Guwahati: A major new seismic study has produced the most comprehensive earthquake catalogue ever assembled for the Shillong Plateau, offering unprecedented insight into one of the world’s most active seismic regions and sharpening the scientific basis for earthquake risk planning across Northeast India.
Authored by Mohd Shahabuddin and Prof. William Kumar Mohanty of IIT Kharagpur, the research—published in Seismological Research Letters (2025)—compiles 6,877 earthquakes of magnitude 3.5 and above spanning nearly 1,200 years, from 825 CE to 2024, making it the longest and most rigorously standardised seismic record for the region to date.
The Shillong Plateau, covering much of Meghalaya and adjoining areas, sits at the complex junction of the Indian–Eurasian plate to the north and the Indian–Burmese plate to the east—a tectonic collision that began around 60 million years ago and continues today. This geological setting has made the region capable of generating large to great earthquakes, including the 1897 Shillong earthquake (Mw 8.1) and the 1950 Assam earthquake (Mw 8.5), which together caused more than 3,000 fatalities and widespread infrastructure damage.
The plateau is bounded by major fault systems—the Dauki Fault to the south, Dhubri Fault to the west, Dhansiri–Kopili Fault to the northeast, and Oldham Fault to the east—placing it entirely within Seismic Zone V, India’s highest-risk category, with expected peak ground acceleration exceeding 0.4g.
Previous earthquake datasets for Northeast India were fragmented, inconsistent, and often limited to the instrumental era. In contrast, this study began with 19,499 reported seismic events gathered from national and international databases and historical literature. After removing duplicates and poorly constrained records, the researchers created a homogeneous, unified catalogue of 6,877 earthquakes.
To achieve this, the team developed 30 new magnitude conversion relationships, bringing earthquakes originally reported in different scales—such as mb, Ms, ML, and Md—onto a single, globally consistent moment magnitude (Mw) scale.
This step corrects long-standing underestimation problems caused by magnitude saturation in older measurement systems.
The catalogue was then declustered—a crucial step for hazard analysis—by removing dependent events such as foreshocks and aftershocks. Using the widely accepted Gardner–Knopoff method, the researchers isolated 4,134 independent mainshocks, enabling clearer identification of long-term seismic patterns.
One of the study’s most significant findings relates to catalogue completeness—how far back earthquake records can be trusted for hazard analysis. The authors show that:
Large earthquakes (Mw ≥ 6.5) now have reliable records extending back over 200 years.
Moderate earthquakes (Mw 5.5–6.0) are consistently recorded for about 100 years.
Smaller but damaging events (Mw 4.5–5.0) show the highest annual activity rates, underscoring frequent stress release in the region.
This level of completeness marks a major improvement over earlier datasets, which often underestimated both the frequency and spatial distribution of seismic events.
Scientists say the updated catalogue provides a critical foundation for modern seismic hazard mapping, probability modelling, and infrastructure risk assessment—especially as Meghalaya and neighbouring states expand roads, urban centres, dams, and hydropower projects in rugged terrain.
“The unified dataset is essential for understanding regional seismicity and for developing realistic earthquake hazard scenarios,” the authors note, adding that the catalogue will directly support earthquake forecasting models and disaster preparedness planning across the Shillong Plateau and adjoining regions.
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