Earthquake | Earthquake Causes, Effects and Facts

Earthquake | Earthquake Causes, Effects and Facts


Earthquakes can happen quickly and without prior notice. A tectonic plate moving along a fault line in the earth's crust causes an earthquake, which is characterised by a severe and rapid shaking of the ground.

Earthquakes can also result in ground tremors, soil liquefaction, landslides, fissures, avalanches, fires, and tsunamis which severely affect the life & nature.

 An earthquake's level of devastation and harm relies on:

  • magnitude
  • intensity and duration
  • the local geology
  • the time of day that it occurs
  • building and industrial plant design and materials
  • the risk-management measures put in place.

Nearly 750 000 people died worldwide from earthquakes between 1998 and 2017, accounting for more than half of all deaths from natural catastrophes. Over 125 million people were impacted by earthquakes during this time period, which means they were hurt, made homeless, moved, or evacuated during the disaster's emergency stage.

Causes of Earthquake

A fault slips suddenly, resulting in an earthquake. Even though the tectonic plates are always moving slowly, but friction makes their edges impenetrable. Waves of energy are created when the stress on the edge of tectonic plates are greater than the friction, and these waves move into the earth's crust to produce the shaking we experience.

Earthquake | Earthquake Causes, Effects and Facts

On Earth, large earthquakes typically occur in belts along the boundaries of tectonic plates.

This has been clear for a while from early earthquake catalogues and is considerably more obvious from modern seismicity maps that include instrumentally determined epicentres. The Circum-Pacific Belt, which impacts a number of inhabited coastal areas surrounding the Pacific Ocean, including as those in New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and on the western shores of North and South America, is the most significant earthquake belt. According to estimates, earthquakes with epicentres in this belt account for 80% of the energy now released in earthquakes. There are several branching and variations in the seismic activity throughout the belt, which is by no means homogeneous. The Circum-Pacific Belt is frequently linked to volcanic activity, which is why it is sometimes known as the "Pacific Ring of Fire."

The Mediterranean region is linked to the Circum-Pacific Belt by the Alpide Belt, which runs across Asia and the East Indies. This region is responsible for about 15% of all earthquake energy released worldwide.

Astounding linked seismic activity bands can also be seen, mostly in the rift valleys of East Africa and along oceanic ridges such as those in the Arctic, Atlantic, and western Indian Oceans. The best way to comprehend this worldwide seismicity distribution is in the context of plate tectonics.

Natural forces

The abrupt release of energy from a specific location of the Earth's crust causes earthquakes. Elastic strain, gravity, chemical processes, or even the movement of large bodies can all release the energy. The most significant of them is the release of elastic strain since it is the only type of energy that can be stored in the Earth in quantities great enough to create significant perturbations. Tectonic earthquakes are earthquakes that occur as a result of this kind of energy release.


After the San Andreas Fault broke in 1906, causing the great San Francisco earthquake, American geologist Harry Fielding Reid developed the so-called elastic rebound theory to explain tectonic earthquakes. The idea states that tectonic earthquakes happen when strains in rock masses build up to the point when the resulting stresses surpass the rock's capacity, leading to abrupt rupture. The fractures spread out quickly through the rock, usually in the same direction, and can occasionally stretch for miles following a specific zone of weakness. For instance, the San Andreas Fault shifted along a 430 km (270 miles) long plane in 1906. The ground was moved horizontally along this line by up to 6 metres (20 feet).

Rock masses are thrown in opposite directions as a fault rupture moves along or up the fault, springing back to a location where there is less strain. This movement could occur in irregular increments rather than all at once at any given site, and these abrupt slowdowns and restarts are what cause the vibrations that seismic waves are made of. The modelling of earthquake causes now takes into account these erratic fault rupture characteristics, both physically and theoretically. Asperities are irregularities along the fault, and fault barriers are regions where the rupture slows or ceases. The earthquake focal, which is typically located close to 5 to 15 kilometres below the surface, is where the fault rupture begins.  The rupture spreads throughout the fault plane in either one or both directions until it is stopped or slowed by a barrier. Sometimes the barrier fails to halt the fault rupture, and it continues on the opposite side; other times, the barrier is broken by stresses in the rocks, and the rupture continues.

The sort of fault slip that creates an earthquake will affect the earthquake's characteristics, as indicated in the picture. A "strike" (i.e., the direction from north taken by a horizontal line in the fault plane) and a "dip" (i.e., the angle from the horizontal exhibited by the sharpest slope in the fault) are two components of the typical fault model.

The footwall is an inclined fault's lowest wall.

The hanging wall is perched over the footwall. Strike-slip faulting is the movement of rock masses past one another on a line with the strike. Dip-slip faulting is movement that occurs parallel to the dip. Depending on whether the block on the other side of the fault from the observer has moved to the right or left, strike-slip faults are either right lateral or left lateral. In dip-slip faults, "normal" faulting occurs when the hanging-wall block moves downward relative to the footwall block; reverse or thrust faulting occurs when the hanging-wall block moves upward relative to the footwall.

Although tectonic movements along faults are frequently gradual and the majority of geologically old faults are now aseismic (that is, they no longer create earthquakes), it is considered that every known fault has once been the epicentre of one or more earthquakes. It is frequently unclear if an earthquake's total energy originates from a single fault plane because the actual faulting that causes it might be complex.

In contrast to the abrupt slide offsets that cause seismic waves, observed geologic faults occasionally exhibit relative displacements on the order of hundreds of kilometres over the course of geologic time. For instance, the causative fault east of Beijing saw a surface strike-slip of around one metre during the Tangshan earthquake in 1976, while the Chelung-pu fault experienced up to eight metres of vertical slide during the Taiwan earthquake in 1999.


Volcanic earthquakes are a distinct subtype of earthquake that are connected to volcanic activity. However, even in these circumstances, it is likely that the disturbance is caused by the abrupt slippage of rock masses close to the volcano and the subsequent release of elastic strain energy. The heat generated by magma moving in r, however, may partially account for the stored energy's hydrodynamic origin.Major earthquakes and volcanoes are clearly correlated with one another's locations, especially in the Circum-Pacific Belt and along oceanic ridges. However, most big shallow earthquakes have epicentres hundreds of kilometres away from volcanic vents, and many earthquake sources are located far from active volcanoes. There is probably no direct causal link between the two events, even when an earthquake's focal point is located immediately beneath structures identified by volcanic vents; most likely, both events are the product of the same tectonic processes.

Artificial induction

Human activities such as the dumping of liquids into deep wells, the detonation of massive underground nuclear explosions, the mining industry, and the filling of enormous reservoirs can occasionally create earthquakes. In the case of deep mining, the strain around the tunnels changes as a result of the removal of rock. It is possible for nearby, existing faults to slip or for rock to break apart and fall into the new voids. As in the case of tectonic earthquakes, it is believed that in the case of fluid injection, the slip is caused by the premature release of elastic strain after the liquid has lubricated the fault surfaces. On existing strained faults close to the test devices, large underground nuclear explosions have been known to cause slip.

Reservoir induction

The filling of big reservoirs is among the most significant of the several earthquake-causing actions mentioned above. There have been more than 20 notable incidents when local seismicity has increased as a result of the storage of water behind towering dams. Because there is sometimes no data to compare earthquake occurrence before and after the reservoir was filled, causality is frequently impossible to prove. The effects of reservoir induction are most noticeable in reservoirs that are deeper than 100 metres (330 feet) and larger than 1 cubic kilometre (0.24 cubic miles). The Hoover Dam in the United States, the Aswan High Dam in Egypt, and the Kariba Dam on the border between Zimbabwe and Zambia are three locations where such connections have very likely taken place.

In these situations, the most widely accepted theory for why earthquakes happen is that the rocks close to the reservoir have already been pushed by regional tectonic forces to the point that surrounding faults are almost ready to slip. The pressure perturbation caused by the reservoir's water causes the fault rupture. The fact that the rocks along the fault have less strength due to higher water-pore pressure may have an even greater impact on the pressure effect. Despite these concerns, the majority of large reservoir fillings have not resulted in earthquakes that are dangerous in size.

In a few instances, the precise seismic source processes linked to reservoir induction have been identified. The evidence supports strike-slip faulting motion for the primary shock at the Koyna Dam and Reservoir in India (1967). The producing technique used at the Kremasta Dam in Greece (1965) and the Kariba Dam in Zimbabwe-Zambia (1961) was dip-slip on typical faults. On the other hand, the thrust mechanisms for the origins of earthquakes at the lake behind Tajikistan's Nurek Dam have been identified. In the first nine years following the water's impoundment in this 317-meter-deep reservoir in 1972, there were more than 1,800 earthquakes, which is four times the typical quake frequency occurs in the area before the reservoir was filled.

Seismology and nuclear explosions

Representatives from a number of nations, including the US and the USSR, met in 1958 to explore the technical foundation for a treaty banning nuclear tests. One of the topics discussed was the viability of creating efficient tools for spotting subterranean nuclear explosions and telling them apart seismically from earthquakes. Following that meeting, seismology received a lot of specialized research, which greatly improved the ability to detect and analyse seismic signals.

Recent seismological work on treaty verification has comprised deploying seismic arrays, estimating explosion yield, researching wave attenuation in the Earth, determining wave amplitude and frequency spectra discriminants, and using high-resolution seismographs in a global network. The results of this research have demonstrated that, in contrast to natural earthquakes, underground nuclear explosions typically produce seismic waves that travel through the body of the Earth that are substantially bigger in amplitude than the surface waves. A global monitoring network of 270 seismographic stations should be able to identify and locate all earthquakes of magnitude 4 and above (equivalent to an explosive output of around 100 tonnes of TNT), according to this telling difference and other types of seismic evidence.

There are many different repercussions of earthquakes, including modifications to the earth's surface, harm to man-made structures, and an impact on both human and animal life. The majority of these consequences take place on solid ground, but as the majority of earthquake foci are actually found beneath the ocean floor, severe effects are frequently seen at ocean borders.

Surface phenomena

Ground movements—vertical or horizontal—along geologic fault traces, rising, falling, and tilting of the ground surface, changes in the flow of groundwater, liquefaction of sandy ground, landslides, and mudflows are among the dramatic geomorphological changes that earthquakes frequently bring about. Geodetic measurements, which are routinely taken in a number of nations severely affected by earthquakes, are helpful in examining topography changes.

Earthquake | Earthquake Causes, Effects and Facts
Building Destroyed in Turkey Earthquake Feb 2023

Buildings, bridges, pipelines, railways, embankments, and other structures are all susceptible to major damage from earthquakes. The kind and level of damage sustained depend on the force of the ground vibrations and how the foundation soils behave. The repercussions of a major earthquake are typically complex and dependent on the topography and the makeup of the surface materials in the area that has been damaged the most, known as the meizoseismal area. On soft alluvium and unconsolidated sediments as opposed to hard rock, they are frequently more severe. Seismic waves travelling along the surface are the primary cause of damage at distances greater than 100 km (60 miles) from the source. In mines, there is typically little damage beyond a few hundred metre depths, despite significant damage to the land surface immediately above.

There are often accounts of unusual sounds and lights during earthquakes. They have been compared to the low-pitched noise of an underground train passing through a station because of their characteristic low pitch. Such sounds are compatible with the ground being penetrated by high-frequency seismic waves. Occasionally, after earthquakes, dazzling flashes, streamers, and bright balls have been observed in the night sky. Electric induction in the atmosphere near the earthquake source is what is thought to be responsible for these lights.


Very long-wavelength water waves in oceans or seas can reach ashore after some earthquakes. Although they are more appropriately referred to as tsunamis or seismic sea waves (tsunami is the Japanese term for "harbour wave"), tidal waves are not formed by the Moon and Sun's gravitational pull. They may be very damaging and occasionally come ashore at tremendous heights—tens of metres above ordinary tide level.

How Do Scientists Predict Tsunamis After Earthquakes?

A quick movement in the seabed that results in the abrupt lifting or lowering of a huge body of water is typically the immediate source of a tsunami. This deformation might be a submarine landslip that results from an earthquake, or it might be the fault that causes the earthquake. There have also been major tsunamis caused by large volcanic eruptions near coastlines, such as those at Thera (about 1580 BCE) and Krakatoa (1883 CE). On December 26, 2004, an earthquake off the Indonesian coast of Sumatra moved the seabed, causing the most catastrophic tsunami yet seen. A succession of waves that inundated coasts from Indonesia to Sri Lanka and even came ashore on the Horn of Africa killed more than 200,000 people.

Water waves propagated in all directions once the sea surface was initially disturbed. The formula (Square root of gh), where g is the acceleration of gravity and h is the sea depth, determines their speed of motion in deep water. When h is 1,000 metres (3,300 feet), this speed—100 metres per second, or 225 miles per hour—might be significant. Although the principal wavelength may be on the order of hundreds of kilometres, the amplitude (or height of disturbance) at the water's surface does not exceed a few metres in deep water. As a result, the principal wave period—the amount of time between the arrival of successive crests—may be on the order of tens of minutes. Because of these, tsunami waves are not noticed by ships far out at sea.

However, the wave amplitude grows as tsunamis approach shallow water. In U- and V-shaped harbours and inlets, the waves can occasionally reach heights of 20 to 30 metres above mean sea level. Around such inlets, they frequently cause significant damage to low-lying ground. The wave front at the entrance is frequently almost vertical, similar to a tidal bore, and the onrush speed can reach 10 metres per second. There may be multiple large waves in some situations, spaced by periods of several minutes or longer. A remarkable retreat of water from the shore may start several minutes or even a half hour before the first of these waves, which is common.

There are organisations that have been established to provide tsunami warnings, particularly in Japan, Siberia, Alaska, and Hawaii. The Seismic Sea Wave Warning System, a system with worldwide assistance intended to lessen fatalities in the Pacific Ocean, is a significant development. It is situated in Honolulu and issues alerts based on reports of earthquakes from seismographic stations around the Pacific.


Seiches, which can occasionally be caused by earthquakes and tsunamis, are rhythmic water movements in almost landlocked bays or lakes. Such oscillations may endure for several hours or even several days.

The big Lisbon earthquake of 1755 generated noticeable oscillations in the waters of lakes and canals as far away as Scotland and Sweden. Seismic surface waves from the 1964 Alaska earthquake began to cause surges in lakes in Texas, in the southwest of the United States, 30 to 40 minutes after the event.

When seismic waves from an earthquake travel through saltwater after being refracted by the seafloor, they have a similar effect. Approximately 1.5 kilometres (0.9 miles) per second, or the speed of sound in water, is the speed of these waves. Such waves can give the impression that a ship has impacted a submerged item if they strike it with enough force. This occurrence is referred to as a seaquake.

 What is the significance of the depth?

Earthquakes take place in the crust or upper mantle, which covers the planet's surface and extends down to a depth of around 800 km (500 miles).

The power of shaking at the surface from an earthquake that happens at 500 km depth is significantly less than if the same earthquake had occurred at 20 km depth because the intensity of shaking from an earthquake reduces with increasing distance from the earthquake's source.

Additionally, the depths of earthquakes reveal vital details about the tectonic environment and the structure of the Earth. This is most visible at subduction zones, when plates are colliding and one plate is being subducted beneath another. By carefully tracking the location and depth of earthquakes linked to the zone, we may identify structural features of a subduction zone, such as how steeply it dips and whether the down-going plate is flat or curved. These details, which are vital, help us to understand the mechanics and characteristics of the deformation in the subduction zone.

The most powerful earthquakes occur deep within subducting slabs, oceanic plates that drop into the Earth's mantle along convergent plate boundaries when a thick oceanic plate collides with a less dense continental plate and falls beneath it.  

A plate boundary contact between two of these plates, which is only active down to a depth of around 60 km, which is fairly shallow, is what produced both the Sumatra 2004 M9.1 earthquake and the Japan 2011 M9.0 earthquake. In contrast, these faults remain brittle and can cause earthquakes to occur as deep as 700 km in deeper subduction zone settings (for example, the Pacific Plate beneath Japan, Kamchatka, and Tonga), when oceanic slabs are comparatively cool in relation to the surrounding mantle.

As the slab sinks further into the mantle, rheology changes (variations in viscosity qualities) take place, which lead to these earthquakes. Subduction zone locations known as "Wadati-Benioff Zones" are where these events seem to occur more frequently.

Faults are exclusively active within continents and along continental plate boundary transform faults like the San Andreas, possibly down to depths of about 20 km.

Unless there is a seismic station nearby and above the epicentre, accurately identifying the depth of an earthquake is often more difficult than determining its position. Therefore, errors on depth calculations tend to be a little bigger than errors on location calculations.


Health risks resulting from earthquakes might vary depending on the earthquake's severity, the built environment (such as substandard housing or urban slums), and any subsequent consequences, such as tsunamis or landslides. Human & Animal health can be affected by earthquakes in both the short and long term.

The following have immediate health effects:

  • trauma-related deaths and injuries from building collapse;
  • secondary effects of the earthquake that cause trauma-related deaths and injuries, such as burns from fires or drowning from tsunamis.

Medium-term health impacts include:

  • Secondary infections from untreated wounds;
  • increased morbidity and risk of complications during pregnancy and childbirth as a result of interrupted obstetric and neonatal services;
  • potential risk of communicable diseases, particularly in areas affected by overcrowding; increased morbidity and risk of complications as a result of discontinuing treatment for chronic diseases;
  • increased psychosocial needs;
  • and potential environmental contamination by chemical/radiological agents.

In addition to harming transport and healthcare infrastructure, earthquakes can also cause service delivery and care access to be hampered. Medical supplies can be misplaced and health personnel might not be able to access hospitals that are still operating.

How to Remain Safe

As the leader of the health cluster for international emergencies, WHO collaborates with partners to prepare for, mitigate, and respond to earthquakes everywhere.

  • Limiting the danger of exposure to earthquakes by enhancing the built environment, with better land-use control, including controlling building construction, and strengthening health emergency risk management systems
  • Ensuring that health facilities are robust to hazards, that they can continue to operate, that they can respond to increased and altered health demands following earthquakes, and that their staff is properly trained in doing so
  • Mobilising medical response teams, setting up field hospitals and temporary health facilities as well as emergency medical supplies.
  • Making investments in community readiness, as locals frequently serve as first responders.

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