Explained | How are earthquakes measured?

What is at the heart of earthquakes? Can they be predicted? Is it possible to set up early warning systems? 

The story so far: Recently a powerful earthquake of magnitude 5.9 on the Richter scale struck a remote town in Afghanistan, killing over a thousand and injuring many more. According to the U.S. Federal Emergency Management Agency, a 5.9 on the Richter scale is roughly equivalent to 37 times the energy released by the atomic bomb dropped on Hiroshima. Experts are still trying to figure out the best early warning system to mitigate the damage caused by earthquakes.

According to the theory of plate tectonics, the 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. The point inside the Earth where the earthquake rupture starts is called the focus or hypocentre. The point directly above it on the surface of the Earth is the epicentre.

The Hindu Science Quiz | How much do you know about earthquakes?

Any elastic material when subjected to stress, stretches in a proportional way, until the elastic limit is reached. When the elastic limit is crossed, it breaks. Similarly, the Earth also 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. These are known as seismic waves.

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. It has a history of about 120 years and its sensors can now detect an earthquake within five to ten minutes.

According to Shyam S. Rai who is a Raja Ramanna Fellow and Professor Emeritus at the Indian Institute of Science Education and Research, Pune, the wave parameters are measured, not the total energy released. He explains that there is a relationship between the quantum of energy released and the wave amplitude. The 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.

This is a measure of the magnitude of an earthquake and was first defined by Charles F. Richter of the California Institute of Technology, U.S., in 1935. The magnitude of an earthquake is the logarithm of the amplitude of the waves measured by the seismographs. Richter scale magnitudes are expressed as a whole number and a decimal part, for example 6.3 or 5.2. 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.