Natural Hazards and Risk Mitigation

Some new insights about the structure and seismotectonics of the Himalaya are given in this chapter including thick or doubling of the crust and the role of detachment surface, Low-Velocity Zones (LVZs), transverse faults and subducting ridges of the Indian plate in Himalayan earthquakes. Strain rates inferred from GPS are described for different sectors of the Himalaya–Myanmar–Andaman belt and estimates of return periods of large earthquakes are highlighted. Seismic gap areas in the Himalaya inferred from past large earthquakes are described.

The Himalayas are facing the maximum number of natural disasters like earthquakes, landslides and floods. The young age of the mountains, active continent–continent collision tectonics and geographical position are all contributory factors. The levels of different hazards in the Himalayas are identified in this chapter.

The 2500 km Himalayan arc spanning Kashmir, Ladakh, in the west to Eastern syntaxis in the east is a tectonically complex and seismically active northern subduction boundary of the Indian plate. Three decades of GPS data give well-constrained surface convergence rates ranging from 10 to 26 mm/yr in the various segments of the Himalayas (Kashmir, Ladakh, Himachal, Garhwal, Kumaun, Nepal, Sikkim, Bhutan, Arunachal and Eastern syntaxis). Arc parallel rates of 3–10 mm/yr is the manifestation of locked curvature of the central Himalayan arc and the E–W extension rate of Tibet. Inverse modelling of surface convergence rates is used to estimate oblique slip rate of 13–20 mm/yr along Main Himalayan Thrust (MHT) at a depth of 15–20 km and locking width of 100–150 km from the frontal Himalayas suggesting that each segment of the Himalaya is unique in nature. Geodetic strain rates derived from the GPS-derived surface convergence rates suggest that the Himalayan region is predominantly under compression with high strain rate coinciding with the northern boundary of sub surface basal decollement (MHT) along which Indian plate subducts below Tibet. Seismic strain rates for each segment of the Himalaya are computed using the instrumental and historical earthquake catalogue. Seismic potential of each segment of the Himalaya is estimated by a combined analysis of geodetic and seismic strain rates and the corresponding moment rates. Further, strain budget and accumulated slip since the last devastating earthquake was used to estimate the recurrence interval and probable magnitude of impending earthquake in the Himalayan segments.

Landslide is one of the deadliest hazards in the tectonically active Himalayan Region, particularly in the Lesser Himalayas. Rainfall-induced weathering and the hydrothermal alteration during the formation of the MCT have resulted in the production of hydrophilic clay minerals, which reduces the frictional strength of the slide planes by absorbing water in their interlayer spaces and thus triggers landslides. However, smectite, illite and vermiculite-like clay minerals with higher interlayer spacing enhance the shear strength with an increase in sliding velocity which gives rise to short-term but large-scale landslides, whereas chlorite crystals retain the strain energy within its lattice due to the presence of hydrogen bonds, which joins the talc-like T-O-T layer and brucite like-octahedral sheet and facilitates viscoelastic behaviour even at high-stress regime inhibiting the occurrences of large-scale landslides. Hence, in the NaMu highway section in Nepal, more active landslides are evidenced in the areas where the soil dominantly contains illite rather than those containing chlorite and kaolinite with little or no illite. Moreover, unlike smectite or other clay minerals, the shear strength of chlorite diminishes with increasing slide velocity and results in slow-motion landslides for longer duration or creep, as also observed frequently in parts of the Lesser Himalayas.

The Toldi slide falling in the Samroli area along the NH-44 in the Udhampur district of Jammu and Kashmir has been plagued by frequent failures during insistent rainfall from last more than three years. This slide falls within the Murree Group of rocks comprising interbedded medium- to fine-grained sandstone and mudstone which is considered as the water-sensitive lithology. The current slide on the basis of its areal content is categorized as a medium slide and covers an area of about 51,888 m². It is also categorized as a rock slide depending on the kind of movement and the geologic material. The fundamental goal of the present study was to comprehensively understand the underlying failure mechanisms, with the aim of proposing immediate remedial measures to mitigate the losses and disruptions experienced by the population. In order to determine the quality of the rock mass and the slope mass's instability, the current study includes extensive field investigations as well as geotechnical studies in the form of rock mechanics (RMR, SMR and kinematic analysis). The basic rock mass rating inferred that the rock mass falls under fair category; however, the slope mass rating indicated a completely unstable slope with a high likelihood of failure (failure probability of 0.9). Kinematic analysis on the other hand uncovered two prevalent modes of failure at the site: planar and wedge failures. The study also inferred that the unsystematic toe cutting of the water-sensitive lithology is the dominant reason behind this landslide. Other causes include uncontrolled blasting, mechanical excavation, subsequent rainfall, improper drainage, steeper slope angle and shear zones around the slide.

Rockfall is a natural mass-wasting phenomenon in mountainous regions that involves the detachment of rock fragments by sliding, toppling and falling. The fragments sliding and toppling the hill slopes are often weakened by improper road-cutting, and sometimes are subjected to sliding by rainfall or earthquake occurrences. To minimize the risk, slope stability analysis and its stabilization play a crucial role. In the present study, two rockfall-prone slopes near Shivpuri along NH-58 in the Garhwal Himalayas, India, have been studied. A field study was carried out along this route. The rock mass classification techniques such as Continuous Slope Mass Rating (CSMR) and Q-slope were used for slope stability assessment. The stability grade of these slopes evaluated from CSMR has been validated with Q-slope rock mass classification. The results of Q-slope techniques have been plotted on the Q-slope stability chart and stability condition of both the slopes have been observed. The safe slope angle for both the slopes has been determined using the Q-slope technique. These rock mass classification systems are much more effective for quick assessment of slope stability. Hence, the methodology present in this study will be beneficial for a critical stability assessment of slope vis-a-vis for selection of suitable control measures.

The Srinagar-Bandipora highway is one of the historical roads of Kashmir Valley and has always remained a supportive lifeline for the people in this region. This road is particularly vulnerable to landslides due to extreme weather conditions, complex mountainous terrain, and anthropological activity. Because of this, there are always frequent traffic disruptions and a severe threat to ongoing traffic and all activities associated with the region's social and economic development. As a result, it is critical to identify and mitigate the region's landslide risk, which causes significant inconvenience and economic and human losses. In the current study, landslide susceptibility mapping (LSM) was performed utilizing a bivariate frequency ratio model (FRM) and a geographical information system (GIS) platform. Besides, the landslide inventory details were prepared from primary and secondary data sources. The landslide susceptibility map (LSM) was generated from landslide inventory data and fifteen landslide causative factors (CFs). Therefore, the frequency ratio model examined the link between landslide CFs and landslides. LSM divided the area into five susceptibility categories, viz., very high (20.17Km²), high (39.22Km²), medium (62.49Km²), low (76.87Km²), and very low (56.90Km²). Subsequently, the landslide susceptibility map was then evaluated and compared to the existing landslide inventory data using the area under the curve (AUC) method. Moreover, the LSM created will be helpful to various stakeholders, including planners, designers, engineers, and the local community, for the upcoming building and maintenance projects in the study region.

The Himalayas contain the largest glacier system outside the Polar regions, serving as a vital water tower for the rivers that drain this massive mountain range and its extensive adjoining alluvial fan. The recent retreat of Himalayan Glaciers has had a significant impact on at least half a billion people. According to the updated glacier inventory data from the Geological Survey of India (GSI; Raina V K and Srivastava D (2008), Glacier Atlas of India. GSI Publication 315) there are more than 9,575 glaciers in the Indian-administered part of the Himalayas, covering an area of approximately 37,000 km². Meanwhile, ICIMOD identifies a staggering 540,000 individual glaciers in the Hindu Kush-Himalayan Region, spanning an area of 60,000 km², with an estimated ice reserve of 6,000 km³. These glaciers release meltwater seasonally into the tributaries of the Indus, Ganges, and Brahmaputra Rivers. Around 300 million people depend upon water from these rivers to sustain agricultural and economic activities on the Indian subcontinent’s plains. Regions where water supply heavily depends on melting snow and ice are projected to experience hydrological disruptions due to recent warming. Beyond being a constant source of water, this glacier system serves as a climate change indicator in the region, influencing river discharge, debris production, and transportation, triggering avalanches, flash floods, and causing siltation in lower valleys. Additionally, it directly and indirectly affects the mountain ecology that sustains life. Historically, Himalayan Glaciers have exhibited substantial fluctuations, a pattern that continues into the present day. The ongoing shrinkage of glaciers due to climate change is expected to bring about a major impact on water resources, precipitating dramatic changes in the environment. This paper provides a brief discussion of some critical aspects of glacier shrinkage and its potential impacts.

The global burden of earthquake hazards has intensified over the decades, particularly in developing countries with inadequate safety measures. Despite two centuries of progress in earthquake science, including mapping fault systems and establishing seismic building codes, many high-risk nations have failed to implement these regulations effectively. This paper advocates for a paradigm shift in addressing earthquake hazards by proposing a dedicated UN forum to enhance global mitigation efforts. Simultaneously, it calls for developing comprehensive nationwide preparedness frameworks aimed at reducing vulnerability and saving lives.

The global burden of earthquake hazards, which have increasingly turned into disasters, has grown over the decades, particularly in countries with inadequate earthquake safety measures. This risk is especially pronounced in much of the developing world. For over two centuries, advances in earthquake science have led to mapping fault systems and establishing seismic building codes, warning systems, and other protective measures. However, in many high-risk countries—such as Pakistan, Afghanistan, India, Nepal, Bangladesh, Indonesia, Haiti, and Morocco—these regulations remain under-implemented despite the known dangers. Recent disasters, such as the 2023 earthquakes in Syria and Turkey, which claimed more than 50,000 lives, and the moderate but deadly earthquake in Morocco that killed over 2,000, underscore the urgent need for a more robust response. Both well-mapped and lesser-known fault systems continue to pose significant threats, especially in the Global South, where the impacts of such disasters disproportionately affect millions of people. This paper argues for a paradigm shift in addressing earthquake hazards, drawing inspiration from the climate change discourse at the United Nations. Establishing a dedicated UN forum for earthquake hazards could catalyze global efforts to enhance mitigation and adaptation strategies, paving the way for a safer future. Additionally, the development of comprehensive, nationwide earthquake preparedness frameworks is critical. Addressing long-standing gaps in mitigation and adaptation could significantly improve countries' vulnerability profiles and save countless lives.

A comprehensive strategy incorporating inputs from geological, geotechnical, and geophysical studies was carried out to account for the local site effects on earthquake ground motion at various sites in and around Gorakhpur City, Uttar Pradesh, India, falling in Seismic Zone-IV of Seismic Zoning Map of India (BIS 2002, 2016). Gorakhpur City, one of the biggest and rapidly developing cities is located on the middle Gangetic Plain, which comprises fluvial sediments from the Older Alluvium (Middle to Late Pleistocene age) and the Newer Alluvium (Holocene age). The M 7.8 (Mw), 2015 Gorkha/Nepal Earthquake, located around 200 km northeast of Gorakhpur, has jolted the city at > VI seismic intensity, with vertical and horizontal peak ground acceleration (PGA) of 0.25733 g and 0.385995 g, respectively. The horizontal-to-vertical-spectral-ratio (HVSR) approach of ambient noise survey and microtremor study is used to evaluate the seismic site amplification of Gorakhpur city. The fundamental soil frequency and HVSR peak amplitude maps demonstrate peak soil amplification in the range from 0.7 to 3.5. The scatter plot between predominant frequency and peak amplification shows that the bulk of sites in Gorakhpur City (65.42%) fall into the medium damage potential zone, followed by the low damage potential zone (31.53%), and the high to extremely high damage potential zone (3.05%). In accordance, with the multichannel analysis of surface waves (MASW) and Standard Penetration Test (SPT) studies, the average shear wave velocity at different locations in and around Gorakhpur city varies between 202.5 m/s and 357.4 m/s, which corresponds to soil type D (stiff soil) of the site classes of the NEHRP Code Provisions (1997). In Gorakhpur, the prevalence of high liquefaction domains can be seen at low-lying places where the groundwater is shallow (= < 10 m depth), as evidenced by liquefaction susceptibility safety factor values for different locations. The Analytical Hierarchy Process and GIS-based integration were applied to produce a final seismic hazard microzonation map of Gorakhpur City. The southern part of Gorakhpur city, particularly the low-lying area, where newer alluvium is exposed, around Ramgarh Tal/lake, has the highest seismic hazard index. A few isolated high seismic microzone patches have been identified in the northern region in Chilua Tal/Lake, Sonbarsa, and Balapar regions. The present study is beneficial for seismic disaster preparedness, urban planning, and the preservation of cultural heritage in the city while designing future major engineering projects, multi-story buildings, etc. to withstand the incoming seismic ground motion caused by the large earthquakes in the region.

In seismically active zones like bridge project sites located at Koyalajan Kamla River in Rasalpur and Koyalajan Road at Block Biraul, Bihar, India structural safety necessitates assessing liquefaction potential. An analysis for a bridge project in these areas, located in seismic zone V, revealed vulnerability due to a previous magnitude 7.5 earthquake with peak ground acceleration (PGA) of 0.36. With groundwater near the surface, two 50-m boreholes were drilled to evaluate site-specific conditions and assess the likelihood of liquefaction. The soil's liquefaction susceptibility was assessed using the Standard Penetration Test (SPT), measuring N-value and fines percentage in line with IS 1893-2016 Annexure-F guidelines for liquefaction potential analysis. The soil investigation report indicated that the site mainly consists of poorly graded silty sand with medium to very dense characteristics (SM). Given the significant foundation load, a well foundation was deemed necessary, and the gross allowable load-bearing pressure at the well tip for bridge abutments and piers was recommended in a range from 85 to 90 T/m². The study's methodology highlighted the importance of being cautious and attentive as the analysis indicated that liquefaction is likely to occur up to 11 m depth below the ground level.

Accelerograms recorded in an earthquake are time-domain signals and do not convey information on the frequency content of the wave. The frequency domain representation of the accelerogram shows the distribution of the frequency content but hides information on the time characteristics. Since both the amplitude and frequency of the seismic wave contribute to structural damage, a comprehensive understanding of the possible structural damage can be understood using the three-dimensional approach of continuous wavelet transform (CWTs). Due to damage in an earthquake, the stiffness of a structure degrades, and the natural period elongates. The vulnerability of the structure thereby also depends on the sequence of arrival of damaging frequencies. In this paper, the strong ground motions, recorded within a fault distance of 20 km, in the devastating 1994 Northridge earthquake (Mw 6.7) are characterized in both the time domain and the frequency domain. The CWTs for these ground motions are generated to understand the time sequence of the arrival of high amplitude frequencies. The information obtained from the CWTs is analyzed and correlated with the observed structural damage, in a variety of buildings having a varied range of natural periods, due to the earthquake.

The dynamic response of the structures, subjected to strong ground motions, depends on the behavior of the soil in and around the structure. The effect of soil-structure interaction is neglected by analysis and designers due to a lack of code provisions. Damage to structures in recent earthquakes has indicated non-uniform distribution of damage, which indicates the effect of soil structure interaction (SSI). In literature, there is no clear consensus on the beneficial or detrimental effect of SSI effect and thus its incorporation in modelling is not prescribed in some seismic codes. In this study, a lumped mass system has been modeled to study the effect of soil-structure interaction. Non-linear time history analysis was performed on this model using ground motions from two major earthquakes in Uttarakhand region. The displacement of the lumped mass was monitored for both cases, i.e., fixed and flexible base conditions. The peak values of displacement time histories are tabulated and plotted versus the corresponding peak ground acceleration. The results indicate that incorporating the effect of soil-structure interaction leads to an increase in the displacement of the structure at certain sites.

The central Himalayan region encompasses the area between the border regions of Jammu and Kashmir, Himachal Pradesh, Uttarakhand, and the Nepal Himalayas.