, northwest of Los Angeles

This is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of the fault that ruptures and the stress drop. Therefore, the longer the length and the wider the width of the faulted area, the larger the resulting magnitude. The topmost, brittle part of the Earth’s crust, and the cool slabs of the tectonic plates that are descending down into the hot mantle, are the only parts of our planet which can store elastic energy and release it in fault ruptures. Rocks hotter than about 300 degrees Celsius flow in response to stress; they do not rupture in earthquakes. The maximum observed lengths of ruptures and mapped faults (which may break in a single rupture) are approximately 1000 km. Examples are the earthquakes in Chile, 1960; Alaska, 1957; Sumatra, 2004, all in subduction zones. The longest earthquake ruptures on strike-slip faults, like the San Andreas Fault (1857, 1906), the North Anatolian Fault in Turkey (1939) and the Denali Fault in Alaska (2002), are about half to one third as long as the lengths along subducting plate margins, and those along normal faults are even shorter.

The most important parameter controlling the maximum earthquake magnitude on a fault is however not the maximum available length, but the available width because the latter varies by a factor of 20. Along converging plate margins, the dip angle of the rupture plane is very shallow, typically about 10 degrees. Thus the width of the plane within the top brittle crust of the Earth can become 50 to 100 km (Japan, 2011; Alaska, 1964), making the most powerful earthquakes possible.

Strike-slip faults tend to be oriented near vertically, resulting in an approximate width of 10 km within the brittle crust, thus earthquakes with magnitudes much larger than 8 are not possible. Maximum magnitudes along many normal faults are even more limited because many of them are located along spreading centers, as in Iceland, where the thickness of the brittle layer is only about 6 km.

In addition, there exists a hierarchy of stress level in the three fault types. Thrust faults are generated by the highest, strike slip by intermediate, and normal faults by the lowest stress levels. This can easily be understood by considering the direction of the greatest principal stress, the direction of the force that “pushes” the rock mass during the faulting. In the case of normal faults, the rock mass is pushed down in a vertical direction, thus the pushing force (greatest principal stress) equals the weight of the rock mass itself. In the case of thrusting, the rock mass “escapes” in the direction of the least principal stress, namely upward, lifting the rock mass up, thus the overburden equals the least principal stress. Strike-slip faulting is intermediate between the other two types described above. This difference in stress regime in the three faulting environments can contribute to differences in stress drop during faulting, which contributes to differences in the radiated energy, regardless of fault dimensions.

Earthquakes away from Plate Boundaries

Where plate boundaries occur within the continental lithosphere, deformation is spread out over a much larger area than the plate boundary itself. In the case of the San Andreas fault continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace (e.g., the “Big bend” region). The Northridge earthquake was associated with movement on a blind thrust within such a zone. Another example is the strongly oblique convergent plate boundary between the Arabian and Eurasian plates where it runs through the northwestern part of the Zagros Mountains. The deformation associated with this plate boundary is partitioned into nearly pure thrust sense movements perpendicular to the boundary over a wide zone to the southwest and nearly pure strike-slip motion along the Main Recent Fault close to the actual plate boundary itself. This is demonstrated by earthquake focal mechanisms.

All tectonic plates have internal stress fields caused by their interactions with neighboring plates and sedimentary loading or unloading (e.g. deglaciation). These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes.

Shallow-Focus and Deep-Focus Earthquakes

Figure 4. Collapsed Gran Hotel building in the San Salvador metropolis, after the shallow 1986 San Salvador earthquake.

The majority of tectonic earthquakes originate at the ring of fire in depths not exceeding tens of kilometers. Earthquakes occurring at a depth of less than 70 km are classified as shallow-focus earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed mid-focus or intermediate-depth earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers).

These seismically active areas of subduction are known as Wadati–Benioff zones. Deep-focus earthquakes occur at a depth where the subducted lithosphere should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure.

Earthquakes and Volcanic Activity

Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, as during the 1980 eruption of Mount St. Helens. Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltmeters (a device that measures ground slope) and used as sensors to predict imminent or upcoming eruptions.

Rupture Dynamics

A tectonic earthquake begins by an initial rupture at a point on the fault surface, a process known as nucleation. The scale of the nucleation zone is uncertain, with some evidence, such as the rupture dimensions of the smallest earthquakes, suggesting that it is smaller than 100 m while other evidence, such as a slow component revealed by low-frequency spectra of some earthquakes, suggest that it is larger. The possibility that the nucleation involves some sort of preparation process is supported by the observation that about 40% of earthquakes are preceded by foreshocks. Once the rupture has initiated, it begins to propagate along the fault surface. The mechanics of this process are poorly understood, partly because it is difficult to recreate the high sliding velocities in a laboratory. Also the effects of strong ground motion make it very difficult to record information close to a nucleation zone.

Rupture propagation is generally modeled using a fracture mechanics approach, likening the rupture to a propagating mixed mode shear crack. The rupture velocity is a function of the fracture energy in the volume around the crack tip, increasing with decreasing fracture energy. The velocity of rupture propagation is orders of magnitude faster than the displacement velocity across the fault. Earthquake ruptures typically propagate at velocities that are in the range 70–90% of the S-wave velocity, and this is independent of earthquake size. A small subset of earthquake ruptures appear to have propagated at speeds greater than the S-wave velocity. These supershear earthquakes have all been observed during large strike-slip events. The unusually wide zone of coseismic damage caused by the 2001 Kunlun earthquake has been attributed to the effects of the sonic boom developed in such earthquakes. Some earthquake ruptures travel at unusually low velocities and are referred to as slow earthquakes. A particularly dangerous form of slow earthquake is the tsunami earthquake, observed where the relatively low felt intensities, caused by the slow propagation speed of some great earthquakes, fail to alert the population of the neighboring coast, as in the 1896 Sanriku earthquake.

Earthquake Clusters

Most earthquakes form part of a sequence, related to each other in terms of location and time. Most earthquake clusters consist of small tremors that cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern.

Aftershocks

An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock.

Earthquake Swarms

Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is obviously the main shock, therefore none have notable higher magnitudes than the other. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park. In August 2012, a swarm of earthquakes shook Southern California’s Imperial Valley, showing the most recorded activity in the area since the 1970s.

Sometimes a series of earthquakes occur in what has been called an earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East.

Common Locations of Earthquakes

Earthquakes and Plate Boundaries

Most, but not all, earthquakes occur at or near plate boundaries. A great deal of stress is concentrated and a great deal of strain, much of it in the form of rupture of the earth, takes place at locations where two plates diverge, transform, or converge relative to each other.

Tension is the dominant stress at divergent plate boundaries. Normal faults and rift valleys as the predominant earthquake-related structures at divergent plate boundaries. Earthquakes at divergent plate boundaries are usually relatively shallow, and, though they can be damaging, the most powerful earthquakes at divergent plate boundaries are not nearly as powerful as the most powerful earthquakes at convergent plate boundaries.

Transform plate boundaries are zones dominated by horizontal shear, with strike-slip faults the most characteristic fault type. Most transform plate boundaries cut through relatively thin oceanic crust, part of the structure of the ocean floor, and produce relatively shallow earthquakes that are only rarely of major magnitude. However, where transform plate boundaries and their strike-slip faults cut through the thicker crust of islands or the even thicker crust of continents, more stress may need to build up before the thicker masses of rock will rupture, and so the magnitudes of earthquakes can be higher than in transform plate boundary zones confined to thin oceanic crust. This is evident in such places as the San Andreas fault zone of California, where a transform fault cuts through continental crust and earthquakes there sometimes exceed 7.0 in magnitude.

Convergent plate boundaries are dominated by compression. The major faults found in convergent plate boundaries are usually reverse or thrust faults, including a master thrust fault at the boundary between the two plates and typically several more major thrust faults running roughly parallel to the plate boundary. The most powerful earthquakes that have been measured are subduction earthquakes, up to greater than 9.0 in magnitude. All subduction zones in the world are at risk of subduction earthquakes with magnitudes up to or even greater than 9.0 in extreme cases, and are likely to produce tsunamis. This includes the Cascadia subduction zone of northern California and coastal Oregon and Washington, the Aleutian subduction zone of southern Alaska, the Kamchatka subduction zone of Pacific Russia, the Acapulco subduction zone of southern Pacific Mexico, the Central American subduction zone, the Andean subduction zone, the West Indian or Caribbean subduction zone, and subduction zones of Indonesia, Japan, the Phillipines, and several more subduction zones in the western and southwestern Pacific Ocean.

Intraplate Earthquakes

Not all earthquakes occur at plate boundaries.
In fact, many quakes take place within plates (intraplate).
If you think about it, this is not that strange.  Faults occur all through the rigid outer shell of the earth that we call the crust.  Where the crust (and underlying lithosphere) is being tugged apart (divergent boundaries), or being smashed together (convergent boundaries), or sliding past (transform boundaries) there is little wonder that earthquakes occur.  But nevertheless, within plates are numerous faults that can undergo movement and generate sizable earthquakes!
Earthquakes can occur wherever there is sufficient stress in the earth’s crust to drive rocks to rupture.

For example, Hawaii is thousands of km (thousands of miles) from any plate boundary, but the volcanoes that compose the islands have built up so rapidly that they are still undergoing gravitational stabilization. Sectors of the Hawaiian islands occasionally slump along normal faults, producing intraplate earthquakes. Most of the earthquakes occur on the big island of Hawaii, which is composed of the youngest, most recently built volcanoes. The geologic record shows that parts of the older islands have undergone major collapses in the last few million years, with sections of the islands sliding out to the seafloor in landslides floored on shallow normal faults.

Another example is the Basin and Range region of the western United States, including Nevada and eastern Utah, where the crust is subjected to tension. Earthquakes occur there on normal faults, far inland from the plate boundaries on the West Coast. The tension in the crust of the Basin and Range province may be partly due to a mid-ocean ridge system that subducted beneath California and is now located beneath the Basin and Range, causing tension in the lithosphere.

The region around Yellowstone National Park also undergoes occasional major earthquakes on normal faults. Earthquakes in that area may be due to the Yellowstone hot spot causing differential thermal expansion of the lithosphere in a broad zone round the hot spot center.

Several East Coast cities, including Boston, New York, and Charleston in South Carolina, have experienced damaging earthquakes in the last two centuries. The faults beneath these cities may date back to the rifting of Pangea and the opening up of the Atlantic Ocean beginning around 200 million years ago.

In the area of the town of New Madrid, along the Mississippi River in southeastern Missouri and western Tennessee, some of the largest earthquakes to have ever occurred in the U.S. took place around the time of the 1811–1812 winter.  Even today, minor to moderate earthquakes continue to occur in this region, a reminder that damaging earthquakes could possibly be in the future.

The fault system beneath this area is very deep-seated and old.  It probably relates to processes that occurred over a billion years ago during formation, and early rifting, of the North American craton.

The fault system at depth could be reacting to stress associated with the massive build-up of sediment in the Mississippi River delta region or simply from flexure and stress changes within the North American Plate– we don’t know for sure!

Earthquakes and Volcanoes

The connections between earthquakes and volcanoes are not always obvious. However, when magma is moving up beneath a volcano, and when a volcano is erupting, it produces earthquakes. Volcanic earthquakes are distinct from the more common type of earthquakes that occur by elastic rebound along faults.

Seismologists can use the patterns and signals of earthquakes coming from beneath volcanoes to predict that the volcano is about to erupt, and can use seismic waves to see that a volcano is undergoing an eruption even if the volcano is at a remote location, hidden in darkness, or hidden in storm clouds.

Volcanic vents, and volcanoes in general, are commonly located along faults, or at the intersection of several faults. Major faults that already exist in the crust may be natural paths to channel rising magma. However, on major volcanic edifices, shallower faults are a product of the development of the volcano. There are feedback effects between the upward pressure of magma buoyancy in the crust, the growth of faults in volcanic zones, and the venting of volcanoes, which is not yet completely understood.

As was noted at the beginning of this section, not quite all earthquakes are due to the slippage of solid blocks of rock along faults. When a volcano undergoes a powerful pyroclastic eruption – in other words, when a volcano explodes – it causes the earth to shake. Earthquakes caused by an explosive volcanic eruptions produce a different seismic signal than earthquakes caused by slippage along faults.

Another example of earthquakes that are caused at least in part by magma movement, rather than by slippage of entirely solid rock along faults, is earthquakes set off by the movement of magma upward beneath a volcano, or up to higher levels in the crust whether or not there is a volcano on top. Such upward movement of magma within the crust is sometimes called magma injection. Seismologists are still researching the interactions between movement of magma in the crust, and related slippage along faults that may be caused by the pressure and movement of the magma.

The Ring of Fire

The Ring of Fire is an area where a large number of earthquakes and volcanic eruptions occur in the basin of the Pacific Ocean. In a 40,000 km (25,000 mi) horseshoe shape, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and/or plate movements. It has 452 volcanoes and is home to over 75% of the world’s active and dormant volcanoes. It is sometimes called the circum-Pacific belt.

Figure 5. The Pacific Ring of Fire

About 90% of the world’s earthquakes and 81% of the world’s largest earthquakes occur along the Ring of Fire. The next most seismically active region (5–6% of earthquakes and 17% of the world’s largest earthquakes) is the Alpide belt, which extends from Java to Sumatra through the Himalayas, theMediterranean, and out into the Atlantic. The Mid-Atlantic Ridge is the third most prominent earthquake belt.

The Ring of Fire is a direct result of plate tectonics and the movement and collisions of lithospheric plates. The eastern section of the ring is the result of the Nazca Plate and the Cocos Plate being subducted beneath the westward moving South American Plate. The Cocos Plate is being subducted beneath theCaribbean Plate, in Central America. A portion of the Pacific Plate along with the small Juan de Fuca Plate are being subducted beneath the North American Plate. Along the northern portion, the northwestward-moving Pacific plate is being subducted beneath the Aleutian Islands arc. Farther west, the Pacific plate is being subducted along the Kamchatka Peninsula arcs on south past Japan. The southern portion is more complex, with a number of smaller tectonic plates in collision with the Pacific plate from the Mariana Islands, the Philippines,Bougainville, Tonga, and New Zealand; this portion excludes Australia, since it lies in the center of its tectonic plate. Indonesia lies between the Ring of Fire along the northeastern islands adjacent to and including New Guinea and the Alpide belt along the south and west from Sumatra, Java, Bali, Flores, and Timor. The famous and very active San Andreas Fault zone of California is a transform faultwhich offsets a portion of the East Pacific Rise under southwestern United Statesand Mexico. The motion of the fault generates numerous small earthquakes, at multiple times a day, most of which are too small to be felt. The active Queen Charlotte Fault on the west coast of the Haida Gwaii, British Columbia, Canada, has generated three large earthquakes during the 20th century: a magnitude 7 event in 1929; a magnitude 8.1 in 1949 (Canada’s largest recorded earthquake); and a magnitude 7.4 in 1970.

SPEAKING OF THE RING OF FIRE–
Take a look at the image below, showing (on right side) the “side-view’ of Pacific Plate subduction beneath North America.
Above the subducting slab is a hypothetical volcano– which forms due to melting of the subducting plate.
But ALSO, we see little dots on the subducting plate, each referring to earthquake locations.  Red symbols are shallow quakes, yellow symbols are for mid-level quakes, and the blue dots are for deep quakes.
In a plan (or map) view, we see a pattern of earthquakes becoming deeper and deeper with distance from the subducting slab.

S. Earl, Univ of British Columbia, Open Text, Physical Geology

This sort of pattern, with earthquakes being deeper with distance from the subduction trench, was first identified by Hugo Benioff (and others) at Caltech University, in the early 1950’s.  It took some time to understand what was going on, but it makes a lot of sense– the subducting slab is getting deeper as it moves away from the trench!

Below, an image of quake depth with distance, at the Kurile Islands, in the Western Pacific.
In this case, the trench itself is around the 700km mark along the A-A’ transect.

Kurile Island Benioff Zone, WikiWand Image

A BIG QUESTION— ARE ALL SUBDUCTION ZONES THE SAME ANGLE OF STEEPNESS?

The Kurile Island Benioff zone, above, is pretty close to about 45 degree angle; suggesting that the down-going slab has that sort of geometry.
But, in fact, subduction zone angle depends on many factors– most importantly the DENSITY of the down-going slab.  If the slab is dense, it will descend at a steep angle (perhaps 60 degrees or more).  If the slab is less dense, or somewhat buoyant, then the subduction angle may be quite shallow!

In the case of Laramide Orogeny (mountain building) in the Rockies– around 70-40 million years ago– most geologists believe that the Farallon Plate (Pacific Ocean Lithosphere) was warm, being near the ridge where it was created, and the angle of subduction was very flat.
Below, an image prepared by Clair Currie, University of Alberta, showing the difference between subduction prior to Laramide time (above) and during Laramide.