We explain what earthquakes are, their causes and consequences. We also explain what to do in the event of an earthquake and which earthquakes were the largest in history.
Earthquakes are sudden and temporary vibrations of the Earth’s crust.
What are earthquakes?
An earthquake is called the vibration of the Earth produced by a rapid release of energy. The most common thing is that earthquakes are caused by the displacement of the tectonic plates that make up the Earth’s surface.
Tectonic plates are large blocks of the Earth’s crust that are in slow but continuous movement. The energy from the forces of thrust and friction between them accumulates and is sometimes released suddenly. This release of energy, which occurs inside the planet, sometimes reaches the surface and causes sudden vibrations known as earthquakes.
Earthquakes can be mild or intense, and they can go unnoticed (hundreds of seismic movements occur every day without being perceived on the surface) or they can have devastating consequences.
How is the magnitude of earthquakes measured? The magnitude of earthquakes is measured with the Richter scaleMagnitude is the amount of energy released by the earthquake, and is measured from 0 to 10. An earthquake of 0 on the Richter scale is imperceptible on the surface, while an earthquake of 10 is equivalent to a maximum magnitude earthquake, and releases energy equivalent to the total annual energy consumption of the United States.
What is seismology?
Seismographs record the seismic activity of regions.
Seismology is the branch of geophysics that studies the seismic waves that produce earthquakes. The study and knowledge of seismic waves allows us to investigate the causes, distribution and effects of earthquakes. Although they cannot yet be predicted too far in advance, seismology has made progress in estimating the probability of earthquakes occurring in certain regions.
To study seismic waves, an instrument called a “seismograph” is used.. A seismograph or seismometer is an instrument invented in 1842 and improved over time whose purpose is to detect seismic waves originating in the Earth’s subsoil.
Since there are two types of waves, P waves (horizontal) and S waves (vertical), two types of seismographs are required. In the case of P waves, a seismograph is used, which consists of a suspended pendulum that swings sideways when it detects the arrival of horizontal waves.
For S waves, a seismograph supported by a hinge that moves up and down when it detects the arrival of vertical waves is used.
The records obtained by seismographs, called seismograms, provide a lot of information about the behavior of seismic waves and earthquakes.
Causes of an earthquake
Most earthquakes originate as a consequence of the movement of tectonic plates. These plates interact with each other at their edges, where energy accumulates due to the push or friction between them.
In addition, there are other phenomena that can cause earthquakes. Some of them are:
- Volcanic eruptions. Earthquakes can occur in the crust when eruptions are explosive and release large amounts of energy.
- Explosions. Mining and oil extraction using hydraulic fracturing often involve breaking up rock to reach the desired resource. This involves detonations with explosives that can generate strong vibrations on the surface even in places far from the explosion.
- Weapons testsExplosions from bomb or missile detonations can generate vibrations on the surface and cause small earthquakes.
- MeteoritesThe fall of a meteorite to Earth can generate seismic waves and vibrations on the surface.
How long does an earthquake last?
Earthquakes usually last a few seconds. Despite that, they are often very destructive. The longest earthquakes can reach 120 seconds. Its duration depends on the amount of energy released and the location where the earthquake occurs.
Sometimes, some minor earthquakes usually precede the main earthquake. Minor vibrations may be felt on the surface a few days or hours before the earthquake occurs. These movements are called “precursor earthquakes.”
There may also be other seismic movements after an earthquake. These smaller seismic movements are called “aftershocks,” and although they are much weaker than the main earthquake, they can cause significant damage because they affect a previously weakened area.
Consequences of an earthquake
The great earthquakes of history devastated entire cities.
The damage caused by earthquakes is measured through the Mercalli intensity scale. Surface damage depends not only on the magnitude of the earthquake but also on the materials and design of the buildings. A low-magnitude earthquake affecting unprepared structures can cause significant damage. On the other hand, a higher-magnitude earthquake on an area prepared with earthquake-resistant buildings can have a lower Mercalli intensity scale.
The Mercalli intensity scale is measured in values from 1 to 12. Intensity 1 is used when the earthquake is practically not felt on the surface and does not cause damage. Intensity 12 is the highest on the scale and is observed when the damage is total. Intensity 6, the intermediate intensity, occurs when the earthquake is felt by everyone, heavy objects such as furniture, trees and even walls move, and some structures are damaged.
Components of an earthquake
The epicenter is the point on the surface where the earthquake vibrations are felt most strongly.
The main components of an earthquake are:
- Hypocenter or focusIt is the place, beneath the earth’s surface, where energy is released.
- Epicenter. It is the place on the surface located directly above the focus or hypocenter. As it is the closest place to the point of energy release, it is where the vibrations are felt the strongest and possibly the most damage occurs.
- Seismic waves. They are the waves through which energy is transmitted from the source to the surface. They are of two types.
- The P or primary waves move horizontally, they are the ones that are felt first and they are the ones that will shake the objects on the surface from one side to the other.
- S or secondary waves take longer to be perceived because they are slower, move vertically, and are the ones that make objects on the surface “jump” up and down.
What are the seismic zones of the world?
The areas with the highest seismic probability are those found on the edges of tectonic plates.
Generally, the areas of greatest seismic risk in the world are those where tectonic plates are pushing against each other. These areas are called “faults.” Faults are found mainly along the edges of tectonic plates.
Some of the most active seismic zones are:
- The Pacific Ring of Fire. It extends along the coast of the Pacific Ocean through North America, South America and Asia. It is located on the edges of the Pacific, Nazca, Philippine and Antarctic tectonic plates. It is known as the “Ring of Fire” because it is an area where there are also many active volcanoes that erupt periodically.
- The Mediterranean region. It extends throughout the countries near the Mediterranean Sea, especially at its eastern end where the edges of the African, Eurasian, Arabian and Anatolian plates converge.
- Central AsiaIt extends around the Himalayas. It is a seismic zone due to the interaction of the edges of the Indian and Eurasian plates.
Real-time seismic monitoring. There are monitors worldwide that keep local and international records of the seismic movements that occur. The institutions that monitor them publish the information in real time, with data on their location and magnitude. Many of them also present real-time maps where earthquakes are marked immediately after they were recorded.
The largest earthquakes in history
The largest earthquakes recorded in history are:
EarthquakeLocationYearMagnitude recorded on the Richter scaleValdivia earthquakeChile19609.5Indian Ocean earthquakeIndonesia20049.3Alaska earthquakeUnited States19649.2Japan earthquakeJapan20119.1Kamchatka earthquakeRussia19529.0Chile earthquakeChile20108.8Sumatra earthquakeIndonesia20058.6Assam earthquakeIndia19508.6
It is believed that other large-magnitude earthquakes have occurred throughout history, such as those in Lisbon in 1755, Lima in 1746 or Arica in 1868, but it is not possible to know their magnitude because in those years there were no instruments to measure the energy released nor was the Richter scale used.
What to do in case of an earthquake?
Governments usually establish a series of recommendations for the population to protect themselves and be safe in the event of an earthquake. Some of those recommendations are:
- Keep calm. Avoid panicking, running and pushing other people.
- Search for a safe place. Try to stay inside buildings, away from windows and objects that could fall such as lamps, mirrors, shelves or pictures.
- Duck and cover. Cover your head with your arms and stay under sturdy tables or desks in case the ceiling collapses and falls.
- Do not use elevators or stairsElevators may stop or become damaged, and stairwells may collapse.
- Do not light matches or electrical appliances. The earthquake may have caused pipes to burst and there may be a gas leak, increasing the risk of explosion.
- Follow evacuation instructions. Once the tremors have stopped, go out into the street in an orderly manner and look for meeting and evacuation points.
Follow with:
References
- Ecoexploratory (sf). What are earthquakes? Eco-exploratory
- NASA Science Space Place (nd). What is an earthquake? Spaceplace
- Tarbuck, E. & Lutgens, F (1999). Earth sciences. An Introduction to Physical Geology. Prentice Hall.