Earthquakes: Causes, Effects & Distribution - UPSC Notes

Earthquakes are considered as one of the most serious natural disasters which affect people all across the globe. It can also be disastrous sometimes even from long distances where earthquakes occur. They turn out to be so dangerous because seismologists, the scientists who take up the study of earthquakes, cannot predict them in time for evacuations or other precautions. It can be sudden shaking of the ground which is caused by the passage of seismic waves through Earth’s rocks. These waves are produced when the energy stored in the Earth crust is released suddenly and this release of energy happens when masses of rock straining against one another suddenly fracture and “slip.” Hypocenter is the point below the earth’s surface where earthquakes start and epicenter is that location which lies directly above the hypocenter on the surface of the Earth.

Go through the NCERT Earthquake Notes UPSC to understand all the topics in detail. This article elaborates on Earthquakes, their types, measurement, & Distribution which is useful for the upcoming UPSC exams.

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What is Earthquake?

Fig: Earthquake Diagram

Earthquakes are sudden, violent ground shakings usually generated by the movement of huge sections of the earth's surface, referred to as tectonic plates. These seismic activities tend to emanate from any accumulation of stress across a geological fault or fracture that eventually leads to the release of energy manifesting in seismic waves. Earthquakes range in intensity from minor tremors barely sensed to disastrous shocks that can bring widespread devastation and loss of life. The point inside the Earth from which the earthquake begins to take place is called the hypocenter, while the point directly above it on the Earth's surface is the epicenter. The nature of earthquakes should be realized so that one can be prepared with regard to disasters resulting from such a sudden natural event, especially for countries at the level of frequent seismic activities.

Why Earthquake Occurs?

When a stress builds up between the plates as one plate passes another, then there occurs an earthquake. Because of this plates do not move one over another smoothly rather than that they just snag and grind, allowing energy to build up. When this movement of plates occurs over and over then it leads to the release of shock or seismic waves through the Earth’s crust.

FOCUS is that point at which this slippage occurs and the point which is located just above the FOCUS on the earth’s surface is called epicenter. Seismic shock waves originate radially in outward direction from these points their energy will reduce with distance.

Also read: Classification of Soil in India for UPSC here!

Faults Causing Earthquakes

There are several faults because of which earthquakes occur. Based on whether fault is occurring because of convergent, divergent, or transform tectonic plates, different types of earthquake.

Strike-Slip Faults

Such faults occur along transform boundaries where movement of tectonic plates are horizontally or parallel to each other. When tectonic plates cross over these fault lines they cause deformation of rivers, roads, etc. One most common examples of strike-slip faults are the San Andreas Fault in the United States and the North Anatolian Fault in Turkey.

Normal Faults

Such faults occur along divergent plate boundaries. As in such faults there occur extensional forces , the footwall is forced upward, while the hanging wall slides occur downward. All this combinely creates a series of valleys known as graben and mountains known as horst. Mountains and valleys are created because of normal faults only like: Grand Tetons, the Basin and Range in the western United States, and the Wasatch Front in Utah.

Reverse Faults

Such faults are caused by compressional forces as tectonic plates collide together forcing one plate to rise above another. Such faults are seen in the Northern Rocky Mountains, the Alps, Himalayas, and the Appalachian mountains.

Check out NCERT notes on Endogenic Processes!

Types of Earthquakes

Types of earthquakes include tectonic (caused by plate movements), volcanic (associated with volcanic activity) and human-induced.

Tectonic Earthquake

It is one of the most common types of earthquakes. Earth is like a solid sphere from the outside but from inside it is extremely active. And there are four basic layers of the Earth: a solid crust, a hot, nearly solid mantle, a liquid outer core, and a solid inner core. There is continuous shifting movement in the tectonic plates as they drift around on the viscous, or slowly flowing, mantle layer below. This drifting movement, again and again, leads to stress on the Earth’s crust and when this stress gets too large it causes cracks which are known as faults. And with movement in tectonic plates it causes movements at the faults. This leads to the slipping of land along the faultline along convergent, divergent and transform boundaries causing earthquakes.

Volcanic Earthquake

Such earthquakes are confined to areas of active volcanoes. They are produced by stress changes in solid rock because of the withdrawal of magma. Such earthquakes cause land to subside and can produce large ground cracks. The main reason behind such earthquakes is when magma leaving the volcano is filled by rocks being pushed to the surface.

Human-Induced Earthquake

The area where intense mining activity like sometimes collapse of underground mining also causes minor tremors. It is also known as collapse earthquakes. Sometimes there is ground shaking because of the intense explosion of chemical and nuclear devices. Such tremblement is known as explosion earthquakes. The earthquake which shows its effect over the area of large reservoirs are induced earthquakes.

You can check NCERT notes on Types of Rocks here.

What are Seismic Waves?

These are the waves full of energy which causes earthquakes. These waves travel through the earth and are recorded on seismographs.

Waves which cause earthquakes are of two types: body waves and surface waves.

Body Waves

These waves are generated because of release of energy at the focus and move in all directions travelling through the body of the earth. Further body waves are of two types. They are called P- and S-waves.

• P-waves: They speed very fast and are the first to arrive at the surface. These are also called ‘primary waves’. These waves show similarity with sound waves as they travel through gaseous, liquid and solid materials.
• S-waves: After a lag of some time these waves arrive at the surface and are also called secondary waves. S-waves can travel only through solid materials.

Surface Waves

When there is the interaction between body waves and surface rock it generates new waves called surface waves. The movement of these waves are along the surface and these waves are the last to report on seismographs. They are considered to be more destructive than body waves. They cause displacement of rocks, and hence, the collapse.

You might also like: NCERT notes on Ocean Waves!

Measurement of Earthquakes

Measuring earthquakes is important, considering the consequences of such disasters and how better to be prepared for future seismic events. Generally speaking, there are two scales used mainly for the accomplishment of this task: the Mercalli Intensity Scale and the Richter Magnitude Scale.

Mercalli Intensity Scale

The Mercalli Intensity Scale, originally proposed in 1902 by Giuseppe Mercalli and revised later, measures the effects and relative damage of an earthquake at particular places. It runs from I, for not felt, to XII for total destruction. This is a subjective scale because it depends on the human observation of earthquake impacts, like the structural damage of buildings, coupled with personal accounts showing the level of the earthquake impact on buildings and populations.

Richter Magnitude Scale

In contrast, the Richter Magnitude Scale, developed by Charles F. Richter in 1935, is a measure of the total energy release at the source of the earthquake. By means of seismographs, it measures the amplitude of seismic waves to provide a rather more objective, fairly accurate analysis of the intensity of the earthquake. The Richter Scale is logarithmic; thus, each whole number increase in magnitude is ten times greater in measured amplitude and about 31.6 times greater in energy release. For example, an earthquake measuring 5.0 on the Richter Scale possesses much more energy than one measuring 4.0.

Read the article on the world distribution of earthquakes!

Distribution of Earthquakes

There is no specific location for earthquakes to occur, but based on history it is observed that a general pattern is repeated year after year. There are mainly three large zones where earthquakes is generally repeated:

• The circum-Pacific seismic belt: It is the world’s greatest earthquake belt which is found along the rim of the Pacific Ocean. Approx 81% of earthquakes occur in this belt only. Because of this reason it is also known as the “ring of fire.”
• The Alpide earthquake belt: This belt extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic.All total 17% of earthquakes occur in this belt and in them some of the earthquakes are highly destructive.

The third belt of earthquakes follows the submerged mid-Atlantic Ridge. This ridge is deep under water and very far from human development.

Also read: NCERT notes on Structure of Earth!

Download the Earthquakes UPSC Notes PDF for Key Takeaways!

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