Assam Earthquake
Context
- Recently, a powerful earthquake measuring 6.4 on the Richter Scale jolted Assam and other parts of Northeast.
- According to the National Centre for Seismology (NCS) report, the tremors have been attributed to Kopili Fault zone closer to Himalayan Frontal Thrust (HFT).
Himalayan Frontal Thrust (HFT):
- HFT, also known as the Main Frontal Thrust (MFT), is a geological fault along the boundary of the Indian and Eurasian tectonic plates.
Kopili Fault Zone:
- The Kopili fault zone is a 300 km long and 50 km wide lineament (linear feature) extending from the western part of Manipur up to the tri-junction of Bhutan, Arunachal Pradesh and Assam.
- The area is seismically very active falling in the highest Seismic Hazard zone V associated with collisional tectonics where Indian plate subducts beneath the Eurasian Plate.
- Subduction is a geological process in which one crustal plate is forced below the edge of another.
- Squeezed between the subduction and collision zones of the Himalayan belt and Sumatran belt, the North East is highly prone to earthquake occurrences.
Back to Basics
Earthquake
- An earthquake in simple words is the shaking of the earth. It is a natural event. It is caused due to release of energy, which generates waves that travel in all directions.
- The vibrations called seismic waves are generated from earthquakes that travel through the Earth and are recorded on instruments called seismographs.
- The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.
Seismic Hazard Map of India
- India is one of the highly earthquake affected countries because of the presence of technically active young fold mountains – Himalaya.
- India has been divided into four seismic zones (II, III, IV, and V) based on scientific inputs relating to seismicity, earthquakes occurred in the past and tectonic setup of the region.
- Previously, earthquake zones were divided into five zones with respect to the severity of the earthquakes but the Bureau of Indian Standards (BIS) grouped the country into four seismic zones by unifying the first two zones.
- BIS is the official agency for publishing the seismic hazard maps and codes.
Measuring earthquakes
- Seismometers are the instruments which are used to measure the motion of the ground, which including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources.
- A Seismograph is also another term used to mean seismometer though it is more applicable to the older instruments.
- The recorded graphical output from a seismometer/seismograph is called as a seismogram. (Note: Do not confuse seismograph with seismogram. Seismograph is an instrument while seismogram is the recorded output)
- There are two main scales used in the seismometers: (1) Mercalli Scale and Richter Scale.
Mercalli Scale
- The scale represents the intensity of earthquake by analyzing the after effects like how many people felt it, how much destruction occurred etc. The range of intensity is from 1-12.
Richter Scale
- The scale represents the magnitude of the earthquake. The magnitude is expressed in absolute numbers from 1-10. Each whole number increase in Richter scale represents a ten times increase in power of an earthquake.