According to the National Center for Seismology (NCS)  , a shallow earthquake of 4.0 magnitude struck Delhi-NCR, with a focal depth of 5 km.

• Another 4.0 magnitude earthquake was recorded in Bihar at 8:02 AM, with a depth of 10 km.

About Earthquake

An earthquake is the shaking of the Earth’s crust caused by a sudden release of energy. This release of energy occurs along a fault, which is a break in the Earth’s crustal rocks. 

How do earthquakes occur?

• Earthquakes are caused by the movement of tectonic plates. 
• When two tectonic plates slide past each other, the jagged edges can get caught, and when they release, the energy is released as vibrations. 
• Earthquakes can also be caused by the injection or withdrawal of magma, or explosions of chemical or nuclear devices. 

Types of earthquakes

• Earthquakes can vary in intensity and duration, ranging from minor tremors to major seismic events. 
• Earthquakes can be categorized into shallow, intermediate, and deep based on their depth. 

What is a Shallow Earthquake?

• Shallow earthquakes originate between 0 to 70 km below the surface.
• Intermediate earthquakes occur at 70 to 300 km, and deep earthquakes at 300 to 700 km.
• The shallower the earthquake, the more destructive it is, as seismic waves lose less energy before reaching the surface.
  • Example: A 5.6 magnitude earthquake in Indonesia (November 2022) killed over 160 people.

Earthquake Waves: Earthquake waves are basically of two types — body waves and surface waves.

• Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Eg. P-waves and S-waves.
• The body waves interact with the surface rocks and generate new sets of waves called surface waves.
• Surface waves, also known as L-waves, travel along the earth’s surface and are the most destructive.

Difference between P and S waves

Types of Wave	P-waves (Primary waves)	S-waves (Secondary waves, shear waves)
Speed	Faster than S-waves	Slower than P-waves
Travel path	Can travel through liquid, gas, and solid	Can only travel through solid
Vibration	Vibrate parallel to direction of propagation	Vibrate perpendicular to direction of propagation
Effect	Cause pressure changes in the medium	Create troughs and crests in the material
Shadow Zones	The shadow zone of is P-waves much smaller than that of the  S-wave.	The shadow zone of S-wave is much larger than that of the P-waves.

• The difference in travel time between P-waves and S-waves can be used to determine the epicenter of an earthquake. 

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