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.