Recent news has been focused on the scientific discovery of the first geological evidence of an earthquake at the border of Assam and Arunachal Pradesh. Known historically as the Sadiya earthquake, this finding is expected to contribute to the seismic map of the eastern Himalaya. This map will aid in regional construction and planning, particularly in the vicinity of the Tuting-Tidding Suture Zone – a major part of the Eastern Himalaya where the Himalayas take a sharp southern turn and link with the Indo-Burma Range.
Key Findings from the Wadia Institute of Himalayan Geology
Scientists from the Wadia Institute of Himalayan Geology (WIHG), an autonomous institution under the Department of Science and Technology (DST), Government of India, led a mega trench excavation in Himebasti village, Arunachal Pradesh. The most recent event recorded the imprints of the 1697 Sadiya earthquake which was then analyzed using modern geological techniques. Large tree trunks found embedded in the youngest flood deposits at the Subansiri River’s exit implied post-seismic aggradation of the river, likely as a result of an array of aftershocks lasting for about six months.
The Role of Past Earthquakes in Determining Future Seismic Activities
Studying past earthquakes helps in determining the seismic potential of a region. This assists in mapping regional earthquake hazards and synchronizing developmental activities accordingly. For instance, Arunachal Pradesh, being near the Indo-China Border, is strategically important and at times becomes the center of ownership disputes. A host of developmental activities like road, bridge, and hydropower projects are underway, highlighting the urgent need for understanding the seismic patterns in this region.
India’s Seismic Hazard Map
India figures among the countries most affected by earthquakes, largely due to the presence of tectonically active young fold mountains, the Himalayas. The country has been divided into four seismic zones (II, III, IV, and V) based on scientific inputs relating to seismicity, past earthquakes, and the tectonic setup of the region. Previously there were five zones, but the Bureau of Indian Standards (BIS) – the official agency for publishing the seismic hazard maps and codes – unified the first two zones.
Understanding the Seismic Zones
Each seismic zone arrives with a different level of risk and potential damage. Seismic Zone II is associated with minor damage earthquakes, while Zones III, IV, and V are known for moderate to major damages. Zone V is the seismically most active region where some of the country’s most powerful shocks have occurred.
Seismic Waves, the Richter Scale, and the Mercalli scale
Seismic waves – vibrations from earthquakes that travel through the Earth – are recorded on instruments called seismographs. These record a zigzag trace reflecting the varying amplitude of ground oscillations beneath the instrument. The magnitude and intensity of earthquake events are either scaled according to the Richter or Mercalli scales. While the Richter scale expresses the energy released during an earthquake in absolute numbers, the Mercalli scale takes into account the visible damage caused by the event.
