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Essay on Earthquakes

Essay # 1. What is an Earthquake?

An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocentre, and the location directly above it on the surface of the earth is called the epicentre.

Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens.

The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place of the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock.

Essay # 2. What Causes Earthquakes and Where do they Happen?

The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet. But this skin is not all in one piece—it is made up of many pieces like a puzzle covering the surface of the earth. Not only that, but these puz­zle pieces keep slowly moving around sliding past one another and bumping into each other.

We call these puzzle pieces tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.

What is a fault and what are the different types?

A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move rela­tive to each other. This movement may occur rapidly, in the form of an earthquake—or may occur slowly, in the form of creep. Faults may range in length from a few millimetres to thousands of kilometres.

Most faults pro­duce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.

Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse, depending on their motion. Faults that move horizontally are known as strike-slip faults and are classified as either right- lateral or left-lateral. Faults, which show both, dip-slip and strike-slip motion are known as oblique-slip faults.

Normal fault:

A dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.

Thrust fault:

A dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block. This type of faulting is common in areas of compression, such as regions where one plate is being subducted under the other as in Japan. When the dip angle is shallow, a reverse fault is often described as a thrust fault.

Strike-slip fault:

A fault on which two blocks slide past one another. The San Andreas Fault is an example of a right lateral fault.

A left-lateral strike-slip fault is one on which the dis­placement of the far block is to the left when viewed from either side.

A right-lateral strike-slip fault is one on which the dis­placement of the far block is to the right when viewed from either side.

Plate Tectonics Theory:

The plate tectonics theory is a starting point for understanding the forces within the Earth that causes earthquakes. Plates are thick slabs of rock that make up the outermost 100 kilometres or so of the Earth. Geologists use term ‘tectonics’ to describe deforma­tion of the Earth’s crust, the forces producing such deformation, and the geologic and structural features that result.

Earthquakes occur only in the outer, brittle portions of these plates, where temperature in the rock is relatively low. Deep in the Earth’s interior, convection of the rocks, caused by temperature variations in the Earth, induces stresses that result in movement of the overlying plates. The rate of plate movements range from 2 to 12 centimetre per year and can now be measured by precise surveying techniques.

The stresses from convection can also deform the brittle portions of overlying plates, thereby storing tremendous energy within the plates. If the accumulating stress exceeds the strength of the rocks comprising these brittle zones, the rocks can break suddenly, releasing the stored elastic energy as an earthquake.

Three major types of plate boundaries are recognized. These are called spreading, convergent, or transform, depending on whether the plates move away, toward, or laterally past one another, respectively. Subduction occurs where one plate converges towards the other plate, moves beneath it, and plunges as much as several hundred kilometres into the Earth’s interior.

Essay # 3. Why the Earth Shake When there does is an Earthquake?

While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released.

The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us!

Essay # 4. How are Earthquakes Recorded?

Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free.

When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

Essay # 5. How do Scientists Measure the Size of Earthquakes?

The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilome­tres deep beneath the earth’s surface. So how do they measure an earthquake? They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was.

A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.

The size of the earthquake is called its magni­tude. There is one magnitude for each earthquake. Scientists also talk about the intensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.

Essay # 6. How Can Scientists Tell where the Earthquake Happened?

Seismograms come in handy for locating earth­quakes too and being able to see the P-wave and the S-wave is important. You learned how P and S waves each shake the ground in different ways as they travel through it. P-waves are also faster than S-waves, and this fact is what allows us to tell where an earthquake was. To understand how this works, let’s compare P and S waves to lightning and thunder.

Light travels faster than sound, so during a thunderstorm you will first see the lightning and then you will hear the thunder. If you are close to the lightning, the thunder will boom right after the lightning, but if you are far away from the lightning, you can count several seconds before you hear the thunder. The farther you are from the storm, the longer it will take between the lightning and the thunder.

P-waves are like the lightning, and S-waves are like the thunder. The P-waves travel faster and shake the ground where you are first. Then the S-waves follow and also shake the ground. If you are close to the earthquake, the P and S wave will come one after the other, but if you are far away, there will be more time between the two.

By looking at the amount of time between the P and S wave on a seismo- gram recorded on a seismograph, scientists can tell how far away the earthquake is from that location. However, they can’t tell in what direction from the seismograph the earthquake is, only how far away it is. If they draw a circle on a map around the sta­tion where the radius of the circle is the determined distance to the earthquake, they know the earthquake lies somewhere on the circle. But where?

Scientists then use a method called triangulation to determine exactly where the earthquake is. It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake. If you draw a circle on a map around three different seismographs where the radius of each is the distance from that station to the earth­quake, the intersection of those three circles is the epicentre!

Foreshocks and Aftershocks:

Foreshocks are smaller earthquakes that may occur in the same area of a larger earthquake that follows. They are caused by minor fracturing of rocks under stress prior to the main break that happens during the largest earth­quake of the series, called the mainshock.

Foreshocks can start up to a year before the mainshock, as was the case before the three large (magnitudes between 6.3 and 6.7) earthquakes near Tennant Creek in January 1988. Not all earthquakes have foreshocks, and sometimes a series of similar sized earthquakes, called an earthquake swarm, happens over months without being followed by a significantly larger mainshock. This limits the useful­ness, at this stage, of foreshocks in earthquake prediction.

Aftershocks are smaller earthquakes that may occur after the mainshock, in the same area. They are caused by the mainshock area readjusting to the fault movement, and some may be the result of continuing movement along the same fault. The largest aftershocks are usually at least half a magnitude unit smaller than the mainshock and the aftershock sequence may continue for months or years after the mainshock.

Essay # 7. How Terminology Involved in Earthquake?

An earthquake can be likened to the effect observed when a stone is thrown into water. After the stone hits the water a series of concentric waves will move outwards from the centre. The same events occur in an earthquake.

There is a sudden movement within the crust or mantle, and concentric shock waves move out from that point. Geologists and Geographers call the origin of the earthquake the focus. Since this is often deep below the surface and difficult to map, the location of the earthquake is often referred to as the point on the Earth surface directly above the focus. This point is called the epicentre.

The strength, or magnitude, of the shockwaves determines the extent of the damage caused. Two main scales exist for defining the strength, the Mercalli Scale and the Richter Scale.

Earthquakes are three-dimensional events, the waves move outwards from the focus, but can travel in both the horizontal and vertical plains. This pro­duces three different types of waves which have their own distinct characteristics and can only move through certain layers within the Earth. Let us take a look at these three forms of shock waves.

Types of Shockwaves:

i. P-Waves:

Primary Waves (P-Waves) are identical in character to sound waves. They are high frequency, short-wavelength, longitudinal waves which can pass through both solids and liquids. The ground is forced to move forwards and backwards as it is compressed and decompressed. This produces relatively small displacements of the ground. P-Waves can be reflected and refracted, and under cer­tain circumstances can change into S-Waves.

ii. S-Waves:

Secondary Waves (S-Waves) travel more slowly than P-Waves and arrive at any given point after the P-Waves. Like P-Waves they are high frequency, short-wavelength waves, but instead of being longitudinal they are transverse.

They move in all directions away from their source, at speeds which depend upon the density of the rocks through which they are moving. They cannot move through liquids. On the surface of the Earth, S-Waves are responsible for the sideways displacement of walls and fences, leaving them ‘S’ shaped.

iii. L-Waves:

Surface Waves (L-Waves) are low frequency transverse vibrations with a long wavelength. They are created close to the epicentre and can only travel through the outer part of the crust. They are responsible for the majority of the building damage caused by earthquakes.

This is because L-Waves have a motion similar to that of waves in the sea. The ground is made to move in a circular motion, causing it to rise and fall as visible waves move across the ground. Together with secondary effects such as landslides, fires and tsunami these waves account for the loss of approximately 10,000 lives and over $100 million property per year.

Essay # 8. How Many Types of Earthquakes are There?

i. Intraplate Earthquakes:

Earthquakes that do not occur on plate margins are called intraplate earthquakes. All earthquakes on mainland Australia and Tasmania are intraplate. On studying these intraplate earthquakes in various continents, seismologists have found that most of them are caused by thrust faulting due to the rocks being squeezed or compressed.

It seems that the movement of the tectonic plates causes the rocks away from their margins to be compressed. Intraplate earthquakes are not as common as those on plate margins, but major earthquakes with magnitudes of 7.0 or more do happen occasionally.

ii. Tectonic Earthquakes:

Tectonic earthquakes are triggered when the crust becomes subjected to strain, and eventually moves. The theory of plate tectonics explains how the crust of the Earth is made up of several plates, large areas of crust which float on the Mantle.

Since these plates are free to move, slowly they can either drift towards each other, away from each other or slide past each other. Many of the earthquakes which we feel are located in the areas where plates collide or try to slide past each other.

The process which explains these earthquakes, known as Elastic Rebound Theory can be demon­strated with a green twig or branch. Holding both ends, the twig can be slowly bent. As it is bent, energy is built within it. A point will be reached where the twig suddenly snaps. At this moment the energy within the twig has exceeded the Elastic Limit of the twig. As it snaps the energy is released, caus­ing the twig to vibrate and to produce sound waves.

Perhaps the most famous example of plates sliding past each other is the San Andreas Fault in California. Here, two plates, the Pacific Plate and the North American Plate, are both moving in a roughly northwesterly direction, but one is moving faster than the other. The San Francisco area is subjected to hundreds of small earthquakes every year as the two plates grind against each other. Occasionally, as in 1989, a much larger movement occurs, triggering a far more violent ‘quake’.

Major earthquakes are sometimes preceded by a period of changed activity. This might take the form of more frequent minor shocks as the rocks begin to move, called foreshocks, or a period of less frequent shocks as the two rock masses tempo­rarily ‘stick’ and become locked together.

Detailed surveys in San Francisco have shown that railway lines, fences and other longitudinal features very slowly become deformed as the pressure build up in the rocks, then become noticeably offset when a movement occurs along the fault.

Following the main shock, there may be further movements, called aftershocks, which occur as the rock masses ‘set­tle down’ in their new positions. Such aftershocks cause problems for rescue services, bringing down buildings already weakened by the main earthquake.

iii. Volcanic Earthquakes:

Volcanic earthquakes are far less common than Tectonic ones. They are triggered by the explosive eruption of a volcano. Given that not all volcanoes are prone to violent eruption, and that most are ‘quiet’ for the majority of the time, it is not surpris­ing to find that they are comparatively rare.

When a volcano explodes, it is likely that the associated earthquake effects will be confined to an area of 10 to 20 miles around its base, whereas a tectonic earthquake may be felt around the globe.

The volcanoes which are most likely to explode violently are those which produce acidic lava. Acidic lava cools and sets very quickly upon contact with the air. This tends to choke the volcanic vent and block the further escape of pressure. For example, in the case of Mt. Pelee, the lava solidified before it could flow down the sides of the volcano.

Instead it formed a spine of solid rock within the volcano vent. The only way in which such a blockage can be removed is by buildup of pressure to the point at which the blockage is literally exploded out of the way. In reality, the weakest part of the volcano will be the part which gives way, sometimes leading to a sideways explosion as in the Mt. St. Helens eruption.

When extraordinary levels of pressure develop, the resultant explosion can be devastating, produc­ing an earthquake of considerable magnitude. When Krakatoa (Indonesia, between Java and Sumatra) exploded in 1883, the explosion was heard over 5000 km away in Australia.

The shockwaves produced a series of tsunami (large sea waves), one of which was over 36 m high; that’s the same as four, two- story houses stacked on top of each other. These swept over the coastal areas of Java and Sumatra killing over 36,000 people.

By contrast, volcanoes producing free flowing basic lava rarely cause earthquakes. The lava flows freely out of the vent and down the sides of the vol­cano, releasing pressure evenly and constantly. Since pressure doesn’t build up, violent explosions do not occur.

Essay # 9. How Earthquake Effects?

Earthquake is a natural hazard.

The following are the immediate hazardous effects of earthquake:

(i) Ground shaking

(ii) Differential ground settlement

(iii) Land and mud slides

(iv) Soil liquefaction

(v) Ground lurchin

(vi) Avalanches

(vii) Ground displacement

(viii) Floods from dam and levee failures

(ix) Fires

(x) Structural collapse

(xi) Falling objects

(xii) Tsunami

The first six listed above have some bearings upon landforms, while others may be considered the effects causing immediate concern to the life and properties of the people in the region. The effect of tsunami would occur only if the epicentre of the tremor is below oceanic waters and the magnitude is sufficiently high.

Tsunamis are the waves generated by the tremors and not an earthquake itself. Though the actual quake activity lasts for a few seconds, its effects are devastating provided the magnitude of the quake is more than 5 on the Richter scale.

The earthquake is a natural hazard. If a tremor of high magnitude takes place, it can cause heavy damage to the life and property of the people. However, not all the parts of the globe necessarily experience major shocks. Note that the quakes of high magnitude, i.e. 8+ are quite rare; they occur once in 1-2 years whereas those of ‘tiny’ types occur almost every minute.