G N Shiny Neharika received her doctorate in Civil Engineering. Her research work was supervised by Prof. K S Rajan. Here’s a summary of her research work Seismic Hazard and Seismic Slope Stability Assessment of Darjeeling Sikkim Himalayas, India.
Earthquakes and landslides are sudden destructive natural disasters that constitute a significant threat to humankind on a global scale, causing a tremendous death toll and claiming thousands of lives each year. The 2500km long and youngest mountainous Himalayan arc region, located in the north of India, is in seismic zones IV and V and is one of the world’s most seismically active regions, with several destructive and highly vulnerable earthquakes and 70% of global fatal deadly landslides. The earthquakes in this region are related to the uplift of mountains along with active faults due to the ongoing collision between the Indian and Eurasian plates. Among many losses during earthquakes in the beautiful Himalayan landscape, the earthquakes triggered landslides which pay a high degree of susceptibility. The slides are further intensified by steep slopes, heavy rains, uneven topography, geology, climatic conditions, and unplanned urbanization. The landslides observed during an earthquake may become unstable or reactivated due to future earthquakes, infrastructure development, and climatic change. According to Khattri (1999), there is a 56% probability that an earthquake with a magnitude greater than 8.5 would strike in the Himalayan seismic gap during the next 100 years. The movement between the plates in the Himalayan region causes faults and stresses in rocks, where future earthquakes are likely to occur. Accordingly, earthquake-induced landslides have become an important problem in the Himalayan region. Hence, the region needs better preparedness against earthquake-induced landslides to reduce social and economic setbacks in the area.
Researchers use numerous approaches to evaluate seismic slide stability under deterministic, probabilistic, and statistical techniques. In most practices, uncertainty in selecting design ground motion remains a subjective matter. Although reliable methods for predicting earthquake occurrence are unavailable, the level of earthquake-induced ground motion at the site can be estimated using seismic hazard studies. Among the challenges in predicting ground intensity, the seismic hazard assessment (SHA) is considered one of the practical solutions that cope with the random and complicated earthquake process. The ground motions obtained from SHA help to provide the specific triggering condition of slope failure in terms of ground shaking intensity. Therefore, seismic hazard studies will serve as the first and fundamental step for quantifying the hazards associated with the selected location. Further, few researchers developed combined seismic hazard assessment to slope properties to evaluate the most probable ground motion scenario for seismically induced landslides. Therefore, advanced and updated seismic hazard studies yield the most important outcomes for future landslide hazard assessment in the earthquake-prone Himalayan region.
Many researchers have carried out SHA studies based on probabilistic (PSHA) and deterministic (DSHA) frameworks for the Himalayan region and developed PGA hazard maps, earthquake catalogs, etc., but from past literature and practical experiences, it was evident that using these ground intensities are either underestimated/overestimated/not reliable in predicting exact seismic vulnerability in a region. Because the developed hazard maps are outdated or at macro-level solutions/global scale, or they are not updated with the yearly earthquake database, or they are not expanded under the updated prediction equations, or they are estimated using Geospatial Information System (GIS) with large scale generalized database, or they have uncertainty in the data or based on stochastic generation of earthquake catalog (EC). The preparation of an earthquake catalog (EC) is the critical input in the SHA, and such EC information is only available up to 2014 in previous studies, while this study is considered up to 2021. Another crucial factor is the depth range of seismogenic sources. Previous researchers have adopted a standard depth range or average depth range for all point and linear sources. In this study, depth is incorporated with appropriate focal depth for the point sources and an average depth of the study area for all the linear sources. Next, the maximum magnitude evaluation of tectonic sources is one of the essential factors in SHA studies, which are generally estimated using deterministic approaches for the study area. The present study used a probabilistic approach, i.e., Regional Rupture Character (RRC). The regional ground motion prediction equation (GMPE) developed by Anbazhagan et al. (2013) is the most suitable equation designed for a distance range of 300 km with a magnitude ranging from 4.5 to 8.7, is used in the present study, while other researchers used different GMPEs. In addition, the ground motion that would trigger a landslide evaluated using a fully probabilistic (FPSHA) technique is not yet studied for the study area.
Therefore, in the present study, the seismic hazard analysis (SHA) is carried out using three different frameworks, namely, DSHA, PSHA, and FPSHA, for a 300 km distance study area and case study (Tindharia landslide) using a refined database and updated approaches. The DSHA provides maximum controlling earthquake, and the design-based earthquake (DBE) ground motion and maximum considered earthquake (MCE) ground motion are obtained from PSHA, and FPSHA finds the total probability of slope failure under various ground-shaking levels and provides the most probable ground motion that would trigger the landslide in next 50 years. The complete details of site characteristics, such as seismicity, geology, topography, and geotechnical factors of the study area, were collected from various sources. The earthquake catalog of the study area for the past 212 years has been prepared based on homogenization, de-clustering, and completeness check, which is the first essential input in SHA studies. The study was carried out to understand the seismic performance of ground motions from three methodologies on landslides in the study area.
The results show a significant difference in the design ground motion that ranges within the study area from the three approaches. The case study performed on the Tindharia landslide using DSHA, PSHA, and FPSHA approaches provided the site-specific design ground motions of 0.90g. 0.30g and 0.20g. The DSHA pointed out that the highest ground motion governed by the seismic sources, which are the most elevated threat to the site, might be due to not considering uncertainties in the earthquake database and GMPE. The deterministic hazard maps are recommended when the consequences of failure are intolerable. Still, it will not be the probability, so we have yet to learn the acceptable hazard and possible future ground motions. The PSHA ground motions are evaluated based on earthquake statistics that provide ground motion that will be exceeded at a given location in a given future time period. The results show that the PSHA ground motions are substantially lower than DSHA ground motions, which might be because of uncertainty in the earthquake database and GMPE. Also, the ground motions are higher than FPSHA because of considering only past historical earthquakes without including the soil properties. Subsequently, the FPSHA probable ground motions are determined by combining the PSHA with the dynamic slope stability model based on Newmark’s approach. The mathematical relationship of Newmark’s equation involved slope properties like topography and geotechnical characteristics. In this study for 3160 slope models, this approach evaluates the most probable ground motion scenario that would trigger the slope in the next 50 years. A considerable difference between ground motion levels in FPSHA with DSHA and PSHA approaches was observed; this might be due to the involvement of uncertainties of the slope models, GMPE, and seismic source models.
From the research, the seismic landslide hazard could be overestimated or underestimated when using design ground motions of DSHA, and PSHA approaches, to select the scenario triggering condition of the landslides. Further, the PSHA provides only earthquake-driven probabilistic ground motions based on past historical earthquakes. Therefore, these ground motions are suggested for general seismic infrastructure design. The FPSHA de-aggregation charts are estimated based on earthquake statistics and soil properties. Thus, the fully probabilistic technique for seismic slope stability provides the best suitable design ground motions that would trigger the landslide by handling all possible earthquake scenarios that may lead to slope instability.
The updated hazard maps and design charts developed in this study are used for general seismic infrastructure designing, hazard zonation mapping, landslide monitoring, seismic slope stability analysis, landuse planning, creating code requirements, and mitigation measures (with better pre-disaster prevention by earthquake-resistant design and preparedness for post-disaster rescue). The findings from the hazard analysis are further used in developing ground motion attenuation relationships for the region, synthetic ground motion generation, and other engineering applications.
May 2023