NCERT Notes: Earthquake - Body Waves, Causes & Types [Geography Notes For UPSC]

NCERT notes on essential topics for the UPSC Civil Services Exam. These notes will also be useful for other competitive exams like banking PO, SSC, etc. This article talks about Earthquakes, their magnitude scale, causes, types, etc.

Earthquake – Body Waves, Causes & Types (UPSC Notes):- Download PDF Here

About Earthquake

• All-natural earthquakes occur in the lithosphere.
• Seismic wave studies offer a full picture of the layered interior.
• An earthquake is, simply put, the shaking of the earth’s crust.
• It is caused due to the energy release, which triggers waves that travel in all directions.
• The emanation of energy occurs along a fault.
• A fault is a sharp break in the crustal rocks.
• Rocks along a fault generally move in opposing directions.

Types of Earthquakes

• Tectonic earthquakes: The most common form of earthquake, is caused by the movement of loose fragmented pieces of land on the earth’s crust known as tectonic plates.
• Volcanic earthquake: The less prevalent compared to the tectonic variety, these earthquakes happen before or after the eruption of a volcano. It is caused when magma leaving the volcano is filled by rocks being pushed to the surface.
• Collapse earthquake: This earthquake occurs in underground mines. The main cause is the pressure generated within the rocks.
• Explosion earthquakes: The occurrence of this type of earthquake is artificial. High-density explosion such as nuclear explosions is the primary cause.

Causes of Earthquakes

• It is caused due to the tectonic movements of the earth.
• The energy release produces waves that travel in all directions.
• The point where energy is released is called the focus or hypocentre. It is generally located at a depth of 60 km.
• This causes a release of energy, and the energy waves travel in all directions.
• The point where the energy is released is called the focus of an earthquake or hypocentre.
• The point on the surface of the earth which is vertically above the focus is called the epicentre. It is the first place to experience the waves.

Body Waves

• Earthquake waves are of two types — body waves and surface waves.

P- Waves

• P-waves are also known as the Primary waves. They are the first waves to arrive at the surface.
• The characteristics of P-waves are like sound waves. They travel through all three mediums- solid, liquid, and gas.
• These waves tend to vibrate parallel to the direction of wave propagation. This causes density differences in the material through which they travel.
• These waves are responsible for elongating and squeezing material.

S- Waves

• S- Waves arrive sometime after the happening of the Earthquake and they are called secondary waves.
• A significant characteristic of these S-waves is that they travel only through a solid medium.
• The direction of vibration of this S-wave is perpendicular to the direction of wave propagation, thereby creating crests and troughs in the material of their transmission.

Shadow Zone

• The shadow zone is the zone of the earth from angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P waves.
• The shadow zone results from P waves being refracted by the liquid core and S waves being stopped completely by the liquid core.
• A zone between 105° and 145° from the epicentre was recognized as the shadow zone for both the wave types.
• The entire zone beyond 105° does not receive S-waves.
• The shadow zone of the S-wave is larger than that of the P-waves.
• The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre.

Effects of an earthquake

The following are the immediate hazardous effects of Earthquake:

• Shaking of ground
• The disparity in ground settlement
• Natural disasters like Tsunami, landslides, mudslides, and avalanches
• Soil liquefaction
• Ground lurching and displacement
• Floods and fires
• Infrastructure collapse.

Measurement of Earthquake

All earthquakes are different in their intensity and magnitude. The instrument for the measurement of vibrations is known as Seismograph.

Magnitude scale

• Richter scale is used to measure the magnitude of the earthquake
• The energy released during a quake is expressed in absolute numbers of 0-10.

Intensity scale

• The Mercalli scale is used to measure the intensity of an earthquake
• It measures the visible damage caused due to the quake.
• It is expressed in the range of 1-12.

Note: Read on to know more about Earthquakes apart from the NCERT source.

Earthquake Fault Types

Normal, reverse (thrust), as well as strike-slip faults are the three primary fault types that can all result in an interplate earthquake. Examples of dip-slip faulting include normal and reverse faulting when movement on the faults contains a vertical component and displacement all along fault is in the plane of dip.

Normal

• Normal faults primarily appear along divergent boundaries or other regions in which the crust is extending.

Reverse

• In regions where the crust is shortening, like near a convergent boundary, reverse faults develop.

Strike-slip

• Transform boundaries are a specific kind of strike-slip fault. Strike-slip faults are steep formations in which the opposing sides of the fault slip past one another horizontally. Movement on faults with both dip-slip and strike-slip components is a common cause of earthquakes.

Shallow-focus and Deep-focus Earthquakes

The bulk of tectonic earthquakes begin at depths of up to ten kilometres in the ring of fire. “Shallow-focus” earthquakes are those that have a focal depth of fewer than 70 km, whereas “mid-focus” earthquakes are those with a focal depth of between 70 and 300 km. Deep-focus earthquakes can develop at significantly deeper depths (ranging from 300 to 700 km) within subduction zones, wherein older and colder oceanic plates drop below another tectonic plate. Due to the tremendous temperature and pressure, deep-focus earthquakes happen in which the subducted lithosphere should no longer be brittle. Faulting brought on by olivine going through a phase transformation into a spinel structure is one potential mechanism for the creation of deep-focus earthquakes.

Earthquake Clusters

The majority of earthquakes are related to one another in terms of space and time and occur in succession. There is a belief that earthquakes can reoccur in a predictable pattern, however, most earthquake clusters are made up of minor vibrations that do little to no harm.

Aftershocks

A further earthquake, known as an aftershock, happens after the mainshock. The main sources of these aftershocks include the crust surrounding the ruptured fault line as it adjusts to the impacts of the mainshock, rapid changes in tension amongst rocks, as well as the stress from the first earthquake. A building that has already suffered damage from the original earthquake might still sustain more damage from an aftershock, despite the fact that they are always lesser in size. When an aftershock is greater than that of the mainshock, the mainshock that originally occurred is reclassified as a foreshock and the aftershock is reclassified as the mainshock. As the crust near the shifted fault plane adapts to the mainshock’s effects, aftershocks are created.

Earthquake Swarms

Series of earthquakes that occur in a specific location over a brief period of time is known as earthquake swarms. The fact that no individual earthquake in the chain is plainly the main shock, hence none has a noticeably higher magnitude than the other, distinguishes them from earthquakes which are succeeded by a string of aftershocks. When several earthquakes hit a fault at once, this phenomenon is known as an earthquake storm. The clusters of earthquakes are each caused by the trembling or stress rearrangement of the preceding earthquakes. These storms, which are similar to aftershocks but on neighbouring fault segments, happen over the period of years, with several of the later earthquakes being just as destructive as the earlier ones.

Induced Seismicity

Tectonic plate movement is the main source of earthquakes, although they can also be brought on by human activity. Constructing reservoirs, mining resources like coal or oil, and pumping fluids subsurface for waste disposal or hydraulic fracturing are a few examples of activities that could alter the stress and strain on the crust. These earthquakes often have minor magnitudes. Studies link Oklahoma’s oil sector to several earthquakes over the previous century, and the 5.7 magnitudes 2011 earthquake there is believed to have been triggered by wastewater from oil extraction being dumped into injection wells. Although the connection between the 2008 Sichuan earthquake and loading from the Zipingpu Dam has not been proven clearly, a Columbia University report made the claim.

Earthquake Prediction

Seismology’s branch on earthquake prediction is engaged with pinpointing the date, place, and magnitude of earthquakes that might happen in future within predetermined ranges. There are numerous techniques for determining when and where earthquakes will occur. Seismologists have put a lot of work into their studies, but it is still not possible to make scientifically valid predictions for a certain day or month.

Earthquake Forecasting

While forecasting is frequently distinguished from prediction, prediction is typically thought of as a sort of forecasting. The probabilistic evaluation of the general earthquake risk, along with the frequency and magnitude of destructive earthquakes in a specific area spanning years or decades, is the focus of earthquake forecasting. The likelihood that a segment will rupture within the next few decades could be calculated for faults that are well-characterised. Persons within the system’s area may be able to seek refuge prior to the earthquake’s influence being felt thanks to the development of earthquake warning systems that may notify a region of an earthquake in progress before the ground has started to move.

Earthquake Preparedness

Engineering for earthquakes has as its goal predicting the effects of earthquakes upon buildings and infrastructure and designing them to reduce the risk of harm. Seismic retrofitting can be used to alter existing buildings to increase their earthquake protection. Building owners may benefit from having earthquake insurance to protect their finances from earthquake-related losses. A government or corporation can use emergency management tactics to reduce risks and get ready for outcomes. The Igor expert system, which is a component of a portable lab which facilitates the processes leading to the seismic analysis of masonry structures and the design of retrofitting activities on them, may aid in the evaluation of structures and the design of preventative activities. People can also take precautions including securing water heaters and large objects that could hurt someone, finding utility shutoffs, and learning what to do when the shaking begins. Preparedness for earthquakes includes the risk of a tsunami brought on by a strong quake in regions close to major bodies of water.

Earthquake – Body Waves, Causes & Types (UPSC Notes):- Download PDF Here

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