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Here's all you need to know about earthquakes

A massive 7.9-magnitude earthquake struck east of Pokhara in Nepal on Saturday, causing widespread damage to buildings in the capital Kathmandu and injuring dozens across the country. People from Delhi-NCR, Ranchi, Kolkata, Guwahati, Patna, Jaipur and Bhopal, among other places, felt the tremor.

india Updated: Apr 26, 2015 13:49 IST

A massive 7.9-magnitude earthquake struck east of Pokhara in Nepal on Saturday, causing widespread damage to buildings in the capital Kathmandu and injuring dozens across the country. People from Delhi-NCR, Ranchi, Kolkata, Guwahati, Patna, Jaipur and Bhopal, among other places, felt the tremor.

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Kathmandu's historic Dharara Tower tower, built in the 19th century, collapses after severe earthquake. (Photos by special arrangement)

Here's all you need to know about earthquakes:

What is an earthquake and what causes them to happen?

An earthquake is caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the earth's crust and cause the shaking that we feel during an earthquake. An earthquake occurs when plates grind and scrape against each other.

What is a fault?

Faults are fractures in Earth's crust where rocks on either side of the crack have slid past each other. Sometimes the cracks are tiny, as thin as hair, with barely noticeable movement between the rock layers. But faults can also be hundreds of miles long.

At what depth do earthquakes occur?

Earthquakes occur in the crust or upper mantle, which ranges from the earth's surface to about 800 kilometers deep (about 500 miles).

What is "surface rupture" in an earthquake?

Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. Not all earthquakes result in surface rupture.

How are earthquakes measured?

The familiar Richter scale (which is not a physical device but rather a mathematical formula) is no longer widely used by scientists to report an earthquake's size. Today, an earthquake's size is typically reported simply by its magnitude, which is a measure of the size of the earthquake's source, where the ground began shaking. While there are many modern scales used to calculate the magnitude, the most common is the moment magnitude, which allows for more precise measurements of large earthquakes than the Richter scale. A network of geological monitoring stations, each with instruments that measure how much the ground shakes over time called seismographs allow scientists to calculate an earthquake's time, location and magnitude. Seismographs record a zigzag trace that shows how the ground shakes beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world.

How are quakes classified?

Based on their magnitude, quakes are assigned to a class. An increase in one number, say from 5.5 to 6.5, means that a quake's magnitude is 10 times as great. The classes are as follows:

Great: Magnitude is greater than or equal to 8.0. A magnitude-8.0 earthquake is capable of tremendous damage.
Major: Magnitude in the rage of 7.0 to 7.9. A magnitude-7.0 earthquake is a major earthquake that is capable of widespread, heavy damage.
Strong: Magnitude in the rage of 6.0 to 6.9. A magnitude-6.0 quake can cause severe damage.
Moderate: Magnitude in the rage of 5.0 to 5.9. A magnitude-5.0 quake can cause considerable damage.
Light: Magnitude in the rage of 4.0 to 4.9. A magnitude-4.0 quake is capable of moderate damage.
Minor: Magnitude in the rage of 3.0 to 3.9.
Micro: Magnitude less than-3.0. Quakes between 2.5 and 3.0 are the smallest generally felt by people.

After an earthquake strikes, its magnitude is continuously revised as time passes and more stations report their seismic readings. Several days can pass before a final number is agreed upon.

What are the seismic zones of India?

The Geological Survey of India (GSI.) first published the seismic zoning map of the country in the year 1935. With numerous modifications made afterwards, this map was initially based on the amount of damage suffered by the different regions of India because of earthquakes. Color coded in different shades of the color red, this map shows the four distinct seismic zones of India. Following are the varied seismic zones of the nation, which are prominently shown in the map:

Zone - II: This is said to be the least active seismic zone
Zone - III: It is included in the moderate seismic zone
Zone - IV: This is considered to be the high seismic zone
Zone - V: It is the highest seismic zone

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