•      Earthquakes are the shaking, rolling or sudden shock of the Earth’s surface.

•      They are the Earth's natural means of releasing stress.

•      They are caused by severe shock waves that travel through solid rocks from underground to the surface.

•      Earthquakes can be felt over large areas although they usually last less than one minute.

•      Earthquakes cannot be predicted.

•      The point under the ground where the earthquake begins is called the focus and the corresponding spot on the surface is called the epicentre.

•      During the Gujarat earthquake of 2001, Bhuj was the epicentre.

•      Sometimes an earthquake has foreshocks.

•      These are smaller earthquakes that happen in the same place as the larger earthquake that follows.

•      It can’t be predicted that an earthquake is a foreshock until the larger earthquake happens.

•      The largest, main earthquake is called the mainshock.

•      Mainshocks always have aftershocks.

•      These are smaller earthquakes that occur afterwards in the same place as the mainshock.

•      This occurs when rocks those have been moved out of their place and start falling back into the same place.

•      Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock.

•      In the Kashmir earthquake of 2005, aftershocks were felt for many days.

•      If an earthquake occurs under the sea, it can cause a tsunami.

•      The most destructive of these are generated from large shallow earthquakes with an epicentre or fault line near or on the ocean floor.

•      The tsunami that hit North-Eastern Japan following the 11 March earthquake was 15 metres high some areas and it travelled 10 km inland.


Measuring an Earthquake:

•      Earthquakes are recorded by instruments called seismographs.

•      The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs freely.


Working of Seismograph:

•      When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not shake.

•      Instead the spring or string that it is hanging absorbs all the movement.

•      So, what is recorded is the difference in position between the shaking part of the seismograph and the motionless part.

•      The seismograph measures the intensity, direction, and duration of earthquakes.

•      Many scales are used in seismograph, but the most common is the Richter scale.

•      The Richter scale has eight levels:

      –  The lowest or 1 indicates a quake that is almost not felt. 

      –  The highest or 8 indicates a massive earthquake.


Effects of an Earthquake:

•      An earthquake, especially a strong one, always causes destruction of property.

•      Buildings may develop cracks or even fall down.

•      Roads and bridges get damaged.

•      Trees get uprooted.

•      Many people lose their lives too by getting trapped under heavy slabs that have fallen.

•      An earthquake, especially an undersea one, can cause other disasters like tidal waves.

•      It can cause landslides and fires.