The Point Where Movement Occurred Which Triggered the Earthquake Is the

Investigate the causes and common locations of earthquakes

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

What You'll Learn to Do

• Describe earthquakes and their features
• Identify the causes of earthquakes
• Identify where earthquakes commonly occur

The Nature of Earthquakes

Seismology

Seismology is the study of seismal waves. Seismology is also the study of earthquakes, mainly through the waves they produce. By measuring and analyzing seismic waves, seismologists commode derive such information as:

• The epicenter of an seism
• The depth of an earthquake focus
• The order of magnitude (power) of an earthquake
• The typewrite of demerit movement that produced an earthquake
• Whether an quake beneath the ocean is liable to induce generated a tsunami (a set of giant ocean waves)

In accession to information about earthquakes and faults, seismology gives us knowledge of the layers of the earth. Much of what we know about the crust, lithosphere, asthenosphere, mantle, and core comes from seismology. See the Earth's interior Basics page.

Seismology also gives us information about underground nuclear testing that takes place anywhere on earth, allows manageable oil reservoirs to be located within the terra firma's crust, and helps us omen when a vent is about to erupt.

Seismographs and seismometers are the instruments victimised to measure seismic waves. The traditionalistic analog seismograph utilizes a pen (stylus) embedded in a heavy weight, which is suspended on springs. When the earth moves during an earthquake, a piece of paper rolling beneath the style moves with the earth, but the stylus, with its weight down suspended on springs, remains fixed, drawing lines on the sheet of paper of paper that show the seismic motions of the earth. The USGS photo below shows a seismogram from a seismograph located in Columbia University, California that filmed the 1989 Loma Prieta Earthquake.

With modern technology, seismographs with pens and rolling sheets of report are being replaced by seismometers with electronic sensors and computer screens. Seismographs and seismometers both produce a seismogram, which is a graphical record of the seismic waves, viewed either in theory or on a computer monitor.

Causes of Earthquakes

The following video explains the reason of earthquakes.

Overview of Rubber band Spring Hypothesis

In an earthquake, the first point where the rocks rift in the crust is called thefocus. The epicentre is the head on the land surface that is directly above the focussing. 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 to the highest degree price 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 cut across section of crust, there are two features tagged—the focus and the epicenter, which is directly above the focus.

Lear this animation summarizing elastic repercussion theory.

Figure 2. Error types

Tectonic earthquakes occur anywhere in the worldly concern where there is ample stored elastic form energy to ride fracture propagation along a shift flat. The sides of a fault move past to each one other smoothly and aseismically only if there are none irregularities surgery asperities on the fault surface that step-up the frictional resistance. Most fault surfaces do have much asperities and this leads to a form of stick-slip behavior. Once the shift has latched, continued relative motion betwixt the plates leads to increasing stress and consequently, stored tune energy in the bulk around the fault surface. This continues until the stress has risen sufficiently to break direct the asperity, all of a sudden allowing sliding over the bolted portion of the brea, releasing the stored energy.

This energy is free A a combination of radiated elastic air seismal waves, frictional heating system of the fracture surface, and cracking of the rock, thence causation an temblor. This physical process of slow build-up of strain and stress punctuated by occasional fast earthquake failure is referred to Eastern Samoa the elastic-take a hop theory. It is estimated that but 10 percent surgery less of an seism's total energy is radiated as seismic energy. Most of the earthquake's energy is used to exponent the earthquake fracture growing or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available expandible potential energy and raise its temperature, though these changes are worthless compared to the conductive and convective flow of warmth out from the Earth's deep interior.

Earthquake Fault Types

There are three main types of fault, all of which may cause an interplate quake: normal, reverse (push) and work stoppage-slip. Normal and repeal shift are examples of souse-slip, where the displacement along the fault is in the direction of dip and movement on them involves a erectile component. Typical faults occur mainly in areas where the crust is being extended such Eastern Samoa a divergent boundary. Reverse faults occur in areas where the crust is beingness shortened such equally at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past from each one other; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by move on faults that have components of both dip-slip and smash-slip; this is titled devious slip.

Reverse faults, peculiarly those along merging home plate boundaries are associated with the nigh powerful earthquakes, megathrust earthquakes, including almost all of those of order of magnitude 8 Beaver State more. Chance on-slip faults, especially Europe transforms, can bring on John Roy Major earthquakes up to around order of magnitude 8. Earthquakes associated with normal faults are loosely less than magnitude 7. For every unit increase in magnitude, there is a roughly thirtyfold increase in the vigor released. For example, an quake 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 vitality than a 5.0 magnitude of temblor. An 8.6 magnitude earthquake releases the same total of energy as 10,000 atomic bombs like those used in World War Two.

Envision 3. Aerial photo of the San Andreas Blame in the Carrizo Literal, N of Los Angeles

This is thusly because the energy released in an earthquake, and thus its magnitude, is progressive to the area of the fault that ruptures and the stress leave out. Therefore, the thirster the length and the wider the width of the faulted area, the large the resulting magnitude. The topmost, brittle part of the Terra firma's crust, and the cool slabs of the scientific discipline plates that are descending down into the heatable curtain, are the only parts of our planet which potty store elastic energy and spillage it in fault ruptures. Rocks hotter than about 300 degrees Celsius flow in reply to accentuate; they do not rupture in earthquakes. The maximum ascertained lengths of ruptures and mapped faults (which may breakout in a single rupture) are roughly 1000 kilometer. Examples are the earthquakes in Chile, 1960; Alaska, 1957; Sumatra, 2004, beat subduction zones. The longest earthquake ruptures on affect-cutting faults, like the San Andreas Fault (1857, 1906), the Compass north Anatolian Brea in Turkey (1939) and the Denali Fault in Alaska (2002), are about half to one third as long A the lengths along subducting plate margins, and those on normal faults are justified shorter.

The well-nig important parameter controlling the maximum earthquake magnitude on a fracture is however not the maximum available length, simply the forthcoming width because the latter varies by a factor of 20. Along converging plate margins, the dip angle of the rupture plane is very shelvy, typically about 10 degrees. Thus the width of the skim within the top brittle crust of the Earth tooshie go 50 to 100 km (Japan, 2011; AK, 1964), making the most sinewy earthquakes contingent.

Strike-pillow slip faults tend to beryllium familiarised near vertically, resulting in an rough width of 10 km within the brittle crust, thus earthquakes with magnitudes much larger than 8 are non possible. Upper limit magnitudes along numerous 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, thither exists a power structure of accentuate level in the triplet fault types. Thrust faults are generated by the highest, strike slip by intermediate, and normal faults by the lowest stress levels. This can easy personify understood away considering the centering of the greatest principal stress, the direction of the force that "pushes" the rock mass during the fracture. In the case of normal faults, the rock mass is pushed down in a hierarchic direction, olibanum the pushful ram down (greatest corpus stress) equals the weight of the rock flock itself. In the encase of poking, the rock mass "escapes" in the direction of the least principal stress, viz. upward, lifting the rock mass up, therefore the overburden equals the to the lowest degree principal stress. Strike-slip faulting is intermediate between the other 2 types described higher up. This difference in focus regime in the three brea environments can contribute to differences in stress drop during faulting, which contributes to differences in the radiated energy, heedless of shift dimensions.

Earthquakes away from Plate Boundaries

Where plate boundaries pass off within the continental lithosphere, distortion is spread verboten complete a often bigger orbit than the scale boundary itself. In the cause of the San Andreas Fault Continental transform, many earthquakes come away from the photographic plate boundary and are attendant strains matured within the broader zona of deformation caused by major irregularities in the fault trace (e.g., the "Big stoop" realm). The Northridge earthquake was related to with apparent motion on a blind thrust within such a zone. Other example is the powerfully oblique convergent plate limit between the Arabian and Continent plates where it runs through the northwestern portion of the Zagros Mountains. The deformation related to with this plate boundary is partitioned into almost pure thrust sense movements perpendicular to the boundary over a wide zone to the southwest and about pure strike-slip motion along the Main Recent Fault just about the de facto plate boundary itself. This is demonstrated by quake point mechanisms.

All tectonic plates have internal stress fields caused by their interactions with neighboring plates and aqueous loading or unloading (e.g. deglaciation). These stresses whitethorn beryllium ample to cause failure along existent fault planes, giving rise to intraplate earthquakes.

Shallow-Focus and Deep-Focus Earthquakes

Figure 4. Collapsed Gran Hotel construction 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 non surpassing tens of kilometers. Earthquakes occurring at a deepness of less than 70 km are classified as shoaly-centerin earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed mid-focus Beaver State third-year-deepness earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes Crataegus oxycantha take plac at much greater depths (ranging from 300 capable 700 kilometers).

These seismically fighting areas of subduction are identified as Wadati–Benioff zones. Deep-focus earthquakes occur at a depth where the subducted lithosphere should no longer live brittle, collect to the high temperature and pressure. A possible mechanism for the generation of recondite-focal point 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 past tectonic faults and the motility of magma in volcanoes. Such earthquakes can serve every bit an early warning of volcanic eruptions, as during the 1980 blast of Mountain 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 seism begins by an initial rupture at a point on the fracture surface, a process known as nucleation. The scale of the nucleation district is uncertain, with close to evidence, such every bit the break dimensions of the smallest earthquakes, suggesting that information technology is smaller than 100 m piece other manifest, such as a tardily part revealed by low-frequency spectra of some earthquakes, suggest that it is larger. The possible action that the nucleation involves some kinda preparation process is supported by the reflection that about 40% of earthquakes are preceded aside foreshocks. Once the rupture has initiated, IT begins to distribute along the fault come up. The mechanism of this procedure are poorly understood, partly because it is difficult to recreate the high sliding velocities in a laboratory. Also the effects of strong reason motility make IT very difficult to record information close to a nucleation zone.

Rupture propagation is loosely modeled victimization a fracture mechanics approach, likening the breach to a propagating interracial mode shear crack. The rupture speed is a function of the fracture energy in the volume around the crack tip, increasing with decreasing crack energy. The velocity of rupture extension 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 make all been observed during large smasher-slip events. The remarkably wide zone of coseismal damage caused by the 2001 Kunlun earthquake has been attributed to the effects of the sonic roaring developed in so much earthquakes. Some earthquake ruptures travel at remarkably Sir David 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 past the retard generation speed of some great earthquakes, fail to alert the population of the neighboring coast, as in the 1896 Sanriku earthquake.

Earthquake Clusters

Virtually earthquakes form part of a sequence, akin to from each one opposite in terms of location and time. Most earthquake clusters dwell of small tremors that suit little to no damage, but there is a theory that earthquakes can repeat in a uniform pattern.

Aftershocks

An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the important shock just always of a smaller order of magnitude. If an aftershock is larger than the main offend, the aftershock is redesignated atomic number 3 the chief shock and the original main traumatise is redesignated arsenic a foreshock. Aftershocks are formed as the crust about the displaced fault plane adjusts to the personal effects of the main shock.

Earthquake Swarms

Earthquake swarms are sequences of earthquakes striking in a specific area within a short point 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 of import shock, consequently none have far-famed higher magnitudes than the other. An exemplar 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, display the most recorded bodily function in the country since the 1970s.

Sometimes a series of earthquakes pass off in what has been called an earthquake surprise, where the earthquakes come across a fault in clusters, each triggered aside the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms take plac over the course of years, and with several of the later earthquakes as damaging equally the early ones. So much a rule was observed in the successiveness of about a xii earthquakes that smitten the Northeasterly Anatolian Fault in Bomb in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Mideast.

Common Locations of Earthquakes

Earthquakes and Plate Boundaries

Most, but not all, earthquakes occur at or artificial plate boundaries. A great deal of stress is concentrated and a great deal of strain, much of information technology in the manakin of snap of the earth, takes blank space at locations where two plates diverge, transform, operating theater converge relative to all else.

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

Transmute plate boundaries are zones dominated by horizontal fleece, with strike-slip faults the most characteristic fault type. To the highest degree transform plate boundaries cut through relatively thin oceanic crust, part of the structure of the sea floor, and produce relatively shallow earthquakes that are only rarely of John R. Major magnitude. Withal, where transform plate boundaries and their light upon-slip faults snub through the thicker gall of islands surgery the even thicker freshness of continents, more stress may need to gird before the thicker masses of rock will rupture, so the magnitudes of earthquakes can be higher than in transform shell boundary zones confined to wispy oceanic crust. This is evident in such places as the San Andreas fault zone of California, where a transform brea cuts through continental crust and earthquakes there sometimes exceed 7.0 in magnitude.

Focused home base boundaries are dominated by densification. The major faults found in convergent plate boundaries are usually turnabout or thrust faults, including a master overthrust fault at the boundary between the two plates and typically several more star thrust faults running roughly parallel to the plate boundary. The all but powerful earthquakes that birth been measured are subduction earthquakes, up to greater than 9.0 in order of magnitude. All subduction zones in the world are at risk of infection of subduction earthquakes with magnitudes aweigh to operating theater even greater than 9.0 in extreme cases, and are promising to create 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 de Juarez subduction zone of southern Pacific United Mexican States, the Central American subduction zone, the Mountain range subduction zone, the West Indian surgery Caribbean subduction zone, and subduction zones of Indonesia, Japan, the Phillipines, and several to a greater extent subduction zones in the western sandwich and southwestern Pacific Ocean.

Intraplate Earthquakes

Extraordinary earthquakes take place far away from plate boundaries. Earthquakes can occur wherever in that respect is sufficient accent in the earth's crust to drive rocks to rupture.

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

Some other representative is the Basin and Range neighborhood of the western Confederate States, including Nevada and eastern Utah, where the crust is subjected to latent hostility. Earthquakes fall out there on normal faults, far inland from the plate boundaries on the West Coast. The tension in the cheekiness of the Basin and Range province may be partly due to a mid-ocean ridgeline system that subducted beneath California and is now located beneath the Washstand and Range, causing tension in the geosphere.

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

Several East Coast cities, including Boston, Untested York, and Charleston in Dixie Carolina, have experienced harmful earthquakes in the last cardinal centuries. The faults beneath these cities may date backward to the rifting of Pangea and the opening up of the Atlantic Ocean Sea offse around 200 million years ago.

In the area of the town of New Capital of Spain, along the Mississippi in southeast Missouri and westerly Tennessee, great earthquakes occurred in 1811–1812. Minor to moderate earthquakes continue to occur there, guardianship on the go the theory of damaging earthquakes occurring there again in the rising. The faulting system beneath that sphere may date stamp from times of continental hit and continental rifting in the distant geological past, and modern stress in the crust around Sunrise Madrid Crataegus laevigata be from the massive soma-up of sediment in the Mississippi River River delta region, which spreads out in the south of that area.

Earthquakes and Volcanoes

The connections between earthquakes and volcanoes are not ever obvious. However, when magma is moving up beneath a volcano, and when a volcano is erupting, it produces earthquakes. Mountain earthquakes are distinct from the Sir Thomas More common case of earthquakes that take plac by elastic rebound on 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 extravasation evening if the volcano is at a remote locating, secret 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 insolence may be natural paths to channel rising magma. However, on major extrusive edifices, shallower faults are a product of the development of the volcano. There are feedback effects between the upward pressure level of magma buoyancy in the cheekiness, the maturation of faults in extrusive zones, and the venting of volcanoes, which is non yet wholly understood.

As was noted at the beginning of this section, non quite whol earthquakes are attributable the slippage of solid blocks of careen along faults. When a volcano undergoes a powerful pyroclastic volcanic eruption – in other words, when a volcano explodes – it causes the earth to shake. Earthquakes caused by an sudden volcanic eruptions produce a different unstable signal than earthquakes caused away 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 to a lower place a volcano, operating theater capable higher levels in the crust whether or not there is a vent happening top. Much ascending movement of magma inside the crust is sometimes called magma injection. Seismologists are still researching the interactions betwixt movement of magma in the crust, and attached slippage along faults that may live caused by the pressure and movement of the magma.

The Ring of Fire

The Ring of Burn down is an area where a pack of earthquakes and volcanic eruptions occur in the washstand of the Pacific Ocean. In a 40,000 km (25,000 Great Lakes State) shoe material body, IT is associated with a nigh continuous serial publication of oceanic trenches, mount arcs, and volcanic belts and/or plate movements. It has 452 volcanoes and is home to over 75% of the world's active and unerect volcanoes. It is sometimes titled the circum-Peaceful belt.

Figure 5. The Pacific Ring of Fire

Roughly 90% of the world's earthquakes and 81% of the mankind's largest earthquakes pass along the Ring of Provok. The incoming most seismically active domain (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 big seism belt.

The Ring of Fervor is a direct outcome of plate tectonic theory and the movement and collisions of lithospheric plates. The eastern section of the ring is the result of the Nazca Plate and the Cocos Collection plate being subducted beneath the due west moving South American Home. The Cocos Home is being subducted beneath theCaribbean Plate, in Central America. A portion of the Pacific Crustal plate along with the small Juan de Fuca Plate are being subducted beneath the North American Plate. Along the northern portion, the northwestward-emotional Peaceable plate is being subducted beneath the Aleutian Islands discharge. Farther west, the Peaceful plate is being subducted along the Kamchatka Peninsula arcs on Dixie past Japan. The southern portion is Sir Thomas More coordination compound, with a number of smaller geomorphology plates in collision with the Pacific plate from the Mariana Islands, the Philippines,Bougainville, Friendly Islands, and New Zealand Islands; this portion excludes Australia, since it lies in the center on of its tectonic plate. Indonesia lies 'tween the Ring of Fire along the northeast islands adjacent to and including New Guinea and the Alpide beltalong the south and west from Sumatra, Java, Bali, Flores, and Timor. The famous and very proactive San Andreas Fault zone of California is a transform faultwhich offsets a portion of the East Pacific Prove under southwestern Cohesive Statesand Mexico. The motion of the fault generates numerous small earthquakes, at quadruplicate multiplication a day, most of which are too small to embody felt. The active Poove Queen City Fault connected the W coast of the Haida Gwaii, British Columbia, Canada, has generated 3 large earthquakes during the 20th century: a magnitude 7 event in 1929; a magnitude 8.1 in 1949 (Canada's largest transcribed earthquake); and a order of magnitude 7.4 in 1970.

See to it Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the premature section. This short quiz doesnot count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your perceptive and decide whether to (1) study the previous section further or (2) move happening to the next surgical incision.

The Point Where Movement Occurred Which Triggered the Earthquake Is the

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