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Introduction to Earthquake

Earthquake is a natural disaster, which causes loss of lives and brings huge destruction to buildings, roads and properties. Earthquake occurs almost all the parts of the world. However some areas are more affected than the others. Those areas which are affected more and more and experience frequent earthquakes are called Earthquake Prone Areas. As in the earlier chapter it has been mentioned that the Plate Tectonics move all the time. If this movement is transvergent and two crustal plates slide past each other, these plates cause a stress between their boundaries, when this stress is suddenly released, strong seismic waves are produced. These waves travel though the body and surface of earth, thus moving the earth’s surface and cause an earthquake.

Earthquake can be defined as:

”Earthquakes are those movements of the earth’s crust that make the ground vibrate and shake.”

OR

”An earthquake is the release of energy that has been slowly built up. during the stress of increasing deformation of rocks.

Earthquake occur when energy stored in elastically-strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can also be generated by bomb blasts, volcanic eruptions, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake-prone areas, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth. The study of how seismic waves behave in the Earth is called seismology.

Seismic Waves:

When a sudden break or shift occurs in the Earth’s Crust , energy radiates out as seismic waves, just as the energy from a disturbance in a body of water radiates out in wave form. In every earthquake, there are several different types of seismic waves.

  • Body Waves: 

    Body waves move through the inner part of the Earth, while surface waves travel over the surface of the earth. Surface waves–sometimes called long waves, or simply L waves– are responsible for most of the damage associated with earthquakes, because they cause the most intense vibrations. Surface waves stem from body waves that reach the surface. There are Two main types of body waves;

    • Primary Waves: Also called P-waves or compressional waves. These are a type of seismic body wave in which rock particles vibrate parallel to the direction of wave travel. Primary waves are alternatively compressional and extensional, and cause the rocks they pass through to change in volume. These waves are the fastest traveling seismic waves and can travel through solids, liquids, and gases. Also called P wave See Note at earthquake.
    • Secondary Waves: These waves are also called S-waves. Secondary waves cause the rocks they pass through to change in shape. These waves are the second fastest traveling seismic waves (after primary waves) and can travel through solids but not through liquids or gases. Also called shear wave S wave.
  • Surface Waves:

Surface waves are something like the waves in a body of water–they move the surface of the Earth up and down. This generally causes the worst damage because the wave motion rocks the foundations of man-made structures. L-waves are the slowest moving of all waves, so the most intense shaking usually comes at the end of the earthquake.

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