Context: Recently, scientists have found the first geological evidence of an earthquake on the border of Assam and Arunachal Pradesh, documented as the Sadiya earthquake in history. This finding could contribute to a seismic hazard map of the eastern Himalaya, which can facilitate construction and planning in the region.

• This site is near the Tuting-Tidding Suture Zone- a major part of the Eastern Himalaya, where the Himalaya takes a sharp southward bend and connects with the Indo-Burma Range.

Key Points

• Scientists from Wadia Institute of Himalayan Geology (WIHG) carried out a mega trench excavation at Himebasti village, Arunachal Pradesh where the most recent event records the imprints of the 1697 Sadiya earthquake and analysed it with modern geological techniques.
• WIHG is an autonomous institute under the Department of Science and Technology (DST), Government of India.
• Scientists found large tree trunks embedded in the youngest flood deposits at the exit of the Subansiri River (Sadiya town is located roughly 145 km southeast of Subansiri river), suggesting the post-seismic aggradation of the river following an array of aftershocks till six months.
• Aggradation is the term used in geology for the increase in land elevation due to deposition of sediment.
• Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. They are smaller than the mainshock and within 1-2 rupture lengths distance from the mainshock.

Significanceof the Study

• Studying the past earthquakes help to determine the seismic potential of the region. This helps in mapping the earthquake hazard of the region and enables syncing the development activities accordingly.
• Arunachal Pradesh, being near the Indo-China Border, and sometimes centre of dispute regarding its ownership, is strategically important.
• A number of development initiatives like constructing roads, bridges and hydropower projects are being undertaken, therefore there is an urgent need for understanding the pattern of seismicity in this region.

Seismic Hazard Map of India:

• India is one of the highly earthquake affected countries because of the presence of tectonically active young fold mountains, Himalayas.
• 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.

Note: BIS is the official agency for publishing the seismic hazard maps and codes.

Measurement: The earthquake events are scaled either according to the magnitude or intensity of the shock.

• The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the earthquake which is expressed in absolute numbers, 0-10.
• The intensity scale or Mercalli scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.

About National Center for Seismology (NCS) 

• It is the nodal agency of the Government of India for monitoring earthquake activity in the country.
• NCS maintains National Seismological Network of 115 stations each having state-of-the-art equipment and spreading all across the country.
• NCS monitors earthquake activity all across the country through its 24×7 round the clock monitoring centre.
• NCS also monitors earthquake swarm and aftershock through deploying temporary observatory close to the affected region.
• Apart from earthquake monitoring, NCS is also actively involved in Seismic Hazard Microzonation and seismological research.
• NCS functions under the Ministry of Earth Sciences, GOI.

Background

• The history of instrumental earthquake monitoring in India dates back to 1898, when the first seismological observatory of the country was established at Alipore  (Calcutta) on 1 December, 1898 after the great  Shillong plateau earthquake of
• The whole country has been divided into four zones viz. zone V, IV, III and II according to the seismic zoning map of India prepared by Bureau of Indian Standards (BIS) based on the historical seismicity and strong ground motions.

Out of these zones, Zone V exhibits the highest seismic risk and zone II has the least.

• Zone II: Remaining parts of Rajasthan, Uttar Pradesh, Gujarat, Haryana, Madhya Pradesh, Maharashtra, Odisha, Andhra Pradesh, Telangana, Karnataka and Tamil Nadu.
• Zone III: Kerala; Goa; Lakshadweep islands; parts of Uttar Pradesh and Haryana; remaining parts of Gujarat and Punjab; parts of West Bengal, western Rajasthan, Madhya Pradesh; remaining part of Bihar; northern parts of Jharkhand and Chhattisgarh; parts of Maharashtra, Odisha, Andhra Pradesh, Telangana, Tamil Nadu and Karnataka.
• Zone IV: Ladakh; Remaining parts of Jammu & Kashmir, Himachal Pradesh and Uttarakhand; Some parts of Haryana, Parts of Punjab; Delhi; Sikkim; northern part of Uttar Pradesh; small portions of Bihar and West Bengal; parts of Gujarat and small portions of Maharashtra near the west coast and small part of western Rajasthan.
• Zone V: Parts of Jammu and Kashmir (Kashmir valley); Western part of Himachal Pradesh; Eastern part of Uttarakhand, Kutch in Gujarat; part of Northern Bihar; all northeastern states of India and Andaman & Nicobar Islands.