Can the Northridge Fault Rupture Again

The following video explains the cause of earthquakes.

Overview of Rubberband Rebound Theory

In an earthquake, the initial point where the rocks rupture in the crust is called thefocus. The epicenter is the point on the land surface that is direct above the focus. In virtually 75% of earthquakes, the focus is in the top ten to 15 kilometers (six to nine miles) of the crust. Shallow earthquakes cause the near damage because the focus is nigh where people live. However, it is the epicenter of an earthquake that is reported past scientists and the media (figure ane).

Effigy 1. In the vertical cross department of crust, there are two features labeled—the focus and the epicenter, which is directly higher up the focus.

Watch this animation summarizing elastic rebound theory.

Figure 2. Mistake types

Tectonic earthquakes occur anywhere in the globe where at that place is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. The sides of a mistake move by each other smoothly and aseismically merely if there are no irregularities or asperities along the error surface that increase the frictional resistance. Almost fault surfaces do have such asperities and this leads to a course of stick-sideslip behavior. Once the error has locked, continued relative movement 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 incommensurateness, suddenly allowing sliding over the locked portion of the error, releasing the stored free energy.^[1]

This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the error surface, and neat of the rock, thus causing an convulsion. This process of gradual build-upward of strain and stress punctuated by occasional sudden earthquake failure is referred to every bit the elastic-rebound theory. It is estimated that but 10 pct or less of an earthquake's full energy is radiated as seismic free energy. Most of the convulsion's free energy is used to power the convulsion fracture growth or is converted into rut generated by friction. Therefore, earthquakes lower the Earth's available rubberband potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the World'southward deep interior.^[ii]

Earthquake Fault Types

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

Opposite faults, particularly those along convergent plate boundaries are associated with the most powerful earthquakes, megathrust earthquakes, including nearly all of those of magnitude viii or more. Strike-slip faults, particularly continental transforms, can produce major earthquakes up to about magnitude 8. Earthquakes associated with normal faults are mostly less than magnitude seven. For every unit increment in magnitude, there is a roughly thirtyfold increment in the energy released. For instance, an convulsion of magnitude 6.0 releases approximately 30 times more free energy than a 5.0 magnitude convulsion and a 7.0 magnitude convulsion releases 900 times (30 × 30) more than free 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 Earth State of war 2.

Effigy 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 expanse of the fault that ruptures^[3] and the stress drib. Therefore, the longer the length and the wider the width of the faulted area, the larger the resulting magnitude. The topmost, brittle office of the Earth'south crust, and the cool slabs of the tectonic plates that are descending down into the hot mantle, are the just parts of our planet which can store rubberband energy and release it in fault ruptures. Rocks hotter than about 300 degrees Celsius menstruation in response to stress; they do not rupture in earthquakes.^[four] The maximum observed lengths of ruptures and mapped faults (which may break in a single rupture) are approximately chiliad km. Examples are the earthquakes in Chile, 1960; Alaska, 1957; Sumatra, 2004, all in subduction zones. The longest earthquake ruptures on strike-sideslip faults, like the San Andreas Fault (1857, 1906), the North Anatolian Fault in Turkey (1939) and the Denali Mistake in Alaska (2002), are well-nigh half to one third every bit long every bit the lengths along subducting plate margins, and those forth normal faults are even shorter.

The virtually of import parameter decision-making the maximum earthquake magnitude on a error is however non the maximum available length, only the available width considering the latter varies by a factor of twenty. Forth converging plate margins, the dip angle of the rupture plane is very shallow, typically about 10 degrees.^[5] Thus the width of the airplane within the top breakable crust of the Earth tin can become 50 to 100 km (Japan, 2011; Alaska, 1964), making the well-nigh powerful earthquakes possible.

Strike-slip faults tend to be oriented near vertically, resulting in an judge width of 10 km within the brittle crust,^{[half dozen]} thus earthquakes with magnitudes much larger than viii are not possible. Maximum magnitudes forth many normal faults are fifty-fifty more limited because many of them are located along spreading centers, as in Republic of iceland, where the thickness of the brittle layer is only about 6 km.^[7]

In improver, 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.^[8] This can easily be understood by because the direction of the greatest principal stress, the direction of the strength that "pushes" the rock mass during the faulting. In the example of normal faults, the rock mass is pushed down in a vertical direction, thus the pushing forcefulness (greatest principal stress) equals the weight of the rock mass itself. In the example 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 primary stress. Strike-slip faulting is intermediate betwixt the other two types described to a higher place. This departure in stress regime in the three faulting environments can contribute to differences in stress drib during faulting, which contributes to differences in the radiated energy, regardless of error 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 instance of the San Andreas error continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused past major irregularities in the fault trace (e.g., the "Big curve" region). The Northridge earthquake was associated with move on a blind thrust inside such a zone. Another example is the strongly oblique convergent plate boundary betwixt the Arabian and Eurasian plates where information technology runs through the northwestern part of the Zagros Mountains. The deformation associated with this plate purlieus is partitioned into near pure thrust sense movements perpendicular to the boundary over a broad zone to the southwest and nearly pure strike-slip motion forth the Chief Recent Fault close to the bodily plate boundary itself. This is demonstrated by convulsion focal mechanisms.^[9]

All tectonic plates have internal stress fields acquired by their interactions with neighboring plates and sedimentary loading or unloading (e.g. deglaciation).^[10] These stresses may exist sufficient to crusade failure along existing error planes, giving rise to intraplate earthquakes.^[eleven]

Shallow-Focus and Deep-Focus Earthquakes

Effigy 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 band 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 seventy and 300 km are commonly termed mid-focus or intermediate-depth earthquakes. In subduction zones, where older and colder oceanic crust descends beneath some other tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers).^[12]

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 breakable, due to the loftier 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.^[13]

Earthquakes and Volcanic Activity

Earthquakes often occur in volcanic regions and are acquired in that location, both past tectonic faults and the movement of magma in volcanoes. Such earthquakes can serve every bit an early warning of volcanic eruptions, every bit during the 1980 eruption of Mount St. Helens.^[14] Earthquake swarms tin can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms tin can exist recorded past seismometers and tiltmeters (a device that measures ground slope) and used every bit sensors to predict imminent or upcoming eruptions.^[15]

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 show, such as a boring component revealed by depression-frequency spectra of some earthquakes, advise that it is larger. The possibility that the nucleation involves some sort of preparation procedure is supported by the ascertainment that well-nigh 40% of earthquakes are preceded past foreshocks. Once the rupture has initiated, information technology 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 potent footing motion make it very difficult to tape information shut to a nucleation zone.^[xvi]

Rupture propagation is generally modeled using a fracture mechanics approach, likening the rupture to a propagating mixed way shear crack. The rupture velocity is a office of the fracture free 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 South-wave velocity, and this is contained 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 broad zone of coseismic damage caused past the 2001 Kunlun earthquake has been attributed to the effects of the sonic smash developed in such earthquakes. Some earthquake ruptures travel at unusually low velocities and are referred to as dull earthquakes. A especially dangerous form of irksome earthquake is the seismic sea wave convulsion, observed where the relatively low felt intensities, caused by the boring propagation speed of some great earthquakes, fail to alert the population of the neighboring coast, every bit in the 1896 Sanriku earthquake.^[17]

Convulsion Clusters

Most earthquakes form function of a sequence, related to each other in terms of location and time.^[18] Most convulsion clusters consist of small tremors that cause little to no harm, but in that location is a theory that earthquakes can recur in a regular pattern.^[xix]

Aftershocks

An aftershock is an earthquake that occurs later on a previous earthquake, the mainshock. An aftershock is in the same region of the primary stupor merely ever 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 equally a foreshock. Aftershocks are formed every bit the crust around the displaced fault plane adjusts to the effects of the chief shock.^[20]

Earthquake Swarms

Earthquake swarms are sequences of earthquakes striking in a specific area within a short menstruation of time. They are different from earthquakes followed by a serial of aftershocks by the fact that no single earthquake in the sequence is plainly the main shock, therefore none have notable higher magnitudes than the other. An example of an convulsion swarm is the 2004 activeness at Yellowstone National Park.^[21] In August 2012, a swarm of earthquakes shook Southern California's Imperial Valley, showing the most recorded activity in the area since the 1970s.^[22]

Sometimes a serial of earthquakes occur in what has been called an earthquake storm, where the earthquakes strike a error 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 equally 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 E.^[23]

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