Assam Earthquake

What is an Earthquake?

• An earthquake is a natural event which 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 epicentre.

Earthquake zones of India

• The major reason for the high frequency and intensity of the earthquakes is that the Indian plate is driving into Asia at a rate of approximately 47 mm/year.
• Geographical statistics of India show that more than 50% of the land is vulnerable to earthquakes.
• The latest version of the seismic zoning map of India divides India into 4 seismic zones (Zone 2, 3, 4 and 5).

Zones of Seismicity

• Zone 1: Currently the Division does not include a Zone 1. NO area of India is classed as Zone 1.
• Zone 2: This region is liable to MSK VI or less and is classified as the Low Damage Risk Zone.
• Zone 3: This zone is classified as a Moderate Damage Risk Zone which is liable to MSK VII.
• Zone 4: This zone is called the High Damage Risk Zone and covers areas liable to MSK VIII. Jammu and Kashmir, Ladakh, Himachal Pradesh, Uttarakhand, Sikkim, the parts of Indo-Gangetic plains (North Punjab, Chandigarh, Western Uttar Pradesh, Terai, North Bengal, Sundarbans) and the capital of the country Delhi fall in Zone 4.
• Zone 5: Zone 5 covers the areas with the highest risks zone that suffers earthquakes of intensity MSK IX or greater. The region of Kashmir, the Western and Central Himalayas, North and Middle Bihar, the North-East Indian region, the Rann of Kutch and the Andaman and Nicobar group of islands fall in this zone.

Himalayan Frontal Thrust (HFT)

• The Main Frontal Thrust (MFT), also known as the Himalayan Frontal Thrust (HFT) is a geological fault in the Himalayas that defines the boundary between the Indian and Eurasian Plates.
• The fault is well expressed on the surface and thus could be seen via satellite imagery.
• It is the youngest and southernmost thrust structure in the Himalayas deformation front. It is a splay branch of the Main Himalayan Thrust (MHT) as the root décollement.
• It runs parallel to other major splays of the MHT; Main Boundary Thrust (MBT) and Main Central Thrust (MCT).
• The MFT accommodates almost the entire rate of subduction of the Indian Plate therefore, it is no surprise that numerous earthquakes have occurred along this fault, and is expected to produce very big earthquakes in the future.
• Many earthquakes associated with the MFT have resulted in visible ground ruptures, as seen in the Bihar earthquake of 1934 and the 1505 magnitude 8.9 earthquake.

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 the 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.

Reasons for Assam Earthquake

• According to the National Centre for Seismology (NCS) report, the tremors have been attributed to the Kopili Fault zone closer to Himalayan Frontal Thrust (HFT).
• NCS is the nodal agency of the Government of India for monitoring earthquake activity in the country. It comes under the Ministry of Earth Sciences.
• An earthquake of magnitude 6.4 on the Richter scale hit Assam.
• The primary earthquake had its epicentre at latitude 26.690 N and longitude 92.360 E, about 80 km northeast of Guwahati, and a focal depth of 17 km.
• The events are located near to Kopili Fault closer to Himalayan Frontal Thrust (HFT).
• The Kopili Fault is a 300-km northwest-southeast trending fault from the Bhutan Himalaya to the Burmese arc.
• The fault is a fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture.
• The area is seismically very active falling in the highest Seismic Hazard zone V associated with collisional tectonics where the Indian plate sub-ducts beneath the Eurasian Plate the NCS report said.
• The area is seismically very active falling in the highest Seismic Hazard zone V associated with collisional tectonics where the Indian plate sub-ducts beneath the Eurasian Plate.
• When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth.

What is the news?

• An unfamiliar lineament is among four factors behind frequent earthquakes in northern Assam’s Sonitpur area.
• A lineament is a linear feature in a landscape dictated by an underlying geological structure such as a fault.
• According to the Geological Survey of India (GSI), Sonitpur district lies within a tectonically complex triangular area bounded by the east-west trending Atherkhet Fault, the northwest-southeast trending Kopili Fault and a north-south trending lineament.
• The two faults and the lineament, along with the oblique convergence of the Indian plate, have caused frequent earthquakes.
• The National Centre of Seismology recorded 29 earthquakes of magnitude varying from 2.6 to 4.7 in Sonitpur after the 6.4 tremblor on April 28 that damaged several buildings, bridges and a river embankment.
• The last of these 29 earthquakes with Sonitpur as the epicentre was recorded early Wednesday morning. Five more of magnitudes 2.6-3.2 were also recorded in neighbouring districts during this period.
• Sonitpur was the epicentre of this huge 6.4 earthquake after 33 years because of the tectonic complexity. The Atherkhet and Kopili faults, the north-south lineament and the oblique convergence of the Indian plate is causing repetitive earthquakes.
• Both the Atherkhet and Kopilli are active but we do not know about the nature of the lineament involved.
• Atherkhet and Kopili are not the only faults that impact the Sonitpur region. The Siang Fracture, Yemla Fault, Namula Thrust and Canyon Thrust are spread across the northeast and are active along with Main Himalayan Thrust, Main Boundary Thrust, Main Central Thrust and several subsidiary faults.
• The northeast is demarcated as Seismic Zone V, which indicates a zone with high vulnerability. The Indian plate is moving northeast toward the Eurasian plate in the Himalayan region, their oblique collision and release of stress and strain accumulated in the local tectonic or fault environments lead to earthquakes.
• He also explained why water was oozing out of the fault in Sonitpur and adjoining affected areas after the April 28 earthquake.
• This was a result of severe liquefaction [the process of making something liquid] from the unconsolidated substratum [underlying layer of soil/rock].
• The GSI specialist advised people not to heed earthquake predictions.
• Unlike a landslip, a surficial process that can be constrained, an earthquake is a process deep inside the earth. But scientists worldwide are trying for a breakthrough. The work to understand the strain localisation of future earthquakes is under progress.