• 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.