Dibang, Shillong, and Guwahati face the highest risk of strong earthquake shaking in Northeast India, a new study found. Researchers used a new scale to estimate ground motion. Ranjit Das said, "The level of earthquake risk is not determined by the strength of an earthquake alone. Soil and rock matter."

A new study finds the three places among Northeast India's areas most exposed to strong earthquake shaking, highlighting soil and seismic risks.

Guwahati, Oct 7: Dibang in Arunachal Pradesh, Shillong and Guwahati are among the areas of Northeast India most exposed to strong earthquake shaking, according to a new study.

Imphal, Itanagar, Agartala and Kohima show moderate hazard, while Aizawl shows comparatively lower hazard.

The study team was led by Ranjit Das of Universidad Católica del Norte, Chile, a former UNESCO expert in earthquake hazard modelling, and included researchers from the IIT-Bombay. The researchers used Probabilistic Seismic Hazard Analysis (PSHA), a widely used method for estimating earthquake hazard that also informs seismic building codes. It estimates how strongly the ground may shake during future earthquakes.

The study uses the Das Magnitude Scale instead of the conventional moment magnitude scale, which the researchers argue can introduce significant inaccuracies below magnitude 7.5. Using a unified catalogue of 9,968 earthquakes, the study found that the conventional earthquake scale produced ground-motion estimates up to 40 per cent higher than those obtained using the Das Magnitude Scale for the 475-year hazard level, particularly for medium-sized earthquakes.

A measure of the severity of earthquake shaking, known as peak ground acceleration, was highest near Dibang (0.908 g), followed by Shillong (0.893 g) and Guwahati (0.811 g). Itanagar, Imphal, Kohima and Agartala recorded values ranging from 0.61 g to 0.70 g, while Aizawl recorded 0.57 g.

"Our study found relatively lower estimated ground shaking along the Himalayan plate boundary segment between the 1987 and 1950 earthquake epicentres, partly because the available earthquake record is limited. Ground shaking approaching 1 g in this segment would not be surprising. The severe shaking during the 1950 Assam earthquake, associated with its shallow focal depth, shows that very strong ground motion is possible in this region," Das told The Assam Tribune.

"The level of earthquake risk is not determined by the strength of an earthquake alone. Soil and rock can greatly increase or reduce shaking," Das said.

Guwahati and Tezpur, located on soft alluvial deposits of the Brahmaputra Basin, face particular concerns because such soils can amplify earthquake shaking and may also be susceptible to liquefaction - a condition in which water-saturated soil temporarily loses its strength during strong shaking. Buildings in such areas may require appropriate foundation design and groundwater management, he said.

Shillong stands on hard Precambrian bedrock near active fault zones, making it particularly exposed to intense, high-frequency ground shaking. "Buildings in Shillong therefore need to be designed with sufficient structural rigidity to withstand such intense shaking, rather than relying primarily on measures intended to address soft-soil amplification," Das said.

Imphal, underlain by extensive fluvio-lacustrine and alluvial sediments, and Agartala, where young alluvial and fluvial deposits occur, may experience significant site and basin effects. Detailed seismic microzonation and site-specific structural design are therefore important in these areas.

Kohima and Itanagar face both strong shaking and earthquake-triggered landslide risk. Gangtok and Aizawl have stronger bedrock in many areas, and their steep terrain makes landslides a concern. Lighter structures and slope protection could help reduce the risk.

The researchers caution that PSHA depends on historical and instrumental earthquake records, which are limited for Northeast India. Large Himalayan earthquakes may recur over timescales of centuries, making extreme events difficult to estimate from the available record alone. The 2011 Tohoku earthquake in Japan also demonstrated the limitations of relying solely on historical records to anticipate exceptionally strong shaking.

The study highlights the importance of combining seismic records with other sources of information when assessing long-term earthquake hazard.

The study has been published in Nature Scientific Reports.