What is an Earthquake ?

Momentary shaking of the ground or vibrations or oscillations of the ground caused by the slip or by volcanic or magnetic activity or other sudden stress changes in the earth are called earthquakes.

Earthquake is one of the most destructive natural hazard. They may occur at any time of the year, day or night, with sudden impact or little warning. 

They can destroy buildings and infrastructure in seconds, killing or injuring the inhabitants. Earthquakes not only destroy the entire habitation but may de-stabilize the government, economy and social structure of the country.


1) Focus

The point within the earth where earthquake rupture starts is called focus or Hypocentre. It is the source of elastic waves inside the earth.

2) Epicentre 

The point on the earth’s surface vertically above the focus of the earthquake is called Epicentre.

3) Focal depth

The depth of the focus from the epicentre is called focal depth. It is an important parameter in determining the damaging potential of earthquake. 

Most of the damaging earthquakes have shallow focus with focal depth less than 70 km.

The maximum energy released by an earthquake progressively tends to become smaller as the focal depth increases. 

Also, seismic energy from a source deeper than 70 km gets largely dissipated by the time it reaches the surface. 

Therefore, the main consideration in the design of earthquake-resistant structures are shallow focus earthquakes.

4) Epicentral distance

The distance from the epicentre to any point of interest on the surface of the earth is called epicentral distance or focal distance.

5) Focus 

The vibrations felt in the bed rock are called shocks.

A number of smaller size earthquakes take place before and after a big earthquake.

Those occuring before the big one are called foreshocks.

Foreshocks are usually interpreted as being caused by plastic deformation or small ruptures.

6) Aftershocks

Smaller size earthquakes occurring after the main shock are called aftershocks. Aftershocks are usually due to fresh ruptures or readjustment of fractured rocks.

7) Focal region

Seismic destruction propagates from the focus through a limited region of the surrounding earth’s body, which is called the focal region.

8) Isoseismal line

A contour or a line on a map joining points of equal intensity for a particular earthquake is called Isoseismal line.

9) Homoseismal line

The line joining locations at which the shock arrives simultaneously is known as the Homoseismal line.

10) Meisoseismal

The region that suffers the strong shaking and significant damage during earthquake is termed as meisoseismal region.

The region surrounding the epicentre is the Meisoseismal region. 

11) Aseismic

A fault on which no earthquakes have been observed is called aseismic.

12) Seismology

The science related with study of earthquakes and the structure of the earth, is known as seismology. It includes the study of seismic waves, origin, intensity of earthquake, forecasting earthquake, etc.

13) Seismicity

The geographic and historical distribution of earthquakes is known as seismicity.

14) Seismograph

A seismograph is an instrument used to measure the vibration of the earth. It records earthquake ground motion in a particular direction as a function of time.

15) Seismogram

It is a record of seismograph in response to ground motion produced by an earthquake.

Seismogram is used for the following purposes :

To determine Epicentre of the earthquake.

They are used for obtaining the seismic parameters which are used in design of structures and also for identifying seismic zones.

They also helps us in studying seismic waves and their nature which helps in assessing the severity of EQ.

16) Accelerometer

These are the instruments having electronic transducers that produce an output voltage proportional to ground acceleration during earthquake, i.e. it measures ground acceleration.

17) Accelerogram

The motion of the ground can be described in terms of displacement, velocity or acceleration. The variation of ground acceleration with time recorded at a point on ground during an earthquake is called an accelerogram.

The nature of accelerogram depends upon the energy released at source, type of slip at fault rupture, geology along the travel path and local soil conditions.

18) Seismic gap

A section of a fault that has produced earthquakes in the past but is now quiet, is called seismic gap.

19) Seismic zone

An area of similar seismic activities is known as Seismic Zone.

20) Seismogenic

Seismogenic means Capable of generating earthquakes.

21) Earthquake size :

Earthquake size is defined in terms of two things :

  • Magnitude
  • Intensity

Earthquake size is a measure of the quantitative and qualitative effects vibrations produced by the earthquake.

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