Mizoram quake zone

Why in news?

• Mizoram experienced at least eight moderate earthquakes between June 21 and July 9. The tremors ranged from 4.2 to 5.5 on the Richter scale

• The epicentre of most of these quakes was beneath Champhai district bordering Myanmar,

• Mizoram’s zone of “scary” earthquakes is caught between two subterranean faults, a geologist assigned to make a preliminary study

What are Earthquake swarms

• It is a series of many (sometimes thousands) low magnitude earthquakes without a discernible main shock.
• They occur in a localised region and over a period of time ranging from days, weeks to even months, without a clear sequence of foreshocks, main quakes and aftershocks.
• When seismic energy piles up inside the Earth and is released in small amounts from certain points, such a series of earthquakes can occur

Do these small earthquakes foretell a bigger one?

• Earthquakes of magnitude 4 or below hardly cause any damage anywhere and are mostly inconsequential for practical purposes.
• Thousands of such earthquakes are recorded around the world every year, and most of them are uneventful.
• They certainly do not signal any big upcoming even
• When a big event happens, all the smaller earthquakes that have occurred in that region in the near past are classified as foreshocks.
• The description does not exist before any big earthquake has happened.
• So, the talk of these being foreshocks of a big earthquake in Delhi have no basis at all.
• A big earthquake might still occur, which no can rule out. But they cannot be predicted. So to say that these small earthquakes are precursors to the big one is totally unscientific.

Earthquake

•  An earthquake  is shaking of the earth. It is a natural event. It is caused due to release of energy, which generates waves that travel in all directions.

• The release of energy occurs along a fault. Rocks along a fault tend to move in opposite directions. 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, alternatively, it is called the hypocentre.

• The point on the surface, nearest to the focus, is called epicentre. It is the first one to experience the waves. It is a point directly above the focus.

Earthquake Waves

• All natural earthquakes take place in the lithosphere.

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

1. 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.

• There are 2 types of body waves and they are, Primary waves [P] and Secondary [S] waves

1. Primary waves are the first to appear on the surface and hence the name P waves.
   • P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation
   • P waves can travel through gaseous, liquid and solid materials.
2. Secondary waves

S waves appear after P waves.  The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass

1. Surface waves

• The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface.

• The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity.

• Their direction also changes as they reflect or refract when coming across materials with different densities.

• Surface waves are considered to be the most damaging waves.