SEISMIC HAZARD AND SEISMIC SLOPE STABILITY ASSESSMENT OF DARJEELING SIKKIM HIMALAYAS, INDIA

Abstract

The Himalayan region, spanning 2,500 kilometers in northern India, is highly prone to seismic activity. Situated in seismic zones IV and V, this region experiences frequent and devastating earthquakes, responsible for 70% of the world's fatal landslides. Factors such as steep slopes, heavy rainfall, uneven topography, geological conditions, climate, and unplanned urbanization exacerbate the susceptibility of the Himalayan landscape to landslides during earthquakes. The ongoing collision between the Indian and Eurasian plates generates faults and stress, making the region a high-risk area for future earthquakes. Khattri's 1999 research suggests a 56% probability of a magnitude 8.5 or greater earthquake occurring in the Himalayan seismic gap within the next century. Therefore, earthquake- induced landslides are a significant concern, necessitating enhanced preparedness to mitigate social and economic setbacks. To assess slope stability under seismic conditions, researchers employ deterministic, probabilistic, and statistical techniques. While predicting earthquakes with absolute certainty is impossible, seismic hazard studies estimate expected ground motion levels. These studies play a vital role in identifying ground shaking intensities that can trigger slope failures, quantifying hazards associated with specific locations. Integrated seismic hazard assessments, considering slope properties, enable the evaluation of likely ground motion scenarios. Therefore, conducting comprehensive and up-to-date seismic hazard studies is crucial to assess future landslide hazards in the earthquake-prone Himalayan region. Several seismic hazard analyses have been conducted in the Himalayan region, resulting in the development of peak ground acceleration for specific locations. However, challenges exist in using these ground intensities to accurately predict seismic vulnerability. Outdated or macro-level hazard maps, infrequent updates to earthquake databases, lack of expanded prediction equations, generalized GIS databases, data uncertainties, and stochastic-based earthquake catalogs contribute to these challenges. Depth ranges and maximum magnitude evaluations are crucial in seismic hazard assessments. Previous studies used standard or average depth ranges, but this study incorporated appropriate focal depths for point and linear sources. It also utilized a probabilistic approach called Regional Rupture Character (RRC) to estimate maximum magnitudes, setting it apart from studies using different ground motion prediction equations (GMPEs). Furthermore, the study introduced a fully probabilistic technique, called fully probabilistic seismic hazard assessment (FPSHA), to assess ground motion triggering landslides, a novel approach for the region. This study conducted seismic hazard analyses for the Darjeeling Sikkim Himalayan region using three frameworks: deterministic seismic hazard analysis (DSHA), probabilistic seismic hazard analysis (PSHA), and fully probabilistic seismic hazard analysis (FPSHA). DSHA emphasized seismic sources as the primary threat but did not consider uncertainties in the earthquake database and GMPE, potentially impacting results. While deterministic hazard maps evaluate intolerable failure consequences, they lack probabilistic information. PSHA, considering uncertainties in the earthquake database, provides estimates of ground motion exceedance over a specific time period. PSHA ground motions were significantly lower than those from DSHA, possibly due to inclusion of uncertainties. However, PSHA ground motions varied compared to FPSHA, which integrated PSHA with a dynamic slope stability model considering slope properties. FPSHA assessed the most probable ground motion scenarios for landslide triggering over the next 50 years for all slope models. Significant differences in ground motion levels were observed between FPSHA and both DSHA and PSHA, attributed to uncertainties in slope models, GMPE, and seismic source models. It is apparent from this research that seismic landslide hazards can be overestimated or underestimated when relying solely on DSHA and PSHA approaches. While PSHA provides probabilistic ground motions based on historical earthquakes, it is recommended for general seismic infrastructure design. FPSHA, on the other hand, estimates ground motions based on earthquake statistics and soil properties, offering suitable design ground motions for landslide triggering conditions. By considering all possible earthquake scenarios leading to slope instability, FPSHA accounts for specific conditions that can trigger landslides. The updated hazard maps and design charts developed in this study have various applications. They can be utilized in seismic infrastructure design, hazard zoning mapping, landslide monitoring, seismic slope stability analysis, land use planning, code requirements, and implementation of mitigation measures. These outcomes play a crucial role in pre-disaster prevention by facilitating earthquake-resistant design and enhancing post-disaster rescue preparedness. Furthermore, the findings can be employed to develop region-specific ground motion attenuation relationships, synthesize ground motions, and inform various engineering applications.