What is an earthquake?
  • An earthquake is what happens when two blocks of the earth suddenly slip past one another.
  • The surface where they slip is called thefault or fault plane.
  • The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.

labeled fault

  • Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows.
  • Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens.
  • The largest, main earthquake is called the mainshock.
  • Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock.
  • Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!
What causes earthquakes and where do they happen?

Diagram of an earthquake

  • Tectonic plates present on the outer part of the earth keep slowly moving around, sliding past one another and bumping into each other.
  •  The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults.
  • Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.


  •  The volcanic eruptions are often very violent and cause vibrations in the earth crust. Sometimes the vent of a volcano is blocked temporarily and explosive eruption takes place suddenly causing tremors in the earth crust. The Krakatoa that erupted in 1883 became the cause of a violent earthquake there.
  • The roofs of underground caves sometimes give way and release great force to cause minor tremors in the earth crust.
  • Nuclear explosions also release massive energy to cause tremors in the earth crust.
Why does the earth shake when there is an earthquake?
  • While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up.
  • When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released.
  • The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond.
  • The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us!
How are earthquakes recorded?
  • Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram.
  • The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.


How do scientists measure the size of earthquakes?
  •  They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was . A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.


  • The size of the earthquake is called its magnitude.
  • There is one magnitude for each earthquake.
  • Scientists also talk about the intensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.
How can scientists tell where the earthquake happened?
  • Seismograms come in handy for locating earthquakes too. To understand how this works, let’s compare P and S waves to lightning and thunder. Light travels faster than sound, so during a thunderstorm you will first see the lightning and then you will hear the thunder. If you are close to the lightning, the thunder will boom right after the lightning, but if you are far away from the lightning, you can count several seconds before you hear the thunder.
  • P waves are like the lightning, and S waves are like the thunder. The P waves travel faster and shake the ground where you are first. Then the S waves follow and shake the ground also. If you are close to the earthquake, the P and S wave will come one right after the other, but if you are far away, there will be more time between the two. By looking at the amount of time between the P and S wave on a seismogram recorded on a seismograph, scientists can tell how far away the earthquake was from that location.
  • However, they can’t tell in what direction from the seismograph the earthquake was, only how far away it was. If they draw a circle on a map around the station where the radius of the circle is the determined distance to the earthquake, they know the earthquake lies somewhere on the circle. But where?


  • Scientists then use a method called triangulation to determine exactly where the earthquake was .
  • It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake.
  • If you draw a circle on a map around three different seismographs where the radius of each is the distance from that station to the earthquake, the intersection of those three circles is the epicenter!
Can scientists predict earthquakes?
  • No, and it is unlikely they will ever be able to predict them.
  • Scientists have tried many different ways of predicting earthquakes, but none have been successful.
  • On any particular fault, scientists know there will be another earthquake sometime in the future, but they have no way of telling when it will happen.
Where are the regions having maximum frequency of earthquakes  ?
  • Circum-Pacific Earthquake Belt: This belt includes all the coastal areas around the vast pacific ocean. This belt extends as an isostatically sensitive zone through the coasts of Alaska, Aleutian Islands, Japan, Philippines, New Zealand, North and South America. This zone accounts for 68% of all earthquakes on the surface of the earth. The most talked about earthquake areas in this zone include Japan, Chile, California and Mexico.
  • Mediterranean-Asia Earthquake Belt: This belt begins from Alps mountain range and passes through Turkey, Caucasus Range, Iran, Iraq, Himalayan mountains and Tibet to China. One of its branches passes through Mongolia and Lake Baikal and another branch extends to Myanmar. About 31% of world’s earthquakes are located in this region.
  • Other Areas: These include Northern Africa and Rift Valley areas of the Red Sea and the Dead Sea. In addition to these, the ocean ridges are also active earthquake zones.
What are the effects of earthquakes ?
  • Loss of Property: Severe earthquakes reduce to rubble human structures ranging from huts to palaces and single storey to multi storey buildings. Even pipelines laid under the ground and railway lines are damaged or displaced.
  • Loss of Life: Earthquake tremors of a few seconds takes the lives of thousands of people.
  • Changes in the course of rivers: On account of the impact of earthquakes, sometimes rivers also change their course. Consequently,  floods come .
  • Tsunamis: The earthquakes in the sea generate massive waves called Tsunami in Japanese language. It sometimes rises to the height of 20-25 metres. It causes great damage to life and property of people living in coastal areas as well as to tourists.
  • Mud Fountains: On account of earthquakes of high intensity, warm water and mud fountains also burst.
  • Cracks in Earth Crust: Earthquake cause cracks in earth’s crust anywhere in fields, roads, parks and even hills. They are thus rendered useless. The San Andreas fault in California, U.S.A. was created in a similar manner.