What Causes Earthquakes?

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 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. There are three main types of fault, all of which may cause an interplate earthquake: 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. This is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of the fault that ruptures [4] and the stress drop. Therefore, the longer the length and the wider the width of the faulted area, the larger the resulting magnitude.

28 Comments

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 degrees Celsius flow in response to stress; they do not rupture in earthquakes. Examples are the earthquakes in Chile, ; Alaska, ; Sumatra, , all in subduction zones. The longest earthquake ruptures on strike-slip faults, like the San Andreas Fault , , the North Anatolian Fault in Turkey and the Denali Fault in Alaska , 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 Along converging plate margins, the dip angle of the rupture plane is very shallow, typically about 10 degrees. 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. 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. 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.

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. The majority of tectonic earthquakes originate at the ring of fire in depths not exceeding tens of kilometers. 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 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 eruption of Mount St. 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. 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 m while other evidence, such as a slow component revealed by low-frequency spectra of some earthquakes, suggest that it is larger.

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. 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 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 Sanriku earthquake. Tides may induce some seismicity , see tidal triggering of earthquakes for details. Most earthquakes form part of a sequence, related to each other in terms of location and time. 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 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 activity at Yellowstone National Park. 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.

Quaking or shaking of the earth is a common phenomenon undoubtedly known to humans from earliest times. Prior to the development of strong-motion accelerometers that can measure peak ground speed and acceleration directly, the intensity of the earth-shaking was estimated on the basis of the observed effects, as categorized on various seismic intensity scales. Only in the last century has the source of such shaking been identified as ruptures in the earth's crust, with the intensity of shaking at any locality dependent not only on the local ground conditions, but also on the strength or magnitude of the rupture, and on its distance.

The first scale for measuring earthquake magnitudes was developed by Charles F. Subsequent scales see seismic magnitude scales have retained a key feature, where each unit represents a ten-fold difference in the amplitude of the ground shaking, and a fold difference in energy. Subsequent scales are also adjusted to have approximately the same numeric value within the limits of the scale. Although the mass media commonly reports earthquake magnitudes as "Richter magnitude" or "Richter scale", standard practice by most seismological authorities is to express an earthquake's strength on the moment magnitude scale, which is based on the actual energy released by an earthquake.

It is estimated that around , earthquakes occur each year, detectable with current instrumentation. About , of these can be felt. The number of seismic stations has increased from about in to many thousands today. As a result, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes.

The United States Geological Survey estimates that, since , there have been an average of 18 major earthquakes magnitude 7. However, accurate recordings of earthquakes only began in the early s, so it is too early to categorically state that this is the case.

What causes an earthquake? - Curious

While most earthquakes are caused by movement of the Earth's tectonic plates , human activity can also produce earthquakes. Four main activities contribute to this phenomenon: The city of Newcastle was built over a large sector of coal mining areas. The earthquake has been reported to be spawned from a fault that reactivated due to the millions of tonnes of rock removed in the mining process. The instrumental scales used to describe the size of an earthquake began with the Richter magnitude scale in the s. It is a relatively simple measurement of an event's amplitude, and its use has become minimal in the 21st century.

Seismic waves travel through the Earth's interior and can be recorded by seismometers at great distances. The surface wave magnitude was developed in the s as a means to measure remote earthquakes and to improve the accuracy for larger events. The moment magnitude scale measures the amplitude of the shock, but also takes into account the seismic moment total rupture area, average slip of the fault, and rigidity of the rock. The Japan Meteorological Agency seismic intensity scale , the Medvedev—Sponheuer—Karnik scale , and the Mercalli intensity scale are based on the observed effects and are related to the intensity of shaking.

Every tremor produces different types of seismic waves, which travel through rock with different velocities:. Propagation velocity of the seismic waves ranges from approx. In the Earth's interior the shock- or P waves travel much faster than the S waves approx. The differences in travel time from the epicenter to the observatory are a measure of the distance and can be used to image both sources of quakes and structures within the Earth.

• Cain and Abel (LGBT BIBLE Book 3)?
• Related Content.
• What is an earthquake and what causes them to happen??
• The nature of earthquakes.
• Three Racing Hearts (The Morning After Book 3);
• Why Do Earthquakes Happen?!
• Works of George Crabbe!

Also, the depth of the hypocenter can be computed roughly. As a consequence, the first waves of a distant earthquake arrive at an observatory via the Earth's mantle. On average, the kilometer distance to the earthquake is the number of seconds between the P and S wave times 8. By such analyses of seismograms the Earth's core was located in by Beno Gutenberg.

S waves and later arriving surface waves do main damage compared to P waves. P wave squeezes and expands material in the same direction it is traveling. S wave shakes the ground up and down and back and forth. Earthquakes are not only categorized by their magnitude but also by the place where they occur. The world is divided into Flinn—Engdahl regions F-E regions , which are based on political and geographical boundaries as well as seismic activity. More active zones are divided into smaller F-E regions whereas less active zones belong to larger F-E regions.

Standard reporting of earthquakes includes its magnitude , date and time of occurrence, geographic coordinates of its epicenter , depth of the epicenter, geographical region, distances to population centers, location uncertainty, a number of parameters that are included in USGS earthquake reports number of stations reporting, number of observations, etc. Although relatively slow seismic waves have traditionally been used to detect earthquakes, scientists realized in that gravitational measurements could provide instantaneous detection of earthquakes, and confirmed this by analyzing gravitational records associated with the Tohoku-Oki "Fukushima" earthquake.

Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude , the distance from the epicenter , and the local geological and geomorphological conditions, which may amplify or reduce wave propagation.

Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits.

Ground rupture is a visible breaking and displacement of the Earth's surface along the trace of the fault, which may be of the order of several meters in the case of major earthquakes.

Earthquake

Ground rupture is a major risk for large engineering structures such as dams , bridges and nuclear power stations and requires careful mapping of existing faults to identify any which are likely to break the ground surface within the life of the structure.

Earthquakes, along with severe storms, volcanic activity, coastal wave attack, and wildfires, can produce slope instability leading to landslides, a major geological hazard. Landslide danger may persist while emergency personnel are attempting rescue. Earthquakes can cause fires by damaging electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started. For example, more deaths in the San Francisco earthquake were caused by fire than by the earthquake itself.

Soil liquefaction occurs when, because of the shaking, water-saturated granular material such as sand temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits. For example, in the Alaska earthquake , soil liquefaction caused many buildings to sink into the ground, eventually collapsing upon themselves. Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water — including when an earthquake occurs at sea.

Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them. Ordinarily, subduction earthquakes under magnitude 7. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly; the rocks catch on each other.

The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again.

The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus on top of the ground is called the epicenter of the earthquake. Earthquake-like seismic waves can also be caused by explosions underground. These explosions may be set off to break rock while making tunnels for roads, railroads, subways, or mines.

These explosions, however, don't cause very strong seismic waves. You may not even feel them. Sometimes seismic waves occur when the roof or walls of a mine collapse. These can sometimes be felt by people near the mine. The largest underground explosions, from tests of nuclear warheads bombs , can create seismic waves very much like large earthquakes. This fact has been exploited as a means to enforce the global nuclear test ban, because no nuclear warhead can be detonated on earth without producing such seismic waves.