What is an Earthquake?
An Earthquake is a sudden trembling and shaking of Earth’s surface caused by the release of a huge amount of energy in lithosphere (crust+solid mantle). The release of energy within the earth’s lithosphere is caused, usually from the slipping of tectonic plates along a fault, creating seismic waves that travel outwards from the source/ focus. The other causes of the earthquakes may be volcanic eruption, faulting, and filling or emptying of aquifers. The energy that causes an earthquake is transferred from the Epicenter to the surface through seismic waves. Earthquakes may be of different intensities, ranging 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 frequency, type, and intensity of the earthquakes experienced at a particular area for a specific period of time is known as the seismic activity of that particular area. The seismicity at a particular location in the Earth is the average rate of seismic energy release per unit volume.
While simplifying the definition, an earthquake is any kind of event on/in the Earth, which generates seismic waves. Moreover, when these seismic waves travel upward to the Earth’s surface bring about shaking and trembling on the surface, which is known as an Earthquake. The point where there is the origin of the Earthquake, is known as Hypocenter or Focus, while the point of Earth surface, which directly above the Hypocenter, is known as Epicenter.
Types of Seismic Wave (Earthquake Waves)
An earthquake generates two types of waves;
- The body waves, which include primary and secondary waves. These waves penetrate into the body of Earth and travel across the entire body of the Earth.
- The Surface Wave, which include Love and Raleigh Waves. These waves are unable to penetrate the Earth’s body, and are confined to the surface.
Body Waves
1. P-Waves
P-waves (Primary waves) are the fastest seismic waves, which are compressional in nature, they tend to compress and stretch the ground in the direction of travel (like sound waves). These waves are able to move through solids, liquids, and gases. They rapidly arrive first at seismographs to signal an earthquake.
The distinctive characteristics of the P-waves are:
- Type: Longitudinal, compressional waves (pressure waves).
- Motion: Shake the ground back and forth (parallel) to the wave’s direction.
- Speed: Fastest seismic waves, first to arrive, but less destructive.
- Medium: Travel through solids, liquids, and gases (unlike S-waves).
- Significance: Help scientists map Earth’s interior, especially the liquid outer core, by how their speed changes and refracts.
2. S-Waves
S-waves, also known as shear waves, are secondary seismic waves that are longitudinal in nature, and move rock particles perpendicular (up-and-down or side-to-side) to the direction of waves. They cause significant shaking, and bring big devastation after P-waves. They are slower than P-waves and, crucially, can only travel through solids, not liquids or gases, making them vital for mapping Earth’s liquid outer core and interior structure.
Distinctive Characters of S-waves are:
- Type of Wave: Transverse wave (particle motion is perpendicular to wave travel).
- Speed: Slower than P-waves, arriving second at seismographs.
- Medium: Travel only through solids; cannot pass through liquids or gases.
- Motion: Cause shearing (side-to-side or up-and-down) motion in the ground, often more destructive.
- Earth’s Interior: Their inability to travel through the liquid outer core provides key evidence for its liquid state.
The secondary waves are significantly important because the “S-wave shadow zone” (areas where they can’t be detected) confirms the liquid outer core. Moreover, the time difference between P-wave and S-wave arrivals helps calculate the distance to an earthquake’s epicenter.
Surface Waves
1. L-Waves:
L-waves, also known as Love waves travel through surface of Earth. They are powerful seismic waves, and like S-waves, they cause horizontal, and side-to-side shaking of the ground during an earthquake. They are confined to the surface, but cause significant demage to the property, and infrastructure. These waves were discovered by a mathematician A.E.H Love, and thus named after him. They move slower than either type of the body waves, but still faster than the Raleigh Waves. Hence, their energy is concentrated at the Earth’s surface, and travel perpendicular to the wave’s direction with a shearing motion, they cause deformation within the materials that slide past each other.
2. R-Waves
During an earthquake, the R-waves or Rayleigh waves come fourth in speed and third in devastation. These waves cause ground motion in a rolling, elliptical (up-and-down and side-t0-side) motion. They are similar to ocean waves, leading to significant shaking and destruction. They are stronger than P-waves, but weaker than S-waves and L-waves in destruction. They travel along the Earth’s surface after faster P and S body waves, arriving last but often causing the most damage due to their large amplitude and prolonged shaking, hence the name R-wave for Rayleigh. These waves were named after the English physicist “John William Strutt, Lord Rayleigh, who discovered them in 1885.
Causes of Earthquakes
The most common cause of an earthquake is tectonic movement, however there are several other causes of the earthquakes, like volcanic eruption, faulting, land sliding, reservior filling, or human activities like fracking, bomb blasts, drilling, etc. Let have a look at these causes in detail.
01. Earthquakes by Tectonic Movement
3. The earthquakes are frequent and often more powerful along transform boundaries (also known as transform faults or conservative boundaries) because tectonic plates slide horizontally past one another, building up massive amounts of stress that is suddenly released when the friction is overcome. The process that leads to earthquakes at transform boundaries is explained by the elastic rebound theory. Notable examples of such earthquakes are;
- San Andreas Fault, USA: The most famous example, where the Pacific Plate slides past the North American Plate, is responsible for California’s significant earthquake hazard.
- North Anatolian Fault, Turkey: This fault system has produced a sequence of damaging earthquakes throughout the 20th century.
- Alpine Fault, New Zealand: This major transform fault runs nearly the entire length of New Zealand’s South Island and causes frequent seismic activity.
Elastic Rebound Theory
Elastic rebound theory explains the mechanism of earthquakes at a transform boundary. A sudden release of built-up stress in the Earth’s crust deforms the rocks elastically (like a stretched rubbler band) under tectonic forces until the friction on a fault line breaks, causing the rocks to snap back to a less strained, original shape, relasing immense energy as seismic waves. This process involves gradual stress accumulation, sudden rupture, and rapid energy release, causing the ground to shake and shift.
- Plate Movement & Friction: Tectonic plates move slowly (a few centimeters per year), but their edges are rough and get stuck due to friction.
- Gradual Stress Buildup: While the fault is “locked,” the plates continue to move, causing stress and strain energy to build up in the adjacent rocks.
- Sudden Rupture: Eventually, the stress exceeds the strength of the rocks and the frictional force holding them together. The fault suddenly slips, releasing the stored energy as seismic waves. This sudden release causes the ground to shake, which is an earthquake.
- Seismic Waves: This rupture releases energy as P-waves, S-waves, and surface waves, which propagate outward, causing the ground to shake.
02. Volcanic Earthquakes
03. Artificial Induction
A couple of human activities, like injection of fluids into deep wells, the detonation of large underground mass-destructive weapons, the excavation of mines, and the filling of large reservoirs, may cause earthquakes. The removal of rock material during the process of deep mining brings changes in the strain around the tunnels, resulting in slip on adjacent, preexisting faults or outward shattering of rock into the new cavities may occur.
By fluid injection, the slip is induced by premature release of elastic strain, as in the case of tectonic earthquakes, after fault surfaces are lubricated by the liquid. Apart from injection of fluid, powerful underground nuclear explosions have been known to produce slip on already strained faults in the vicinity of the test devices.
- Filling of large reservoirs
Of the various human-induced earthquakes cited above, the filling of large reservoirs is among the most common. A couple of significant seismic incidents have been documented, in which local seismicity has increased following the impounding of water behind high dams. Reservoirs exceeding 330 feet (100 metres) in depth, and 1 cubic kilometer in volume, usually induce seismic effects. There are three proven sites, where such connections have very probably occurred. These are Hoover Dam in the United States, the Aswan High Dam in Egypt, and the Kariba Dam on the border between Zimbabwe and Zambia. The most generally accepted explanation for earthquake occurrence in such cases assumes that rocks near the reservoir are already strained from regional tectonic forces to a point where nearby faults are almost ready to slip. Water in the reservoir adds a pressure perturbation that triggers the fault rupture. The pressure effect is perhaps enhanced by the fact that the rocks along the fault have lower strength because of increased water-pore pressure. These factors notwithstanding, the filling of most large reservoirs has not produced earthquakes large enough to be a hazard.
Some other sites are associated with seismic source are Koyna Dam in India (1967), Kremasta Dam in Greece (1965), and Kariba Dam in Zimbabwe-Zambia. The specific seismic source mechanisms associated with reservoir induction have been established in a few cases. The seismic incidents at Koyna Dam and Reservoir in India (1967) favour the evidence of strike-slip faulting motion. While at Kremasta and Kariba Dam, the seismic generating mechanism was dip-slip on normal faults. Unlike the Kremasta, Kariba, and Koyna, the source of earthquake induced by Nurek Dam in Tajikistan, was based on thrust mechanisms. Meanwhile, more than 1,800 earthquakes occurred during the first nine years after water was impounded in this 317-metre-deep reservoir in 1972, a rate amounting to four times the average number of shocks in the region prior to filling.
- Nuclear Explosions
Underground nuclear explosions generate seismic waves similar to earthquakes, often causing small, and localized tremors, and can even trigger nearby fault slippage, but these earthquakes, generally aren’t powerful enough to trigger major tectonic earthquakes far from the blast site. They merely act more like an aftershocks. The shockwaves can release stress in nearby rock, leading to induced seismicity, nevertheless distant tectonic quakes aren’t typically caused by these blasts. In 1958 representatives from several countries, lead by both United States and the Soviet Union, gathered to discuss the technical points of nuclear test-ban treaty. Among the discussion, emphasis was put on the seismic induction of the underground nuclear blast. In the meeting it was pin-pointed that the test acts like a massive hammer blow, either shaking the ground violently or nudging a precariously balanced geological fault into sudden movement.
Bad Effects of Earthquakes
Earthquakes, deeply affect the lithosphere, atmosphere, hydrosphere, and biosphere of the earth. These effects include changes to the geologic features, damage to man-made structures, and threats to human, animals, and plants life. Earthquakes, mostly disturb the physical features, but since most earthquakes occur beneath the ocean waters, severe impacts are often observed along the margins of oceans. Earthquakes, generally put the following impacts on the planet Earth.
01. Damage to Lithosphere:
Earthquakes cause drastic geomorphological changes, including ground movements (verticle or horizontal), faulting, folding, landsliding, ruptures & fissures, changing the flow of groundwater, and soil liquefaction, etc.
Violent shakings;
- causes the ground to move horizontally and vertically, creating visible cracks (fissures) and fault lines that tear the surface apart.
- loosens soil and rock on slopes, causing mass movement down hillsides.
- causes the loss of strength of the saturated, sandy soils, which behave like a liquid, thus causing the buildings to sink or tilt.
- uplift and subsidence of sections of the grounds, and elevation change as well.
- alter groundwater levels, change river courses, or create new lakes.
02. Changes to the Earth’s Interior:
Earthquakes do not fundamentally alter the Earth’s deep interior (mantle, and core), but rather reveal its structure through seismic waves and significantly cause long-lasting changes to the crust. The changes brought about in the crust include the creation of faults, uplift/subsidence of ground sections, landslides/avalanches, release of stored energy in tectonic plate movement (that leads to new earthquakes), and volcanic eruption, etc.
03. Damage to the Man-made Structure:
Earthquakes can do significant damage to buildings, bridges, pipelines, railways, embankments, and other structures. The type and extent of damage inflicted are related to the strength of the ground motions and to the behaviour of the foundation soils. In the most intensely damaged region, called the meizoseismal area, the effects of a severe earthquake are usually complicated and depend on the topography and the nature of the surface materials. They are often more severe on soft alluvium and unconsolidated sediments than on hard rock. At distances of more than 100 km (60 miles) from the source, the main damage is caused by seismic waves traveling along the surface. In mines there is frequently little damage below depths of a few hundred metres even though the ground surface immediately above is considerably affected.
04. Threat to Lives:
Earthquakes are a significant threat to human, animal and plants life, primarily through the collapse of buildings, natural structures, and man-made infrastructures. While the ground shaking itself rarely causes direct injury, the secondary effects triggered by the shaking are responsible for millions of deaths throughout history. The most usual cause of death and injury is the failure of man-made structures. Buildings not designed with earthquake-resistant codes are particularly vulnerable to collapse, crushing occupants or trapping them in rubble. Moreover, earthquakes can rupture gas lines and damage electrical wiring, sparking fires. The problem is often compounded when water mains also break, leaving no water supply to extinguish the flames. Underwater earthquakes can displace large volumes of water, generating massive ocean waves that travel rapidly and cause devastating flooding and drowning in coastal areas. In hilly or unstable terrain, shaking can trigger landslides, mudslides, and avalanches that bury communities. In areas with water-saturated, loose soil, liquefaction can occur, causing the ground to behave like a liquid and structures to sink or tip over.
The aftermath of an earthquake often leads to a secondary health crisis due to damaged health facilities and infrastructure. This can result in wound infections, the spread of communicable diseases in overcrowded shelters, and the inability to manage chronic medical conditions.
Apart from the physical harm, there is much deep psychological effect of the earthqauke, which derails the process of normal life for both animals and human being. Survivors often experience severe psychological impacts, including post-traumatic stress disorder (PTSD), anxiety, and depression, which can affect their well-being for years after the event.
05. Impact on the Atmosphere:
Earthquakes have equal impacts on all spheres of the earth including the atmosphere. Earthquakes release massive amount of dust, debris, and other gases like methane and radon into the atmosphere. These gases worsen the air quality, and add greenhouse gases in the atmosphere, while also triggering phenomena like lanslides and fires that add pollutants. In many cases, the earthquakes trigger the volcanic activities, which release dust, plumes, and other toxic gases (Carbon Dioxide, Sulphur Dioxide, Hydrogen Sulphide, Hydrogen Chloride, Hydrogen Floride, Methane, and Radon), in the air. Indirectly, seismic activity can cause subtle, but detectable changes in near-Earth atmospheric pressure, temperature, and ionization levels (lithosphere-atmosphere coupling), and major quakes can even slightly alter the Earth’s rotation and shape, affecting atmospheric dynamics.
06. Impact on Hydrosphere:
Earthquakes, widely affect the hydrosphere by triggering massive tsunamis from underwater quakes, altering groundwater levels, creating new lakes and sag ponds, changing river courses, moving glaciers, and inducing lake seiches (oscillations). These impacts range from temporary changes in water flow and clarity to permanent landscape shifts, affecting water resources, ecosystems, and infrastructure globally. Earthquakes also change the quality of water by making it murky and chemically different.
How to escape during an Earthquakes
It is not the Earthquakes that cause casualties and fatalities, but rather the man-made structures, which do not withstand Earthquakes. During an earthquake, the key to protect oneself is “Drop, Cover, and Hold On”. Read more…
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