Seismic Hazard Assessment and Source Zone Delineation in Northeast India: A Case Study of the Kopili Fault Region and Its Vicinity

The present study aims to delineate potential seismic zones within a 300 km radius centered at Tezpur in the Kopili fault region, recognizing that the region has experienced numerous devastating earthquakes due to its complex tectonic features. The high seismicity rate in the area results from these complex geological conditions. The study area, characterized by its complex tectonic structure and varied seismic movements, has been divided into four distinct seismic zones, namely (1) the Northeast Himalayan seismic zone (NEHSZ), (2) the Shillong-Assam seismic zone (SASZ), (3) the Bengal subsurface seismic zone (BSSZ), and (4) the Indo-Burma seismic thrust zone (IBSTZ). To evaluate the potential seismic risk attributes for each seismic source zone, the study utilized an exhaustive and consistent earthquake catalog specific to each zone. These attributes encompass factors such as the maximum earthquake magnitude (M-max), return period, mean annual activity rate, and likelihood of exceeding. The study determined the b-values for all identified seismic zones using the maximum likelihood technique. The calculated b-values are as follows: 0.83 +/- 0.04 for NEHSZ, 0.91 +/- 0.04 for SASZ, 0.81 +/- 0.07 for BSSZ, and 0.94 +/- 0.03 for IBSTZ. The highest rate of earthquake occurrence is observed in the IBSTZ. Similarly, the study estimated the maximum credible earthquake (M-max) for each zone and found that the SASZ has the largest M-max, with a magnitude of M-W 8.6 (+/- 0.52). This finding is supported by historical data from the 1897 Shillong earthquake, which had a magnitude of M-W 8.1, serving as evidence of the region's seismic potential. To estimate the most likely annual largest earthquake (H), the study employed the Gumbel extreme value method (EVT), resulting in values of 5.3, 5.4, 4.7, and 5.5 for NEHSZ, SASZ, BSSZ, and IBSTZ, respectively. The study also estimated the probability of earthquake occurrences within the magnitude range M-W 4.0-8.5 in the next 100 years for the four seismic zones. The implications drawn from this study provide valuable insights into earthquake risks and their potential impacts, facilitating effective mitigation efforts, as well as prudent preparedness and response measures. These outcomes are envisioned to contribute to the overarching goal of reducing loss of life and property associated with seismic events while ultimately enhancing the overall resilience of the region.