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Earthquakes – Its Causes and Features

Grade 7
Jun 5, 2023


Scientists have known for decades that earthquakes do not occur at random. They typically occur in well-defined zones. The zones where earthquakes take place are the boundaries of Earth’s plates. Indeed, earthquake data assisted geologists in understanding the structure of the Earth’s ocean floor and inferring the structure and motion of Earth’s plates.


The unexpected shaking of the ground that happens when masses of rock change their position below the Earth’s surface is known as an earthquake. The shifting masses send out shock waves that may be powerful enough to change the Earth’s surface, pushing up cliffs and making big cracks in the ground.

Causes of Earthquakes

Let us take the example of rubber bands. Rubber bands stretch when you pull them. They return to their original shape once the force is removed because they are elastic. A rubber band, on the other hand, will break if stretched too far. A wooden craft stick works similarly.

When a force is applied to the stick for the first time, it will bend and change shape. The energy required to bend the stick is stored as potential energy within the stick. If the force holding the stick bent is removed, the stick will revert to its original shape, and the stored energy will be released as motion energy.


Formation of Faults

A wooden craft stick has a maximum bend radius. This is referred to as its elastic limit. When the stick’s elastic limit is reached, it either remains bent or breaks.

Formation of Faults

Rocks exhibit similar behavior. Applied forces cause rocks to bend and stretch to a point. This process is known as elastic deformation. The rocks may break if the elastic limit is exceeded. When rocks fracture, they move along surfaces known as faults.

Enormous force is required to overcome the strength of rocks and cause movement along a fault. In relation to the rock on the other side of a fault, the rock on one side of the fault can move up, down, or sideways.


What Causes Faults?

What causes the forces that cause rocks to fracture and faults to form? The Earth’s surface is constantly moving due to forces within the planet. These forces cause plates on the Earth’s surface to move.

The rocks near the plate edges are stressed as a result of this movement. The rocks bend, compress or stretch to relieve stress. The rocks will break if the force is strong enough. An earthquake is caused by the vibrations caused by rock breaking.


Occurrence of Earthquake

When rocks move past each other along a fault, their rough surfaces scratch, temporarily stopping movement. However, forces continue to push the rocks around. This action causes stress to build up at the points where the rocks are stuck. Because of the stress, the rocks bend and change shape.


When stressed past their elastic limit, the rocks can break down, move all along the fault, and come back to their original shapes. The result is an earthquake. Earthquakes can range from insignificant vibrations to devastating energy waves. Most earthquakes, regardless of their magnitude, are caused by rocks moving over, under, or past each other along fault surfaces.

Occurrence of Earthquake

Types of Faults

On rocks, three types of forces act tension, compression, and shear. Tension is the force that separates rocks, while compression is the force that brings rocks together. The force that causes rocks on either side of a fault to slide past each other is known as shear. There are three types of faults.

Types of faults

Normal Fault (Divergent fault)

Tensional forces within the Earth pull rocks apart. When these forces stretch the rocks, a normal fault can form. Along a normal fault, the movement of rocks takes place where rock above the fault surface moves downward in relation to the rock below the fault surface.

Normal fault (Divergent fault)

Reverse Fault (Thrust or Convergent Fault)

Compression forces squeeze rock, resulting in reverse faults. When a rock breaks due to opposing forces, the rock above a reverse fault surface is forced up and over the rock below the fault surface.

Reverse fault (Thrust or convergent fault)

Strike-slip Fault (Transform Fault)

Rocks on either side of a strike-slip fault slide past one another with little upward or downward movement. The San Andreas Fault, which stretches more than 1,100 kilometers through California, is depicted in the photo below. The San Andreas Fault is the boundary between two plates moving sideways past each other on Earth.

Strike-slip Fault (Transform fault)
Strike-slip Fault (Transform fault)

Features of Earthquake

Seismic Waves or Earthquake Waves

When two people hold opposite ends of a rope and shake one end, as shown in Figure 10, they send waves of energy through the rope. Seismic waves generated by earthquakes travel through Earth in the same way that rope waves do.

The ground moves forward and backward, heaves up and down, and shifts from side to side during a strong earthquake. The ground’s surface can ripple in the same way that waves do in the water. Consider trying to stand on a beach with waves crashing on it. This is what you might feel during a powerful earthquake.

Seismic waves

Origin of Seismic Waves or Earthquake Waves

You already know that rocks move past each other along faults, creating stress where their irregular surfaces collide. The stress builds up until the elastic limit is reached, at which point energy is released in the form of seismic waves.

The focus (plural, foci) of the earthquake is the location where this energy release first occurs. Most earthquakes have their epicenters within 65 kilometers of the Earth’s surface. A few have been discovered as far down as 700 kilometers.

Seismic waves

Seismic waves are generated and propagate away from the epicenter of an earthquake.

Seismic waves are classified into two types: those that travel within the planet and can cross its entire width (even through a metallic core) and those that travel along the surface but do not attempt to penetrate the subsurface.

Body waves: P (Primary) waves and S (Secondary) waves are body waves.

P Waves

When there is a quake, P-waves (primary waves) are the first to reach seismometers, followed by S-waves (secondary waves), which arrive after the P-waves and thus reach seismic stations second.

Because P-waves travel faster than S-waves, they always arrive at seismometers first. They travel at speeds ranging from 6 to 14 kilometers per second on average and can traverse any material, solid or liquid. The faster they travel, the denser the material. When the material is less dense, as it is near the surface, they slow down.

S Waves

S-waves travel at a slower rate than P-waves (around 4 to 6 kilometers per second on average) and cannot cross liquid. As a result, the Earth’s outer core, which is molten metal, serves as an impenetrable barrier for them.

P waves
S waves

Surface Waves

Surface waves can only travel along the ground’s surface. They move more slowly than the body waves. They can wreak absolute havoc on their path and are responsible for the widespread devastation that frequently accompanies violent earthquakes.

Surface waves cause most of the destruction resulting from earthquakes. Surface waves move rock particles in a backward, rolling motion and a side-to-side, swaying motion. Many buildings are unable to withstand intense shaking because they are made with stiff materials. The buildings fall apart when surface waves cause different parts of the building to move in different directions.

Surface waves are produced when earthquake energy reaches the surface of the Earth. Surface waves travel outward from the epicenter. The earthquake epicenter is the point on Earth’s surface immediately above the earthquake focus.

Surface waves

Locating an Epicenter

Different seismic waves travel at different speeds through the Earth. The fastest waves are primary waves; secondary waves are slower, and surface waves are the slowest.

Scientists have discovered how to use the varying speeds of seismic waves to calculate the distance to an earthquake’s epicenter. When an epicenter is located a long distance away, the primary wave has more time to put distance between itself and the secondary and surface waves, just like the fastest runner in a race.

The Mercalli scale is used to explain the intensity of earthquakes based on observed effects. The Richter magnitude scale, which is based on the height of the lines on the seismogram, is used to describe the strength of an earthquake.

seismic activity

Earthquakes Have the Following Effects on the Earth’s Geography

Ground Tremors

  • Earthquakes frequently cause ground shaking.
  • Buildings and roads can be destroyed as a result of ground shaking, and in some cases, people have died horribly after being discovered under a building.

Rupture of the Ground

  • When earthquakes occur, the ground begins to swell.
  • The ground will begin to rupture as earthquakes move along a fault, breaking the Earth’s surface into small pieces and causing ruptures in the Earth’s surface.
  • The pipelines in the city will be completely destroyed, as will the wires in the ground.
  • In some countries, underground tunnels exist; these tunnels will be destroyed beyond all expectations. Underground tunnels are very expensive to rebuild, so this is a huge loss.

Land Sliding

  • Ground shaking and ground rupture both cause landslides.
  • Landslides form when the tectonic plates beneath the Earth graze against each other, causing the unstable slopes to shake continuously.
  • Landslides can easily destroy city buildings, roads, and railroad lines, as well as hilltop homes, causing them to tumble down from the hills.


  • The most devastating effect of earthquakes is a tsunami.
  • Tsunamis, also known as tidal waves, are a type of water wave that occurs when the sea bed or sea floor moves vertically upwards during an earthquake.
  • Tsunamis have the power to destroy a city in minutes. Tsunamis pose a serious threat to many parts of the world.
  • Tsunamis have the ability to travel long distances in a short amount of time. Tsunamis can travel up to 700 km/hr in the ocean, which is comparable to the speed of some jet planes.


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