Seismicity and Earthquake

  • Seismicity is a measure of the frequency of earthquake occurrences, mechanisms, and magnitude at a given geographical location. As such, it summarizes a region’s seismic activity.   Beno Gutenberg and      Charles Richter coined the term seismicity in 1941.
  • It is shaking of the Earth, which happens due to the release of energy along a fault line.
  • Hypocentre or Focus: The point where the energy of an earthquake is released
  • Epicentre: It is the point on the surface, directly above the focus, the first one to experience the waves.

Earthquake waves

  1. P-waves: They are called Primary waves. They move faster and are first to arrive at the surface. They are similar to sound waves and can travel through Solid, Liquid and Gaseous materials. P waves vibrate parallel to the direction of the wave which causes stretching and squeezing of the material
  2. S-waves:   They are    called Secondary waves which arrive at a time lag with Primary waves. They can travel only through solid materials. S-waves vibrate in perpendicular to the wave direction which creates crests and troughs.

Surface waves: the body waves interact with the surface rocks and generate surface waves which move along the surface rocks. They are the last to report on the Seismograph and are the most destructive. They cause displacement of rocks and structural collapse. Surface waves vibrate perpendicular to the wave direction.

Shadow Zone

Shadow zones are those specific areas where seismic waves are not reported. These zones are distinct for P and S-waves.

  • Within the distance, up to 105° from epicenter has recorded the arrival of both waves.
  • The zone between 105° – 145° from the epicenter is identified as a shadow zone for both types of waves (P & S).
  • Beyond 105° Zone does not receive S-waves. Thus, the shadow zone of S-wave is much larger than P-wave.
  • P-wave appears after 145° from the epicenter.

Types of Earthquake

  • Tectonic: They are caused due to the sliding of rocks along a fault plane.
  • Volcanic: they are confined to areas of active volcanoes. They are caused due to the eruption of volcanoes and the corresponding tectonic disturbances.
  • Collapse: they are caused in areas of intense mining    activities where the underground mines collapse causing minor tremors
  • Explosion: they are caused due to the explosion of chemical or nuclear devices.

Measurement of Earthquakes

  • A seismograph is an instrument that records the waves reaching the surface.
  • Richter Scale: it is known as Magnitude scale as it measures the energy released during the quake. It is expressed in absolute numbers 0-10.
  • Mercalli Scale: it is called an Intensity scale as it measures the visible damage caused by the earthquake. The range is from 1 – 12.


  • These are long-wavelength, long-period sea waves or tidal waves produced by the sudden or abrupt displacement of large volumes of water (including when an earthquake occurs at sea.)
  • The effect of Tsunami would occur only if the epicentre of the tremor is below oceanic waters, and the magnitude is sufficiently high.

Effects of Earthquakes

  • Ground Shaking
  • Differential ground settlement
  • Land and mudslides
  • Soil liquefaction
  • Ground lurching
  • Avalanches
  • Ground displacement
  • Floods from the dam and levee failures
  • Fires
  • Structural collapse
  • Falling objects
  • Tsunami

Global warming and Earthquakes

  • In recent research, geologists claim that global warming is one of the reasons for increased seismic activity.
  • Reason: Melting glaciers and rising sea levels disturb the balance of pressure on Earth’s tectonic plates which causes an increase in the frequency and intensity of earthquakes.
  • The world’s distribution of earthquakes coincides very closely with that of volcanoes.
  • Regions of greatest seismicity are Circum-Pacific areas, with the epicenters and the most frequent occurrences along the “Pacific Ring of Fire”. It’s said that as many as 70% of earthquakes occur in the Circum-Pacific belt.
  • Another 20% of the earthquakes take place in the Mediterranean-Himalayan belt including Asia Minor, the Himalayas and part of North-Western-China.