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.
How do earthquakes happen?
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?
What are seismic waves?
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.
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. 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.
What is the Richter magnitude scale?
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.
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. The problem with earthquakes is that they are heavily dependent on the material property, which varies from place to place, says Professor Rai.
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, and we have efficient computer systems, all the inputs and excellent data collection, 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. “This has worked in some locations, but not on a large commercial basis,” says Prof. Rai.
“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,” he points out.
