Earthquakes refer to the violent shaking of the ground, which can often lead to mass destruction. Earthquakes can be caused by movement within the earth’s crust, volcanic eruption, or artificial reasons. 

In Geography Class 11: Earthquake notes, we will study the type of earthquake waves, the propagation of the earthquake waves, the different types of earthquakes, and how to measure earthquakes.  

Effects of Earthquakes 

• Earthquakes can lead to avalanches, tsunamis, land and mudslides, fires, and structural collapse
• Earthquakes cause the ground to shake, leading to soil liquefaction, ground lurching, and ground displacement
• Earthquakes can cause dams to flood, destroy communications and transportation lines, and lead to differential ground settlement. 

Causes of Earthquakes

• Earthquakes are caused due to tectonic movement within the earth
• During this movement, there is a release of energy that travels in all directions. 

Types of earthquake waves 

• Body Waves: A body wave is a seismic wave that travels into the earth’s interior, as opposed to a surface wave, which travels near the earth’s surface. Body waves are P and S waves. The earth is shaken in various ways depending on the kind of wave
• P-waves or primary waves are the first waves to arrive at the earth’s surface. Primary waves travel through all mediums-solid, liquid, and gas. They elongate and squeeze the material and vibrate parallel to the direction of the wave propagation.
• S-waves or secondary waves are those body waves that arrive after the earthquake. Unlike primary waves, secondary waves travel only through the solid medium. Secondary waves create crests and troughs in the material as the direction of the s-wave is always perpendicular to the direction of the wave propagation
• Surface Waves – When the body waves interact with the rocks on the surface, they generate surface waves. Surface waves travel along the surface. The velocity with which surface waves travel depends on the density of the medium through which they are travelling. The velocity of the surface waves is higher when it travels through denser materials. The direction of surface waves changes when they come across materials of different densities as they reflect or refract. 

Propagation of the earthquake waves 

• As earthquake waves propagate, they cause different vibrations in the rocks they travel through 
• Since primary waves travel parallel to the direction of the way, the pressure they exert on the material is in the direction of the propagation. This leads to the stretching and squeezing of materials
• Since the secondary waves travel perpendicular to the direction of the wave in a vertical plane, they basically create troughs along with crests in the material. 

The Emergence of Shadow Zone

Shadow Zone

• Shadow zones are zones where earthquake waves are not reported. It has been observed that for each earthquake, there exists a different shadow zone
• Seismographs have recorded P and S waves within a distance of 105° from the epicentre
• The seismographs also recorded P-waves beyond 145° from the epicentre. S-waves were not recorded beyond 105°. Therefore, a shadow zone between 105° and 145° was created for both of the waves
• It is observed that the shadow zone of P-waves appears as a band around the earth. 

Types of Earthquakes 

• Tectonic earthquakes- The movement of the tectonic plates causes these types of earthquakes. Tectonic earthquakes occur frequently and are very common

• Collapse earthquakes- Areas of intense mining activities are prone to minor tremors caused by the roofs collapsing. The tremors caused by these events are known as collapse earthquakes
• Volcanic earthquakes- Volcanic earthquakes are those earthquakes that happen before, after, or during a volcanic eruption. These earthquakes are confined to active volcanic zones and are caused by rocks being pushed to the surface by the magma during an explosion
• Reservoir-induced earthquakes- Reservoir-induced earthquakes are those earthquakes that occur in areas of high or large reservoirs
• Explosion earthquakes- These earthquakes are artificial and are caused by nuclear or chemical explosions.

Measuring Earthquake

• Earthquakes are measured based on the magnitude and intensity of the shock
• The magnitude scale at which earthquakes are measured is called the Richter Scale
• The Richter scale determines the magnitude (energy released during the quake) of the shock on a scale of 0 to 10
• The intensity scale at which earthquakes are measured is called the Modified Mercalli Intensity scale developed by Italian volcanologist Giuseppe Mercalli
• The Mercalli intensity scale determines the intensity (visible damage caused by the quake) on a scale of 1 to 12. 

Conclusion

When two blocks of earth abruptly glide past one other, an earthquake occurs. Earthquakes may occur at any time of day or night, unexpectedly, fiercely, and without warning. When an earthquake strikes a crowded location, it may result in property damage, injuries, and even fatalities. A tsunami is a real danger if you live near the shore. Earthquake damage might also result in floods or fires.

Although there are no assurances of safety during an earthquake, you may take precautions. You should have a catastrophe plan in place. Being prepared may assist to alleviate dread, anxiety, and loss. It is natural to be anxious if you are involved in a crisis.