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.
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.
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.
Why Earthquake in Afghanistan?
- Afghanistan is earthquake-prone because it’s located in the mountainous Hindu Kush region, which is part of the Alpide belt — the second most seismically active region in the world after the Pacific Ring of Fire.
- The Alpide belt runs about 15,000 kilometers, from the southern part of Eurasia through the Himalayas and into the Atlantic.
- Along with the Hindu Kush, it includes a number of mountain ranges, such as the Alps, Atlas Mountains and the Caucasus Mountains.
- Additionally, the Earth’s crust is especially lively in Afghanistan because it is where the Arabian, Indian and Eurasian tectonic plates meet.
- The boundary between the Indian and Eurasian plates exists near Afghanistan’s border with Pakistan.
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.
Issues with Earthquake measurement
- Everywhere, the wave parameters are measured, not the total energy released.
- There is a direct 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 we predict 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.
- 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.
- 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.
Successful attempts made so far
- 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.
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