Earthquake

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Earthquakes
Types Mainshock Foreshock Aftershock Blind thrust Doublet Interplate Intraplate Megathrust Remotely triggered Slow Submarine Supershear Tsunami Earthquake swarm
Causes Fault movement Volcanism Induced seismicity
Characteristics Epicenter Epicentral distance Hypocenter Shadow zone Seismic waves P wave S wave
Measurement Seismometer Seismic magnitude scales Seismic intensity scales
Prediction Coordinating Committee for Earthquake Prediction Forecasting
Other topics Shear wave splitting Adams–Williamson equation Flinn–Engdahl regions Earthquake engineering Seismite Seismology
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An earthquake – also called a quake, tremor, or temblor – is the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they cannot be felt, to those violent enough to propel objects and people into the air, damage critical infrastructure, and wreak destruction across entire cities. The seismic activity of an area is the frequency, type, and size of earthquakes experienced over a particular time. The seismicity at a particular location in the Earth is the average rate of seismic energy release per unit volume.

In its most general sense, the word earthquake is used to describe any seismic event that generates seismic waves. Earthquakes can occur naturally or be induced by human activities, such as

, landslides, and other seismic events. The frequency, type, and size of earthquakes in an area define its seismic activity, reflecting the average rate of seismic energy release.

Significant historical earthquakes include the

.

Efforts to manage earthquake risks involve prediction, forecasting, and preparedness, including

moonquakes

, have been observed on other celestial bodies, indicating the universality of such events beyond Earth.

Terminology

An earthquake is the shaking of the surface of Earth resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes may also be referred to as quakes, tremors, or temblors. The word tremor is also used for non-earthquake seismic rumbling.

In its most general sense, an earthquake is any seismic event—whether natural or caused by humans—that generates seismic waves. Earthquakes are caused mostly by the rupture of geological

is the point at ground level directly above the hypocenter.

The seismic activity of an area is the frequency, type, and size of earthquakes experienced over a particular time. The seismicity at a particular location in the Earth is the average rate of seismic energy release per unit volume.

Major examples

One of the most devastating earthquakes in recorded history was the 1556 Shaanxi earthquake, which occurred on 23 January 1556 in Shaanxi, China. More than 830,000 people died.^[2] Most houses in the area were yaodongs—dwellings carved out of loess hillsides—and many victims were killed when these structures collapsed. The 1976 Tangshan earthquake, which killed between 240,000 and 655,000 people, was the deadliest of the 20th century.^[3]

The

is simultaneously one of the deadliest earthquakes in history.

Earthquakes that caused the greatest loss of life, while powerful, were deadly because of their proximity to either heavily populated areas or the ocean, where earthquakes often create

that can devastate communities thousands of kilometers away. Regions most at risk for great loss of life include those where earthquakes are relatively rare but powerful, and poor regions with lax, unenforced, or nonexistent seismic building codes.

Occurrence

Reverse

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 where movement on them involves a vertical component. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip. The topmost, brittle part of the Earth's crust, and the cool slabs of the tectonic plates that are descending into the hot mantle, are the only parts of our planet that can store elastic energy and release it in fault ruptures. Rocks hotter than about 300 °C (572 °F) flow in response to stress; they do not rupture in earthquakes.^[11][12] The maximum observed lengths of ruptures and mapped faults (which may break in a single rupture) are approximately 1,000 km (620 mi). Examples are the earthquakes in Alaska (1957), Chile (1960), and 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.

Normal faults

Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Earthquakes associated with normal faults are generally less than magnitude 7. 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 six kilometres (3.7 mi).^[13][14]

Reverse faults

Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. 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. Megathrust earthquakes are responsible for about 90% of the total seismic moment released worldwide.^[15]

Strike-slip faults

Thus, earthquakes with magnitudes much larger than 8 are not possible.

In addition, there exists a hierarchy of stress levels in the three fault types. Thrust faults are generated by the highest, strike-slip by intermediate, and normal faults by the lowest stress levels.^[17] 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, and 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.

Energy released

For every unit increase in magnitude, there is a roughly thirty-fold increase in the energy released. For instance, an earthquake of magnitude 6.0 releases approximately 32 times more energy than a 5.0 magnitude earthquake and a 7.0 magnitude earthquake releases 1,000 times more energy than a 5.0 magnitude earthquake. An 8.6-magnitude earthquake releases the same amount of energy as 10,000 atomic bombs of the size used in World War II.^[18]

This is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of the fault that ruptures^[19] and the stress drop. Therefore, the longer the length and the wider the width of the faulted area, the larger the resulting magnitude. The most important parameter controlling the maximum earthquake magnitude on a fault, however, is 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.^[20] Thus, the width of the plane within the top brittle crust of the Earth can reach 50–100 km (31–62 mi) (such as in Japan, 2011, or in Alaska, 1964), making the most powerful earthquakes possible.

Focus

The majority of tectonic earthquakes originate in the Ring of Fire at depths not exceeding tens of kilometers. Earthquakes occurring at a depth of less than 70 km (43 mi) are classified as "shallow-focus" earthquakes, while those with a focal depth between 70 and 300 km (43 and 186 mi) 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 to 700 km (190 to 430 mi)).^[21] 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.^[22]

Volcanic activity

Earthquakes often occur in volcanic regions and are caused there, both by

Rupture dynamics

A tectonic earthquake begins as an area of initial slip on the fault surface that forms the focus. Once the rupture has been initiated, it begins to propagate away from the focus, spreading out along the fault surface. Lateral propagation will continue until either the rupture reaches a barrier, such as the end of a fault segment, or a region on the fault where there is insufficient stress to allow continued rupture. For larger earthquakes, the depth extent of rupture will be constrained downwards by the

In most cases, the rupture speed approaches, but does not exceed, the shear wave (S-wave) velocity of the surrounding rock. There are a few exceptions to this:

Supershear earthquakes

Supershear earthquake ruptures are known to have propagated at speeds greater than the S-wave velocity. These have so far all been observed during large strike-slip events. The unusually wide zone of damage caused by the 2001 Kunlun earthquake has been attributed to the effects of the sonic boom developed in such earthquakes.

Slow earthquakes

Slow earthquake ruptures travel at unusually low velocities. 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.^[25]

Co-seismic overpressuring and effect of pore pressure

During an earthquake, high temperatures can develop at the fault plane, increasing pore pressure and consequently vaporization of the groundwater already contained within the rock.^[27][28][29] In the coseismic phase, such an increase can significantly affect slip evolution and speed, in the post-seismic phase it can control the Aftershock sequence because, after the main event, pore pressure increase slowly propagates into the surrounding fracture network.^[30][29] From the point of view of the

.

Tidal forces

.

Clusters

Most earthquakes form part of a sequence, related to each other in terms of location and time.[31] 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.^[32] Earthquake clustering has been observed, for example, in Parkfield, California where a long-term research study is being conducted around the Parkfield earthquake cluster.^[33]

Aftershocks

An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. Rapid changes of stress between rocks, and the stress from the original earthquake are the main causes of these aftershocks,^[34] along with the crust around the ruptured fault plane as it adjusts to the effects of the mainshock.^[31] An aftershock is in the same region as the main shock but always of a smaller magnitude, however, they can still be powerful enough to cause even more damage to buildings that were already previously damaged from the mainshock.^[34] If an aftershock is larger than the mainshock, the aftershock is redesignated as the mainshock 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 mainshock.^[31]

Swarms

Earthquake swarms are sequences of earthquakes striking in a specific area within a short period. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is the main shock, so none has a notably higher magnitude than another. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park.^[35] In August 2012, a swarm of earthquakes shook Southern California's Imperial Valley, showing the most recorded activity in the area since the 1970s.^[36]

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, 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.^[37][38]

Frequency

It is estimated that around 500,000 earthquakes occur each year, detectable with current instrumentation. About 100,000 of these can be felt.^[4][5] Minor earthquakes occur very frequently around the world in places like California and Alaska in the U.S., as well as in El Salvador, Mexico, Guatemala, Chile, Peru, Indonesia, the Philippines, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, India, Nepal, and Japan.^[40] Larger earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur than earthquakes larger than magnitude 5.^[41] In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are: an earthquake of 3.7–4.6 every year, an earthquake of 4.7–5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years.^[42] This is an example of the Gutenberg–Richter law.

The number of seismic stations has increased from about 350 in 1931 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 (USGS) estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0–7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable.^[43] In recent years, the number of major earthquakes per year has decreased, though this is probably a statistical fluctuation rather than a systematic trend.^[44] More detailed statistics on the size and frequency of earthquakes is available from the United States Geological Survey.^[45] A recent increase in the number of major earthquakes has been noted, which could be explained by a cyclical pattern of periods of intense tectonic activity, interspersed with longer periods of low intensity. However, accurate recordings of earthquakes only began in the early 1900s, so it is too early to categorically state that this is the case.^[46]

Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-kilometre-long (25,000 mi), horseshoe-shaped zone called the circum-Pacific seismic belt, known as the

With the rapid growth of mega-cities such as Mexico City, Tokyo, and Tehran in areas of high seismic risk, some seismologists are warning that a single earthquake may claim the lives of up to three million people.^[50]

Induced seismicity

While most earthquakes are caused by the movement of the Earth's

Measurement and location

The instrumental scales used to describe the size of an earthquake began with the

are based on the observed effects and are related to the intensity of shaking.

Intensity and magnitude

The shaking of the earth is a common phenomenon that has been experienced by humans from the earliest of times. Before the development of strong-motion accelerometers, the intensity of a seismic event was estimated based on the observed effects. Magnitude and intensity are not directly related and calculated using different methods. The magnitude of an earthquake is a single value that describes the size of the earthquake at its source. Intensity is the measure of shaking at different locations around the earthquake. Intensity values vary from place to place, depending on the distance from the earthquake and the underlying rock or soil makeup.^[56]

The

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

Seismic waves

Every earthquake produces different types of seismic waves, which travel through rock with different velocities:

• Longitudinal
  P-waves
  (shock- or pressure waves)
• Transverse
  S-waves
  (both body waves)
• Surface waves – (Rayleigh and Love waves)

Speed of seismic waves

can be computed roughly.

P-wave speed

• Upper crust soils and unconsolidated sediments: 2–3 km (1.2–1.9 mi) per second
• Upper crust solid rock: 3–6 km (1.9–3.7 mi) per second
• Lower crust: 6–7 km (3.7–4.3 mi) per second
• Deep mantle: 13 km (8.1 mi) per second.

S-waves speed

• Light sediments: 2–3 km (1.2–1.9 mi) per second
• Earths crust: 4–5 km (2.5–3.1 mi) per second
• Deep mantle: 7 km (4.3 mi) per second

Seismic wave arrival

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.[60] Slight deviations are caused by inhomogeneities of subsurface structure. By such analysis of seismograms, the Earth's core was located in 1913 by Beno Gutenberg.

S-waves and later arriving surface waves do most of the damage compared to P-waves. P-waves squeeze and expand the material in the same direction they are traveling, whereas S-waves shake the ground up and down and back and forth.^[61]

Location and reporting

Earthquakes are not only categorized by their magnitude but also by the place where they occur. The world is divided into 754

(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

Although relatively slow seismic waves have traditionally been used to detect earthquakes, scientists realized in 2016 that gravitational measurement could provide instantaneous detection of earthquakes, and confirmed this by analyzing gravitational records associated with the 2011 Tohoku-Oki ("Fukushima") earthquake.^[63][64]

Effects

The effects of earthquakes include, but are not limited to, the following:

Shaking and ground rupture

Shaking and

.

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

motion from hard deep soils to soft superficial soils and the effects of seismic energy focalization owing to the typical geometrical setting of such 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. Ground rupture is a major risk for large engineering structures such as

Soil liquefaction

Soil liquefaction occurs when, because of the shaking, water-saturated

Human impacts

Physical damage from an earthquake will vary depending on the intensity of shaking in a given area and the type of population. Undeserved and developing communities frequently experience more severe impacts (and longer lasting) from a seismic event compared to well-developed communities.^[68] Impacts may include:

• Injuries and loss of life
• Damage to critical infrastructure (short and long-term)
  • Roads, bridges, and public transportation networks
  • Water, power, sewer and gas interruption
  • Communication systems
• Loss of critical community services including hospitals, police, and fire
• General property damage
• Collapse or destabilization (potentially leading to future collapse) of buildings

With these impacts and others, the aftermath may bring disease, a lack of basic necessities, mental consequences such as panic attacks and depression to survivors,^[69] and higher insurance premiums. Recovery times will vary based on the level of damage and the socioeconomic status of the impacted community.

Landslides

Earthquakes can produce slope instability leading to landslides, a major geological hazard. Landslide danger may persist while emergency personnel is attempting rescue work.^[70]

Fires

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 1906 San Francisco earthquake were caused by fire than by the earthquake itself.^[71]

Tsunami

2004 Indian Ocean earthquake

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. In the open ocean, the distance between wave crests can surpass 100 kilometres (62 mi), and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600–800 kilometers per hour (373–497 miles per hour), depending on water depth. 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.^[72]

Ordinarily, subduction earthquakes under magnitude 7.5 do not cause tsunamis, although some instances of this have been recorded. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.^[72]

Floods

Floods may be secondary effects of earthquakes if dams are damaged. Earthquakes may cause landslips to dam rivers, which collapse and cause floods.^[73]

The terrain below the Sarez Lake in Tajikistan is in danger of catastrophic flooding if the landslide dam formed by the earthquake, known as the Usoi Dam, were to fail during a future earthquake. Impact projections suggest the flood could affect roughly five million people.^[74]

Management

Prediction

Forecasting

While forecasting is usually considered to be a type of prediction, earthquake forecasting is often differentiated from earthquake prediction. Earthquake forecasting is concerned with the probabilistic assessment of general earthquake hazards, including the frequency and magnitude of damaging earthquakes in a given area over years or decades.^[77] For well-understood faults the probability that a segment may rupture during the next few decades can be estimated.^[78][79]

have been developed that can provide regional notification of an earthquake in progress, but before the ground surface has begun to move, potentially allowing people within the system's range to seek shelter before the earthquake's impact is felt.

Preparedness

The objective of

strategies can be employed by a government or organization to mitigate risks and prepare for consequences.

Artificial intelligence may help to assess buildings and plan precautionary operations. The Igor expert system is part of a mobile laboratory that supports the procedures leading to the seismic assessment of masonry buildings and the planning of retrofitting operations on them. It has been applied to assess buildings in Lisbon, Rhodes, and Naples.^[80]

Individuals can also take preparedness steps like securing water heaters and heavy items that could injure someone, locating shutoffs for utilities, and being educated about what to do when the shaking starts. For areas near large bodies of water, earthquake preparedness encompasses the possibility of a tsunami caused by a large earthquake.

In culture

Historical views

From the lifetime of the Greek philosopher

Mythology and religion

In

In Greek mythology, Poseidon was the cause and god of earthquakes. When he was in a bad mood, he struck the ground with a trident, causing earthquakes and other calamities. He also used earthquakes to punish and inflict fear upon people as revenge.^[83]

In

In popular culture

In modern popular culture, the portrayal of earthquakes is shaped by the memory of great cities laid waste, such as Kobe in 1995 or San Francisco in 1906.^[85] Fictional earthquakes tend to strike suddenly and without warning.^[85] For this reason, stories about earthquakes generally begin with the disaster and focus on its immediate aftermath, as in Short Walk to Daylight (1972), The Ragged Edge (1968) or Aftershock: Earthquake in New York (1999).^[85] A notable example is Heinrich von Kleist's classic novella, The Earthquake in Chile, which describes the destruction of Santiago in 1647. Haruki Murakami's short fiction collection After the Quake depicts the consequences of the Kobe earthquake of 1995.

The most popular single earthquake in fiction is the hypothetical "Big One" expected of California's

Contemporary depictions of earthquakes in film are variable in the manner in which they reflect human psychological reactions to the actual trauma that can be caused to directly afflicted families and their loved ones.[87] Disaster mental health response research emphasizes the need to be aware of the different roles of loss of family and key community members, loss of home and familiar surroundings, and loss of essential supplies and services to maintain survival.^[88][89] Particularly for children, the clear availability of caregiving adults who can protect, nourish, and clothe them in the aftermath of the earthquake and help them make sense of what has befallen them has been shown to be more important to their emotional and physical health than the simple giving of provisions.^[90] As was observed after other disasters involving destruction and loss of life and their media depictions, recently observed in the 2010 Haiti earthquake, it is also believed to be important not to pathologize the reactions to loss and displacement or disruption of governmental administration and services, but rather to validate the reactions to support constructive problem-solving and reflection.^[91]

Outside of earth

Phenomena similar to earthquakes have been observed in other planets (e.g.,

).

See also

References

Sources

• Allen, Clarence R. (December 1976), "Responsibilities in earthquake prediction", Bulletin of the Seismological Society of America, 66 (6): 2069–2074,
  doi:10.1785/BSSA0660062069
  .
• Bolt, Bruce A. (1993), Earthquakes and geological discovery, Scientific American Library,
  ISBN 978-0-7167-5040-6
  .
• Chung, D.H.; Bernreuter, D.L. (1980), Regional Relationships Among Earthquake Magnitude Scales.,
  OSTI 5073993, archived
  from the original on 2020-01-22, retrieved 2017-07-21, NUREG/CR-1457.
• Deborah R. Coen. The Earthquake Observers: Disaster Science From Lisbon to Richter (University of Chicago Press; 2012) 348 pages; explores both scientific and popular coverage
• Geller, Robert J.; Jackson, David D.; Kagan, Yan Y.; Mulargia, Francesco (14 March 1997), "Earthquakes Cannot Be Predicted" (PDF), Science, 275 (5306): 1616,
  S2CID 123516228, archived from the original
  (PDF) on 12 May 2019, retrieved 29 December 2016.
• International Commission on Earthquake Forecasting for Civil Protection (30 May 2011), "Operational Earthquake Forecasting: State of Knowledge and Guidelines for Utilization" (PDF), Annals of Geophysics, 54 (4): 315–391,
  S2CID 129825964, archived
  (PDF) from the original on 17 July 2021.
• Kanamori, Hiroo (2003), "Earthquake Prediction: An Overview", International Handbook of Earthquake and Engineering Seismology, International Geophysics, 616: 1205–1216,
  ISBN 978-0-12-440658-2
  .
• Wood, H.O.; Gutenberg, B. (6 September 1935), "Earthquake prediction", Science, 82 (2123): 219–320,
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Further reading

Library resources about
Earthquakes

---

• Online books
• Resources in your library
• Resources in other libraries

• Hyndman, Donald; Hyndman, David (2009). "Chapter 3: Earthquakes and their causes". Natural Hazards and Disasters (2nd ed.). Brooks/Cole:
  ISBN 978-0-495-31667-1
  .
• Liu, ChiChing; Linde, Alan T.; Sacks, I. Selwyn (2009). "Slow earthquakes triggered by typhoons". Nature. 459 (7248): 833–836.
  S2CID 4424312
  .

External links

Wikiquote has quotations related to Earthquake.

Wikimedia Commons has media related to Earthquake.

Wikivoyage has a travel guide for Earthquake safety.

Look up earthquake in Wiktionary, the free dictionary.

• Earthquake Hazards Program of the U.S. Geological Survey
• IRIS Seismic Monitor – IRIS Consortium