SEISMIC HAZARD AND RISK ASSESSMENT OF NORTHEAST INDIAN REGION-A PROBABILISTIC APPROACH

Abstract

Earthquakes are deadly natural disaster that have affected many people in recent decades. Efforts to reduce their impact have become increasingly focused in developed and developing countries. Despite advances in scientific knowledge, accurately predicting earthquakes remains challenging. However, seismic hazard analysis can be used to forecast future ground shaking for specific or multiple locations. The northeast India region is highly seismically active and contains several distinct tectonic features. Despite improvements in building codes and preparedness, mortality rates are still alarming, especially in areas with vulnerable building construction practices. Conducting seismic hazard and risk assessments is crucial in predicting potential losses and assisting local authorities in formulating effective disaster management plans. Previous seismic hazard studies in the region have mainly focused on areal seismic sources, the dimensions and boundaries of which generally remain subject to personal experience and judgment. For this purpose, the study aims to evaluate seismic hazard in northeast India using a smoothed gridded seismicity model, areal source and fault zone models, and estimate seismic risk in Mizoram, a region with limited attention in previous studies. The primary objectives of this study are to conduct a Probabilistic Seismic Hazard Assessment (PSHA) of northeast India and estimate seismic risk in Mizoram. The study is divided into three major parts to achieve the objectives and presented as contributing chapters in the thesis. The first part is to conduct a PSHA of northeast India considering three seismic source models, i.e., areal sources, fault zone sources, and a smoothed gridded seismicity model. The study area covers the region between 87°–98° E longitude and 20°–30° N latitude. The region has been divided into six areal source zones; twenty-eight identified fault sources have been modeled as fault zone source models, while the gridded seismicity model has been modeled as a point source model. Seismicity parameters have been calculated for each source model using a complete (time and size) earthquake catalog treated for homogenization and declustering. Five Ground Motion Prediction equations (GMPEs) for the active shallow region and three GMPEs for the Indo-Burma subduction zone have been used to evaluate seismic hazards. Hazard assessment has been conducted at the reference rock condition (Vs30 = 760 m/s). A logic tree framework has been implemented in source models and GMPEs to account for uncertainties. Peak Ground Acceleration (PGA) and Spectral Acceleration (SA) have been estimated for various cities in the northeastern states, considering a 2% and 10% Probability of Exceedance (PoE) in 50 years. Hazard curves and uniform hazard spectra (UHS) have also been generated. The seismic hazard results have been used to develop disaggregation plots to quantify the contributions of seismic hazard in terms of magnitude and distance combination. The seismic hazard maps generated from the study indicate a range of PGA values varying from 0.11g to 0.43g for a 10% PoE in 50 years. This shows spatial variability in ground shaking intensity across different locations within the study area. Some areas have lower PGA values, while others have higher PGA values compared to the estimates provided by the existing building code. The comparison of the estimated hazard values with other studies showed some variations, which can be attributed to differences in earthquake catalogs, seismic activity input parameters, and source models used. These factors are crucial in influencing hazard estimation. Imphal stands out with the highest PGA level of 0.40g at a 10% PoE among the UHS developed for the seven cities. This is due to the hazard contribution from two specific seismic sources: the Churachanpur Mao Fault and the eastern subduction zone. The highest hazard levels are observed in eastern Manipur, Mizoram, southern Nagaland, and western Arunachal Pradesh, close to the Indo-Myanmar region and Himalayan thrust.