Earthquake: Geography NCERT Notes For UPSC
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 to be so dangerous because seismologists, the scientists who study 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 notes below on the Earthquake for UPSC preparation to understand all the topics in detail.
Why Do Earthquakes Happen?
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.
Types of Faults Which Causes Earthquakes
There are several faults because of which earthquakes occur. Based on whether fault is occurring because of convergent, divergent, or transform tectonic plates forcing there occurs different types of earthquake.
1. 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.
2. 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.
3. 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.
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Types of Earthquakes
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 cause earthquakes.
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.
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.
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.
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.
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 lag of some time these waves arrive at the surface and are also called secondary waves. S-waves can travel only through solid materials.
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.
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Earthquake NCERT Notes FAQs
The energy which travels through the earthquake in the form of vibration called seismic waves. Seismic waves are measured using seismometers that can record time, location and intensity of an earthquake. Events of earthquakes recorded on the basis of its magnitude or intensity of the schock.The magnitude scale is known as the Richter scale.
The effects which are caused because of earthquakes are: Ground Shaking, Differential ground settlement, Land and mudslides, Fires, Ground lurching, Avalanches, Ground displacement, Floods from dam and levee failures, Structural collapse, Tsunami.
India comes under one of those countries where earthquakes are more frequent because of the presence of technically active young fold mountains – Himalaya. In India there are four seismic zones (II, III, IV, and V) based on scientific inputs relating to seismicity, earthquakes occurred in the past and tectonic setup of the region.
The energy which travels through the earthquake in the form of vibration called seismic waves. An instrument which is used to measure seismic waves is called seismometers
Check yourself and others for injuries. Check water, gas, and electric lines for damage. Stay out of damaged buildings.
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