- 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.