New Study Reveals Earthquake Risks in Northeast India
Earthquake Hazard Assessment in Northeast India
Representational image
Guwahati, Oct 7: A recent study has identified Dibang in Arunachal Pradesh, along with Shillong and Guwahati, as regions in Northeast India that are particularly vulnerable to strong earthquake activity.
Areas such as Imphal, Itanagar, Agartala, and Kohima are categorized as having moderate earthquake hazards, while Aizawl is considered to have a lower risk.
The research was spearheaded by Ranjit Das from Universidad Católica del Norte in Chile, a former expert in earthquake hazard modeling for UNESCO, and included contributions from IIT-Bombay researchers. They employed Probabilistic Seismic Hazard Analysis (PSHA), a standard technique for estimating earthquake risks that also aids in developing seismic building codes. This method predicts the potential ground shaking during future seismic events.
Instead of the traditional moment magnitude scale, the study utilized the Das Magnitude Scale, which the researchers claim reduces inaccuracies for earthquakes below magnitude 7.5. By analyzing a comprehensive catalogue of 9,968 earthquakes, they discovered that the conventional scale often overestimated ground motion by up to 40% at the 475-year hazard level, especially for medium-sized quakes.
The peak ground acceleration, which measures the intensity of earthquake shaking, was found to be highest near Dibang (0.908 g), followed closely by Shillong (0.893 g) and Guwahati (0.811 g). Other cities like Itanagar, Imphal, Kohima, and Agartala recorded values between 0.61 g and 0.70 g, while Aizawl had a lower reading of 0.57 g.
Das noted, “Our findings indicate a lower estimated ground shaking along the Himalayan plate boundary segment between the 1987 and 1950 earthquake epicenters, likely due to limited earthquake records. However, ground shaking nearing 1 g in this area is plausible. The severe shaking from the 1950 Assam earthquake, linked to its shallow focal depth, illustrates the potential for very strong ground motion in this region.”
He further explained that earthquake risk is influenced not just by the earthquake's strength but also by the geological conditions, as soil and rock types can significantly alter shaking intensity.
Guwahati and Tezpur, situated on soft alluvial deposits of the Brahmaputra Basin, are particularly at risk since such soils can amplify shaking and may be prone to liquefaction, where water-saturated soil loses strength during intense shaking. Proper foundation design and groundwater management are essential for buildings in these areas.
Shillong, built on hard Precambrian bedrock near active fault lines, is especially susceptible to severe, high-frequency shaking. “Structures in Shillong must be designed with adequate rigidity to endure such intense shaking, rather than relying solely on measures for soft-soil amplification,” Das advised.
Imphal, which has extensive fluvio-lacustrine and alluvial sediments, along with Agartala, where young alluvial deposits are found, may experience significant site effects. Therefore, detailed seismic microzonation and tailored structural designs are crucial in these locations.
Kohima and Itanagar are at risk from both strong shaking and landslides triggered by earthquakes. Gangtok and Aizawl, with their stronger bedrock and steep terrains, also face landslide threats. Implementing lighter structures and slope protection measures could mitigate these risks.
The researchers warned that PSHA relies on historical and instrumental earthquake data, which is limited for Northeast India. Major Himalayan earthquakes may occur over centuries, making it challenging to predict extreme events based solely on available records. The 2011 Tohoku earthquake in Japan highlighted the limitations of relying only on historical data for anticipating exceptionally strong shaking.
This study underscores the necessity of integrating seismic records with other information sources to accurately assess long-term earthquake hazards.
The findings have been published in Nature Scientific Reports.