Measuring Earthquakes

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Measuring Earthquakes

  • A powerful earthquake of magnitude 5.9 on the Richter scale struck a remote town in Afghanistan.

How do earthquakes happen?

  • According to plate tectonics theory, Earth’s crust and upper mantle are made of large rigid plates that can move relative to one another.
  • Slip-on faults near the plate boundaries can result in earthquakes.
  • Hypocentre: Point inside Earth where the earthquake rupture starts is called the focus or hypocentre.
  • It is directly above it on the surface of the Earth is the epicentre.

What are seismic waves?

  • Earth has an elastic limit and when the stress is higher than this limit, it breaks.
  • Then there is a generation of heat, and energy is released.
  • Since the material is elastic, the energy is released in the form of elastic waves.
  • These propagate to a distance determined by the extent of the impact known as seismic waves.

How are earthquakes measured?

  • Earthquakes are measured by seismographic networks, which are made of seismic stations, each of which measures the shaking of the ground beneath it.
  • In India, the National Seismological Network does this work.
  • Its sensors can now detect an earthquake within five to ten minutes.
  • There is a relationship between quantum of energy released and wave amplitude.
  • Amplitude of the wave is a function of the time period of the wave.
  • It is possible to convert the measured wave amplitude into the energy released for that earthquake.
  • This is what seismologists call the magnitude of the earthquake.

What is the Richter magnitude scale?

  • This is a measure of the magnitude of an earthquake and was first defined by Charles F. Richter in 1935.
  • The magnitude of an earthquake is the logarithm of the amplitude of the waves measured by the seismographs.
  • Since it is a logarithmic scale, an increase of the whole number by one unit signifies a tenfold increase in the amplitude of the wave and a 31-times increase of the energy released.

How are zones designated?

  • Based on seismicity, intensity of earthquakes experienced, and geological and tectonic qualities of a region, countries are divided into several zones. In India, for example, there are four zones, designated Zone II-Zone V. Among these, Zone V is the most hazardous and Zone II the least hazardous.

Can you build early warning systems for earthquakes?

  • Since parameters of the earthquake are unknown, it is near impossible to predict an earthquake.
  • Earthquakes are heavily dependent on the material property, which varies from place to place.
  • If there are elastic waves propagating through a material, there are two kinds of waves — the primary wave which reaches first, and the second one called the secondary wave, which is more destructive.
  • Suppose the primary wave is measured, then it can be said that a possible earthquake of this much magnitude and energy has occurred and this could lead to a ground amplitude which could be destructive.
  • If it is known that the amount of energy released is extremely high, trains and power grids can be shut down and the damage minimised.
  • The most successful early warning systems are in Japan.
  • They have several hundreds of thousands recording devices.
  • Responses are sent to a central point where they estimate whether it is large enough to form a tsunami or some other hazard, and precautionary steps are taken.