Earthquake

Earthquake

An earthquake is one of the most devastating natural phenomena.

Geography

Keywords

earthquake, plate tectonics, seismometer, epicentre, hypocentre, Earth's crust, tectonic plate, earthquake-resistant construction, volcanic activity, wave, tsunami, physical geography, geography

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Questions

  • In which region of the Earth do earthquakes occur most frequently?
  • Along which type of plate boundaries do the most powerful earthquakes occur?
  • What is not true of earthquakes?
  • Shallow-focus earthquakes have a focal depth less than ...
  • What is the epicentre of an earthquake?
  • What is the hypocentre of an earthquake?
  • What is the focal depth of an earthquake?
  • The energy released during an earthquake propagates in the form of ...
  • Which of these waves is not a body wave?
  • Which of these waves is not a surface wave?
  • What is a seismograph?
  • Which of these criteria is NOT important in earthquake-resistant buildings?
  • Which building material makes a building more earthquake-resistant?
  • Is it true that only a small number of earthquakes occur along plate boundaries?
  • Is it true that shallow earthquakes most often occur along divergent plate boundaries?
  • Is it true that it is possible to determine the intensity of a mainshock based on that of the foreshocks?
  • Is it true that P-waves are the first to be detected by instruments?
  • Is it true that body waves cause the greatest damage on the Earth's surface?
  • Is it true that the Richter scale is based on instrumental measurements?
  • Is it true that the Mercalli scale shows the extent of the damage caused by earthquakes?

Scenes

Earthquakes and plate tectonics

An earthquake is a short, elastic motion in the Earth's crust. Earthquakes can occur either on the Earth's surface, e.g. due to landslides, or below the Earth's surface.

Most of the earthquakes occur along the edges of the Pacific Ocean basin. Other seismically active areas are located between the Mediterranean Sea and the Indonesian archipelago and along the edge of oceanic ridges. Like volcanoes, earthquakes are not randomly distributed on Earth.

The most common type of earthquakes are tectonic earthquakes, caused by the movement of tectonic plates; these occur along plate boundaries.

Major earthquakes in the past 30 years

Based on the focal depth, earthquakes are classified into three types: shallow-focus, mid-focus and deep-focus earthquakes. Shallow-focus earthquakes have focal depths less than 70 km, mid-focus earthquakes occur at depths of 70–300 km, while deep-focus earthquakes occur deeper than 300 km.

Earthquakes occurring at divergent plate boundaries, i.e. on the edge of oceanic ridges, are usually shallow-focus earthquakes and are weaker. At convergent plate boundaries, shallow as well as medium or deep-focus earthquakes can occur. Here, shallow and medium-focus earthquakes are generally stronger, deep-focus earthquakes are less strong.
The strongest earthquakes are triggered by the collision of two tectonic plates.
When an earthquake is triggered below the ocean, it can cause a tsunami, that is, very large waves with enormous destructive power.

Generally, an earthquake is a series of shocks. The greatest amount of energy is released during the mainshock, which is sometimes preceded by weaker foreshocks. A mainshock is usually followed by several aftershocks of gradually decreasing magnitude. Classifying the shocks of a particular earthquake is only possible after the seismic waves have completely died down.

What causes earthquakes

Tectonic earthquakes occur when tension in colliding tectonic plates has accumulated to a point where the resulting stress exceeds the plates' resistance and their ability to deform. The tension is released suddenly (just like when a stick that is bent too far breaks) then it spreads out in all directions in the form of waves.

The point of origin of an earthquake, where permanent deformation occurs, is called the focus or hypocentre. The point on the Earth's surface nearest to the focus is the epicentre. Here the earthquake has the greatest strength and destructive power. The distance between the hypocentre and the epicentre is the focal depth of the earthquake.

Seismic waves

The energy released in the focus (or hypocentre) spreads in the form of waves. These waves travel through the interior of the Earth and spread out in all directions. They are called body waves. There are two types of body waves: longitudinal and transverse waves. Their names derive from the direction of particle motion.

The path of longitudinal waves is characterised by alternate regions of compression and rarefaction. Transverse waves have two types: in the first type particles move in a horizontal plane, while in the second type particles travel in a vertical plane, perpendicular to the direction in which the wave travels.

The speed of longitudinal waves is higher, so they are the first waves to be detected by instruments. This is why they are called P-waves, that is, primary waves, while transverse waves are called S-waves, that is, secondary waves.

Waves travelling along the surface of the Earth are called surface waves. They result from the interference of the P-waves with the S-waves.
The interference of P-waves with vertical S-waves generates Rayleigh waves, while the interference of P-waves with horizontal S-waves results in Love waves. They were named after the physicists who described them first. Surface waves travel at lower speeds than body waves, but their amplitude is greater; they cause the greatest damage.

Measuring earthquakes

Several thousand earthquakes occur daily on Earth. Most of them are so weak that they can only be detected by instruments. These instruments, called seismographs, measure and record the ground motion caused by seismic waves during an earthquake.

A seismograph consists of a base fixed to the ground, a paper roll rotating on a cylinder attached to the base, and a weighted pen attached to a frame with a spring. During an earthquake, the cylinder moves together with the Earth, while the weighted pen stays in place due to its inertia and records the motion of the ground on the paper rotating on the cylinder. Each seismic station is equipped with at least three seismographs that record the vibrations in three directions: two horizontally, in North-South and East-West, and one vertically.

To calculate the distance to the earthquake's epicentre, seismologists measure the time delay between the arrival of the P-waves and that of the S-waves. When they know the distance, they draw a circle around the seismic centre. To locate the epicentre exactly, the data from three seismic stations is compared, as the intersection of the three circles reliably determines the location of the epicentre.

The Modified Mercalli scale (MM) classifies earthquakes based on their intensity. This twelve-degree scale shows the effects of an earthquake at a given location. It is not based on instrumental measurements, but on observed effects. The advantage of using this scale is that it makes it possible to classify even earthquakes that occurred centuries ago. However, there is no linear correlation between the intensity of an earthquake and the damage it causes. The extent of damage depends on the type of rock, population density and building methods.

The Richter scale is based on instrumental measurements. It indicates the amount of energy released during an earthquake, i.e. the magnitude, measured by seismometers. Each unit increase in the Richter scale represents a 32-fold increase in the energy released. The magnitude is independent of the effects of the earthquake on the surface.

Earthquake engineering

Although today we have a thorough knowledge of seismic regions and the nature of earthquakes, it is still impossible to predict the exact time an earthquake will occur or its intensity at a specific location. Therefore, the best way to protect against earthquakes in seismic areas is to build earthquake-resistant buildings. The architectural design of buildings, their reinforcement, the building materials used and seismic isolation and damping structures are all important in terms of seismic resistance.

Earthquake-resistant buildings have simple floor plans, low centres of gravity and small windows. Solid reinforced concrete slabs and wall reinforcement play an important role. With regard to building materials, light-frame buildings (e.g. steel or timber-frame buildings) are the most earthquake-resistant, since they are made of flexible materials. Base isolation systems and damping counterweights provide earthquake resistance to tall buildings.

Narration

An earthquake is a short, elastic motion in the Earth's crust. The most common type of earthquake is the tectonic earthquake, caused by the movement of tectonic plates; this occurs along plate boundaries. The strongest earthquakes are triggered by the collision of two tectonic plates.

Tectonic earthquakes occur when tension in colliding tectonic plates has accumulated to a point where the resulting stress exceeds the plates' resistance and their ability to deform. The tension is released suddenly (just like when a stick that is bent too far breaks) then it spreads out in all directions in the form of waves.

The point of origin of an earthquake, where permanent deformation occurs, is called the focus or hypocentre. The point on the Earth's surface nearest to the focus is the epicentre. Here the earthquake has the greatest strength and destructive power. The distance between the hypocentre and the epicentre is the focal depth of the earthquake.

The energy released in the focus (or hypocentre) spreads in the form of waves. These waves travel through the interior of the Earth and spread out in all directions. They are called body waves.

There are two types of body waves: longitudinal and transverse waves. Their names derive from the direction in which the particles move.

The speed of longitudinal waves is higher, so they are the first waves to be detected by instruments. That is why they are called P-waves, that is, primary waves; while transverse waves are called S-waves, that is, secondary waves.

Waves travelling along the surface of the Earth are called surface waves. They result from the interference of the P-waves with the S-waves. Surface waves travel at lower speeds than body waves, but their amplitude is greater; they cause the greatest damage.

Several thousand earthquakes occur daily on Earth. Most of them are so weak that they can only be detected by instruments. These instruments, called seismographs, measure and record the ground motion caused by seismic waves during an earthquake.

A seismograph consists of a base fixed to the ground, a paper roll rotating on a cylinder attached to the base, and a weighted pen attached to a frame with a spring.

The Modified Mercalli scale (MM) classifies earthquakes based on their intensity. This twelve-degree scale shows the effects of an earthquake at a given location.

The Richter scale is based on instrumental measurements. It indicates the amount of energy released during an earthquake, i.e. the magnitude, measured by seismometers. Each unit increase in the Richter scale represents a 32-fold increase in the energy released.

Although today we have a thorough knowledge of seismic regions and the nature of earthquakes, it is still impossible to predict the exact time an earthquake will occur or its intensity at a specific location. Therefore, the best way to protect against earthquakes in seismic areas is to build earthquake-resistant buildings. The architectural design of buildings, their reinforcement, the building materials used and seismic isolation and damping structures are all important in terms of seismic resistance.

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