Geography Set 15: Indian Physiography and Earthquake Hazard Zones MCQ
Welcome to Geography Set 15 of our daily GK series. In this comprehensive set, we dive into the core concepts of Indian Physiography, the Origin and Evolution of the Great Plains, and Earthquake Hazard Zones. Mastering these geological and tectonic frameworks is absolutely crucial for exams like WBCS, SSC, and UPSC.
Below, you will find important Indian Geography objective questions along with deep-dive, fully detailed background explanations to boost your competitive exam preparation. Use our interactive practice quiz, flashcards, and mind maps to master these topics!
Detailed Study Material: Indian Geography
🌍 Section 1: Origin and Evolution of the Great Plains of India (Q1 – Q5)
Origin & Evolution • Great Plains
Q.1) Which fundamental mechanism did the Austrian geologist Eduard Suess propose for the origin of the Great Plains of India?
Ans > The deposition of detritus within a foredeep synclinorium
- The Foredeep Synclinorium Concept: Eduard Suess advanced the hypothesis that the Great Plains of India originated from the continuous infilling of a vast “foredeep” that formed ahead of the high crust-waves of the Himalayas. This massive structural depression was created as a direct consequence of the southward advance of the Himalayan orogeny, which was ultimately obstructed by the rigid, ancient landmass of the Peninsular Plateau. Suess characterized this foredeep structurally as a massive synclinorium—a broad, regional syncline featuring numerous smaller, localized anticlines and synclines at its basement level, resulting in highly uneven bottom topography.
- Topographical Gradients and Sedimentation: The geomorphology of this proposed foredeep exhibited a gentle slope towards the north, whereas the side adjacent to the Peninsular block possessed a much steeper gradient, resting on a basement of hard crystalline rocks. Over millions of years, the major river systems descending from the newly uplifted Himalayan ranges, including the Indus, Ganga, and Brahmaputra, transported tremendous volumes of detritus into this depression. The continuous sedimentary alluviation of this foredeep ultimately generated the expansive, highly fertile alluvial plains seen today, which span an area of approximately 7.8 lakh square kilometers.
- Geological Critiques and Limitations: Modern geological and geophysical scrutiny highlights critical evidentiary gaps in Suess’s theory. The hypothesis struggles to adequately explain the highly uneven, irregular slope of the plain’s basement bed inferred from modern seismic soundings. Furthermore, definitive geophysical evidence demonstrating a continuous crustal connection between the Himalayan and Peninsular blocks exclusively through this plain’s basement structure remains lacking, leading researchers to explore alternate flexural subsidence models.
Origin & Evolution • Great Plains
Q.2) Sir Sydney Burrard attributed the formation of the Indo-Gangetic Plains to which of the following geological processes?
Ans > The infilling of a giant rift valley between parallel faults
- The Rift Valley Hypothesis: Sir Sydney Burrard proposed a fundamentally different tectonic origin, suggesting that the Great Plains of India are the result of the infilling of a massive structural rift valley. He postulated that during the primary tectonic elevation of the Himalayas, extreme bending and curbing movements of the northern continental wall created a deep sub-crustal fracture or fissure, sinking several thousands of meters deep into the earth’s crust.
- Parallel Fault Mechanics: According to Burrard’s structural model, this giant rift valley was established between two major parallel faults of continental scale. One fault line was positioned along the southern boundary of the Shiwalik hills, acting as the northern wall, and the other ran along the northern boundary of the Peninsular block, serving as the southern wall. The intervening land block subsided between these vertical dislocations, forming an immense structural trough.
- Fluvial Infilling and Modern Rejection: Once this colossal rift valley was formed, the Himalayan river systems acting as erosional agents transported vast quantities of terrestrial detritus into the hollow. The subsequent accumulation of thousands of meters of alluvium entirely concealed the deep sub-crustal rift beneath a flat depositional surface. However, Burrard’s hypothesis has been overwhelmingly rejected by modern geologists. The primary criticism is grounded in global tectonic comparisons; nowhere on Earth does a rift valley of such extraordinary length and width exist without associated widespread volcanism or specific structural hallmarks, which are absent in the Indo-Gangetic subsurface.
Origin & Evolution • Great Plains
Q.3) Which theory suggests that the Great Plains of India are the residual remnants of a massive prehistoric marine body?
Ans > The Remnant of the Tethys Theory
- The Post-Upheaval Marine Trough: A prominent geomorphological theory asserts that the Great Plains of India represent the final remnants of the prehistoric Tethys Sea. Following the dramatic tectonic upheaval of the Shiwalik ranges during the later phases of the Himalayan orogeny, the unconsolidated marine remnants of the Tethys were left as a massive, elongated trough. This residual sea was initially joined to the Bay of Bengal in the east and the Arabian Sea in the west, forming a continuous marine channel across northern India.
- River Rejuvenation and Accelerated Deposition: Because the Himalayas were actively rising during this specific geological period, the rivers originating from these newly formed mountains experienced significant kinetic rejuvenation. This increased gradient and velocity allowed the fluvial systems to erode greater quantities of material, rapidly transporting heavy sediment loads into the marine trough. According to recent research, sediment deposited at the bed of the pre-historic Tethys folded towards the northern end due to the continuous northward drift of the Indian plate.
- Marine Recession and Terrestrial Emergence: As the rivers continually deposited their massive detrital loads into the central part of the trough, the marine waters located on the eastern and western margins were gradually displaced and began to recede. This relentless displacement of seawater by fluvial sediments ultimately brought the terrestrial Great Plains of India into existence. While elegantly explaining the marine origin of specific basal sedimentary layers, the theory struggles to account for the highly irregular basement topography and specific crystalline structures discovered through deep drilling in the central portion of the plain.
Geology • Indo-Gangetic Plain
Q.4) Which of the following subsurface features fundamentally divides the Indo-Gangetic Plain and limits the deep sedimentary basins?
Ans > The Delhi-Sargodha Ridge
- Subsurface Basement Topography: Contrary to the perfectly flat surface topography, the geological basement of the Indo-Gangetic Plain is an intricate landscape marked by concealed ridges and deep depressions. These subsurface ridges are rigid crystalline extensions of the Peninsular block that effectively compartmentalize the massive foreland basin into distinct sedimentary zones and sub-basins.
- The Delhi-Sargodha and Delhi-Hardwar Ridges: The Delhi-Sargodha Ridge (DSR) is a major structural feature extending in a northwest direction, which deep magnetotelluric studies reveal bifurcates from the Delhi-Hardwar Ridge (DHR) at a crustal depth of 12-13 km. The DHR strikes NE-SW and exhibits a highly shallow central axis, often less than 400 meters below the surface alluvium. Together, these ridges act as a significant structural barrier, delimiting the western boundary of the Ganga Basin and causing older sedimentary sequences to thin out.
- Seismotectonic Implications of Concealed Ridges: These concealed subterranean ridges are far from dormant; their intersections form highly active seismotectonic junctions. The interaction between the continuous northward subduction of the Indian plate and these rigid transverse structural ridges causes intense localized stress concentrations. This structural friction renders the overlying alluvial plains highly susceptible to enhanced seismic wave amplification and triggers dangerous intraplate earthquakes within the plain itself.
- Delhi-Hardwar Ridge (NE-SW): Delimits western boundary of the Ganga Basin.
- Delhi-Sargodha Ridge (NW-SE): Divides Punjab plains; influences western seismicity.
- Faizabad Ridge (NE-SW): Separates the Sarda depression from the Gandak depression.
- Munger-Saharsa Ridge (N-S / NE-SW): Delimits eastern margin; transition to Bengal Basin.
Geology • Indo-Gangetic Plain
Q.5) What is the general pattern of sediment thickness observed beneath the Indo-Gangetic Plain from the Peninsular margin to the Himalayan foothills?
Ans > The sediments progressively thicken from southwest to northeast.
- The Asymmetric Sedimentary Wedge: Advanced geophysical investigations, including high-frequency receiver function (PRF) inversion and deep borehole data from the Oil and Natural Gas Corporation (ONGC), reveal a highly asymmetric, wedge-shaped sedimentary profile beneath the plains. The thickness of the alluvial deposits increases progressively and substantially from the southwest margin near the rigid Peninsular plateau towards the northeast, directly abutting the Himalayan foothills.
- Basement Depth Variations: The depth to the crystalline basement varies significantly across this transect. In the Central Alluvium Plain, depths range between 1.5 to 1.7 km, expanding to between 1.8 and 2.8 km in the Zone of Terminal Fans. The sediment column achieves its maximum thickness, ranging from 3.8 km to over 5.0 km, adjacent to the Himalayan Frontal Thrust (HFT), accommodating the massive vertical load and detritus flux from the mountain streams.
- Seismic Amplification by Soft Alluvium: Shear-wave inversion models have identified that the top 400–700 meters of these quaternary sediments possess extremely low shear wave velocities (Vs < 0.5 km/s) and a high Vp/Vs ratio. This highly unconsolidated, soft alluvial surface layer acts as a powerful amplifier for seismic waves generated by distant Himalayan earthquakes. The velocity-depth structure confirms that the densely populated urban areas situated over these deep, loose sediments face a dramatically elevated risk of soil liquefaction and structural resonance.
🏞️ Section 2: Physiographic Divisions of the Great Plains (Q6 – Q15)
Physiography • Great Plains
Q.6) Which physiographic division of the Great Plains is characterized by a narrow, highly porous belt of gravel and un-assorted sediment where streams frequently disappear underground?
Ans > Bhabar
- Proximity to the Himalayan Foothills: The Bhabar plain is a narrow, highly specialized geomorphological belt, typically ranging from 7 to 15 kilometers in width, located immediately south of the Shiwalik foothills. It stretches in an arcuate east-west direction from Jammu in the west to Assam in the east, functioning as the primary depositional transition zone where high-energy Himalayan rivers first enter the flat plains.
- Gravel and Porous Sediment Composition: As mountain rivers descend rapidly from steep Himalayan gradients, they abruptly lose their kinetic transporting capacity upon encountering the flat topographical profile of the plains. This sudden drop in energy forces the massive deposition of gravel, pebbles, rocks, and un-assorted coarse sediments in the form of alluvial fans, creating a highly porous geological stratum.
- Subterranean Fluvial Dynamics: Due to the extreme porosity of the un-assorted gravel and boulders that dominate this tract, the water of numerous smaller streams and tributaries percolates downward and effectively disappears from the surface. These streams flow subterraneanly through the Bhabar, leaving the surface relatively dry and characterized by braided, dry riverbeds except during periods of extreme monsoon flooding. Because of its coarse, rocky soil composition, the region is fundamentally unsuitable for standard agricultural cultivation and is naturally dominated by large trees with extensive root systems.
Physiography • Great Plains
Q.7) The marshy and swampy physiographic tract located immediately south of the Bhabar, characterized by re-emerging streams and dense forests, is known as:
Ans > Terai
- The Re-emergence of Subterranean Streams: The Terai tract is positioned directly to the south of the highly porous Bhabar belt. The defining hydrological feature of this region is the resurfacing of the subterranean streams that had vanished into the gravels of the Bhabar. As the topography flattens further and the sediment profile transitions to finer silts and clays, the groundwater table intersects the surface, causing rivers to re-emerge without well-defined, entrenched channels.
- Swampy Ecology and High Humidity: The resurfacing of vast amounts of water on a relatively flat terrain creates a persistently waterlogged, marshy, and swampy environment. The region receives heavy annual rainfall, which, combined with the swampy ground conditions, results in excessive atmospheric humidity. Historically, this unique microclimate supported thick wet evergreen vegetation and a remarkably rich array of flora and fauna, while the poor drainage fostered a highly malarial climate that deterred dense human settlement.
- Anthropogenic Agricultural Reclamation: In recent decades, substantial portions of the Terai tract—particularly in states like Haryana, Punjab, Uttarakhand, and Uttar Pradesh—have undergone massive anthropogenic transformation. The dense forests have been systematically cleared for agriculture. Because the soil is highly rich in humus and decomposed organic matter from centuries of forest cover, it has been transformed into highly productive agricultural land optimally suited for cultivating water-intensive crops like rice, sugarcane, and maize.
Physiography • Great Plains
Q.8) Which alluvial plain represents the older, elevated upland tracts of the Great Plains containing calcareous nodules known as ‘Kankars’?
Ans > Bhangar
- Pleistocene Origin and Upland Elevation: The Bhangar (or Bangar) plains represent the older, elevated alluvial terraces of the Indo-Gangetic basin. These extensive plains are composed of older alluvium that was primarily deposited by river systems during the middle Pleistocene period. Because of their relative elevation, the Bhangar lands are situated distinctly above the flood limits of contemporary rivers.
- Pedological Composition and Kankar Nodules: Because the Bhangar tracts do not receive annual replenishments of fresh silt from seasonal flooding, their pedological evolution is highly distinct from the active floodplains. A defining characteristic of the Bhangar soil profile is the heavy presence of calcium carbonate nodules, locally referred to as ‘Kankars’. These impure calcareous concretions are formed deep in the soil profile due to the long-term downward leaching and subsequent precipitation of calcium compounds in these older soils.
- Agricultural Utility and Drainage: Despite lacking the annual fresh silt deposits that characterize the newer plains, the Bhangar soils remain rich in humus and are extremely well-drained due to their elevated topographical position. This structural stability prevents waterlogging, and when supported by modern tube-well and canal irrigation techniques, the Bhangar plain yields massive agricultural output, making it one of the most intensely cultivated regions on the subcontinent.
Physiography • Great Plains
Q.9) Which section of the Great Plains is composed of newer, highly fertile alluvial soil that is enriched by fresh silt deposits annually?
Ans > Khadar
- Contemporary Riverine Deposition: The Khadar plains constitute the contemporary, active floodplains located directly along the courses of the major Himalayan river systems. Unlike the elevated and older Bhangar terraces, the Khadar consists almost exclusively of newer, younger alluvial deposits. These low-lying areas form the immediate lateral margins of the rivers and represent the most dynamic geological environment within the plains.
- Annual Silting and Supreme Fertility: Because these low-lying areas are highly susceptible to frequent, often annual monsoon flooding, the rivers continually blanket the land with fresh, nutrient-rich deposits of silt, mud, clay, and fine sand. This regular pedological renewal prevents the soil from degrading or leaching heavily over time, making the Khadar the most fertile soil profile within the entire Indo-Gangetic plain.
- Regional Terminology and Intensive Cultivation: In specific regions such as the Punjab, these highly fertile, newly formed Khadar floodplains are locally known as “Bet lands” or “Bets”. Given its supreme natural fertility and immediate proximity to reliable riverine water sources, the Khadar plain is subjected to incredibly intensive agriculture. The land is predominantly devoted to the high-yield cultivation of vital cash and staple crops, including sugarcane, rice, wheat, maize, and oilseeds, serving as a critical breadbasket for the nation.
- Age of Alluvium: Bhangar: Older (Middle Pleistocene). Khadar: Newer (Recent/Holocene).
- Topography: Bhangar: Elevated upland terraces. Khadar: Low-lying active floodplains.
- Flood Inundation: Bhangar: Rarely flooded. Khadar: Flooded frequently (often annually).
- Soil Composition: Bhangar: Contains calcareous ‘Kankar’ nodules. Khadar: Fine silt, mud, clay; lacks Kankars.
- Fertility: Bhangar: Highly fertile (requires fertilizer/irrigation). Khadar: Supremely fertile (naturally renewed).
Physiography • Rajasthan Plain
Q.10) In the Rajasthan Plain, the semi-arid tract located to the east of the Thar Desert, drained by short seasonal streams creating fertile patches, is known as:
Ans > Rajasthan Bagar
- Climatic Division of the Rajasthan Plain: The Rajasthan Plain represents the westernmost extremity of the Great Plains of India. Climatologically and geomorphologically, it is broadly bisected by the 25cm isohyet into two distinct sub-divisions: the hyper-arid Marusthali to the west, and the semi-arid Rajasthan Bagar to the east. The Bagar stretches eastward from the margins of the deep desert up to the foothills of the Aravalli Range.
- Drainage by Seasonal Streams: Unlike the entirely dry, dune-covered expanse of Marusthali, the Bagar region is characterized by a network of short, seasonal fluvial streams that descend from the adjacent Aravalli Range during the monsoon. The Luni River serves as the most prominent and critical example of this drainage system, flowing through the semi-arid region before ultimately emptying into the Rann of Kutch.
- The Formation of Rohi Patches: As these short seasonal streams traverse the Bagar landscape, they deposit crucial moisture and fine silt onto the surrounding terrain. This intermittent hydrological activity gives rise to localized, highly fertile green patches within the otherwise semi-arid landscape, which are locally termed “Rohi”. In stark contrast, the Marusthali to the west remains a true desert zone dominated by shifting sand dunes known locally as “Dhrian” and ancient rocky outcrops that geologically link it to the Peninsular Plateau.
Physiography • Punjab Plain
Q.11) In the geomorphology of the Punjab-Haryana Plain, what term is used to describe the broad flood plains flanked by bluff-like old banks?
Ans > Dhayas
- Fluvial Architecture of the Punjab Plain: The Punjab-Haryana plain, extending 640 km in a northwest-to-southeast direction, was meticulously carved by the depositional and erosional forces of five major rivers of the Indus system: the Sutlej, Beas, Ravi, Chenab, and Jhelum. The interaction of these powerful rivers created a highly specific micro-geomorphology across the region.
- Dhayas and River Terracing: Within this expansive plain, rivers tend to deeply entrench their courses over geological time due to regional uplift and variable sediment loads. The steep, bluff-like old banks that physically flank the active, broad floodplains are locally referred to as “Dhayas”. These elevated bluffs represent the older, scarped edges of river terraces, delineating the boundaries of historical flood events.
- Erosional Chos and the Water Divide: Conversely, the Shiwalik foothills adjacent to the northeastern edge of the Punjab-Haryana plain are subjected to intense fluvial erosion by a multitude of short, seasonal, torrential streams. These destructive streams and the deep gullies they carve into the soft sediments are locally known as “Chos”. To the southeast lies the Haryana Tract, a vital geomorphological feature situated between the Ghaggar and Yamuna rivers, which serves as a fundamental continental water divide separating the Indus river system from the Ganga river system.
Physiography • Ganga Plain
Q.12) Which division of the Ganga plain is characterized by an undulating terrain composed of old lateritic alluvium located on its western margin?
Ans > The Rarh Plain
- Lateritic Alluvium of the Rarh Plain: The Rarh Plain is a distinct physiographic subdivision located on the western margin of the Gangetic delta, situated predominantly within the state of West Bengal. Unlike the flat, grey alluvial soils of the central Ganga plain, the Rarh is characterized by a highly undulating, rolling terrain composed of old, highly oxidized lateritic alluvium.
- Geological Transition Zone: This specific plain serves as a crucial geological and topographical transition zone. It physically and structurally links the elevated, hard-rock Precambrian formations of the Chhotanagpur Plateau in the west to the flat, newly formed active deltaic floodplains (the Diara) of the Ganges to the east. The rivers draining the Rarh plain emerge from the Chhotanagpur plateau, cutting through the lateritic soils.
- Comparison with the Barind Tract: While the Rarh Plain forms the western margin of the delta, the northern part of the Bengal Plains hosts a structurally similar feature known as the Barind Tract. The Barind is also composed of old Pleistocene alluvium, but it is located in North Bengal and forms a slightly elevated, terrace-like inlier surrounded entirely by newer river deposits. The lateritic soils of the Rarh plain are generally less fertile than the newer Ganga alluvium due to extensive leaching of nutrients and high concentrations of iron oxides, necessitating specialized agricultural practices.
Physiography • Delta Plains
Q.13) In the Delta Plains, the marshy, low-lying waterlogged areas formed by river deposition are locally referred to as:
Ans > Bills
- Depositional Dynamics of the Lower Reaches: The Delta Plain is geomorphologically considered a vast, seaward extension of the Khadar land. Situated in the extreme lower reaches of the Ganga and Brahmaputra rivers, this area is overwhelmingly depositional. Because the rivers lose almost all of their kinetic energy near sea level, they drop immense quantities of sediment, comprising a complex matrix of old mud, new mud, and expansive marshes.
- Micro-Relief: Chars and Bills: Due to the sluggishness of the rivers and the sheer volume of sediment, deposition creates highly varied micro-relief across the delta. The elevated, sandy, or muddy uplands formed directly within the river channels and serving as deltaic islands are locally called “Chars”. Conversely, the adjacent low-lying, marshy, and chronically waterlogged depressions situated between river channels are known as “Bills” (or Beels).
- Tidal Interaction and Agriculture: The delta is a highly dynamic geomorphological environment shaped by the constant interaction between massive fluvial sediment discharge and powerful marine tidal forces from the Bay of Bengal. This interaction creates the world’s largest deltaic system. The constant influx of fine silt makes the higher grounds (Chars) and the reclaimed margins of the Bills exceptionally fertile, fundamentally optimizing the region for the intensive cultivation of highly water-dependent crops, specifically paddy (rice), tea, and jute.
Physiography • Doab
Q.14) In the Punjab Plain, the fertile land located between the Beas and Ravi rivers is designated as which Doab?
Ans > Bari Doab
- The Geographic Concept of a Doab: In the geographic and hydrological lexicon of the Indian subcontinent, a “Doab” translates literally to “two waters”. It formally refers to the highly fertile, alluvial tract of land situated immediately between two converging rivers. The vast Punjab plain is fundamentally structured around five such massive Doabs, which dictate the region’s agricultural and cultural geography.
- Nomenclature and the Bari Doab: The specific tract of land situated between the Beas River and the Ravi River is formally known as the Bari Doab. The nomenclature of the Doabs in this region is typically a portmanteau created by seamlessly combining the first syllables or letters of the two flanking rivers (Ba from Beas, Ri from Ravi).
- Agricultural and Economic Heartland: Because these Doabs are flanked by perennial Himalayan rivers fed by glaciers, they are guaranteed a continuous supply of both freshwater and fresh alluvial silt during seasonal flood events. This unique hydrological positioning makes the Doabs the supreme agricultural heartland of the Punjab-Haryana plain, historically supporting incredibly dense human populations and highly intensive farming systems.
- Bist-Jalandhar Doab: Flanked by Beas and Sutlej.
- Bari Doab: Flanked by Beas and Ravi.
- Rachna Doab: Flanked by Ravi and Chenab.
- Chaj Doab: Flanked by Chenab and Jhelum.
- Sind Sagar Doab: Flanked by Jhelum/Chenab and Indus.
Physiography • Soil Types
Q.15) In the Indo-Gangetic Plain, highly degraded, saline, and alkaline soils formed due to intense capillary action in dry regions are known as:
Ans > Reh or Kollar
- Arid Climates and Capillary Action: In the drier, more arid western and central tracts of the Indo-Gangetic Plain (particularly parts of Punjab, Haryana, and western Uttar Pradesh), high surface temperatures trigger rapid evaporation of soil moisture. This intense evaporation creates a hydrological deficit that draws subterranean groundwater to the surface through capillary action, pulling dissolved salts upward through the soil profile.
- Formation of Toxic Efflorescence: When the capillary water reaches the surface and evaporates, it leaves behind a highly concentrated, toxic white efflorescent crust composed of sodium, calcium, and magnesium salts. These highly degraded, saline, and alkaline soil patches are locally identified by farmers and geologists as “Reh,” “Kollar,” or “Usar” lands.
- Impact on Soil Structure and Reclamation: The unchecked accumulation of these toxic salts completely destroys the soil structure, drastically reduces permeability, and severely disrupts the osmotic balance required by plant roots. As a result, Reh lands are notoriously infertile, rendering vast tracts of otherwise flat, arable plains completely barren. Reclaiming these lands is highly resource-intensive, requiring the application of chemical amendments like gypsum to neutralize the alkalinity, followed by massive flushing with fresh water to physically leach the accumulated salts back below the root zone.
🌋 Section 3: Earthquakes in India (Q16 – Q30)
Earthquakes • IS Codes
Q.16) According to the IS 1893:2016 code, what is the assigned Zone Factor (Z) representing the peak ground acceleration for Seismic Zone V?
Ans > 0.36
- The Peak Ground Acceleration Parameter: In the realm of structural engineering, the Bureau of Indian Standards (BIS) designates the Zone Factor (Z) as a numerical representation of the Maximum Considered Earthquake (MCE) ground motion. For Seismic Zone V, representing the highest baseline risk category in the 2016 code, the assigned Z factor is 0.36. This parameter indicates an effective peak horizontal ground acceleration of 36% of gravity (0.36g) that a structure might experience during an MCE event.
- Geographical Coverage of Zone V: Zone V encompasses the most seismically volatile regions of the Indian subcontinent, covering approximately 11% of the country’s landmass. This includes the highly active tectonic collision boundary of the entire Himalayan belt, the North-Eastern states, parts of Jammu and Kashmir, Uttarakhand, the Rann of Kutch in Gujarat, and the Andaman & Nicobar Islands.
- Implications for Structural Design: Because the Z factor directly dictates the design base shear formula (V_B), structures situated in Zone V must be engineered to resist massive horizontal earthquake forces. This demands the most rigorous implementation of ductile detailing, mandating the use of Special Moment-Resisting Frames (SMRF) with maximum ductility to prevent catastrophic structural collapse, a requirement validated by historical tragedies like the 2001 Bhuj earthquake.
- Zone II (Low): Z=0.10. Examples: Hyderabad, Bengaluru, large parts of Peninsular India.
- Zone III (Moderate): Z=0.16. Examples: Mumbai, Kolkata, Chennai, Pune.
- Zone IV (Severe): Z=0.24. Examples: Delhi NCR, Patna, Dehradun, Punjab.
- Zone V (Very Severe): Z=0.36. Examples: Guwahati, Srinagar, Kutch, Andaman & Nicobar.
Earthquakes • IS Codes
Q.17) In the revisions made to the IS 1893 seismic zoning map from 2002 onwards, which previous seismic zone was completely eliminated and merged?
Ans > Zone I was merged into Zone II
- Evolution of the Seismic Zoning Map: Historically, earlier editions of the Indian seismic zoning map classified the country into five (and previously more) distinct zones based on perceived macro-seismic risk. However, driven by emerging empirical data and the destructive impacts of unexpected intraplate earthquakes, the Bureau of Indian Standards substantially revised this map in the pivotal 2002 edition (IS 1893 Part 1:2002).
- The Elimination of Zone I: The most critical macro-zonation change instituted during this revision was the complete elimination of Seismic Zone I, which previously represented areas of “minimal” or “zero” seismic risk. The geographic areas formerly designated as Zone I were merged entirely into Zone II. Consequently, Zone I no longer exists in Indian engineering codes, making Zone II (Z=0.10) the new baseline representing the lowest permissible level of seismic hazard calculation.
- Impact on Peninsular Engineering: This merger profoundly impacted structural engineering practices across Peninsular India. Previously believed to be a highly stable, aseismic craton, devastating events like the 1993 Latur earthquake (magnitude 6.2), which occurred in a region historically perceived as safe, shattered the paradigm of absolute tectonic stability. Upgrading the baseline ensures that every engineered building constructed in India must account for a minimum lateral earthquake force, legally eradicating the concept of an “earthquake-proof” geographical zone.
Earthquakes • Engineering
Q.18) Which Indian Standard (IS) code explicitly provides the mandatory requirements for the ductile detailing of reinforced concrete structures subjected to seismic forces?
Ans > IS 13920
- The Ductile Detailing Code: While IS 1893 determines the magnitude of the seismic forces a building will encounter, IS 13920 is the dedicated Code of Practice that dictates exactly how the structure’s physical detailing must be executed to resist those forces. First published in 1993 and significantly revised in 2016, IS 13920 provides the mandatory guidelines for the ductile design and detailing of monolithic reinforced concrete (RC) structures.
- The Strong Column-Weak Beam Principle: A fundamental engineering concept legally enforced by IS 13920 is the “Strong Column-Weak Beam” capacity design. Clause 7.2 of the code mandates that the sum of the moment capacities of columns at a structural joint must be at least 1.4 times the sum of the moment capacities of the beams framing into it (ΣMc ≥ 1.4 ΣMb). This critical requirement ensures that under severe seismic loading, yielding and plastic hinge formation occur in the horizontal beams (which is highly ductile and survivable) rather than the vertical columns (which causes catastrophic, pancake-style collapse).
- Strict Implementation Constraints: The rigorous ductile detailing specified by IS 13920 is legally mandatory for all multi-story RC buildings constructed in the high-risk Seismic Zones III, IV, and V. To prevent explosive concrete crushing during cyclic shaking, the code prescribes ultra-tight lateral confinement reinforcement rules, explicitly mandating minimum 135-degree hooks for stirrups and restricted spacing within the critical hinge zones at beam-column junctions.
Earthquakes • Engineering
Q.19) In the base shear calculation formula of IS 1893, what does the Response Reduction Factor (R) account for?
Ans > The perceived ductile deformation and energy dissipation capacity of the structure
- The Architecture of the Base Shear Formula: According to IS 1893:2016, calculating the total lateral force or ‘Base Shear’ (V_B) relies on determining the Design Horizontal Seismic Coefficient (A_h). This coefficient is calculated using the formula: A_h = (Z/2) × (I/R) × (S_a/g). Each variable in this equation fundamentally scales the lateral force the building must be engineered to resist.
- The Role of the Response Reduction Factor: The ‘R’ factor acts as a denominator in the formula; thus, a higher ‘R’ value mathematically reduces the required design base shear force. The R factor represents the structure’s inherent capability to undergo ductile deformation and dissipate massive amounts of seismic energy through controlled cracking, yielding, and plastic hinge formation without suffering total structural collapse.
- Dependence on Structural Detailing: The permissible value of R is strictly dependent on the structural system employed. For an Ordinary Moment Resisting Frame (OMRF) that lacks special seismic detailing, R is set at 3.0. However, for a Special Moment Resisting Frame (SMRF) that strictly adheres to the rigorous ductile detailing protocols of IS 13920, the R factor is increased to 5.0. Utilizing an R factor of 5.0 results in a substantial 40% reduction in design base shear compared to R=3.0, providing an economic incentive for engineers to implement high-ductility designs.
Earthquakes • Historic Events
Q.20) Which historic Indian earthquake was pioneeringly analyzed by R.D. Oldham, leading to the identification of the Chedrang fault and the distinction of different seismic wave types?
Ans > 1897 Assam Earthquake
- The Oldham Fault and the Chedrang Tear Fault: The Great Assam Earthquake of 12 June 1897, with an estimated moment magnitude between Mw 8.15 and 8.35, remains one of the most powerful intracontinental earthquakes ever recorded. Pioneering geologist R.D. Oldham proposed that the earthquake ruptured on a south-dipping fault near the northern edge of the Shillong Plateau, subsequently named the Oldham fault. Geodetic analysis demonstrated an astonishing average slip of 25 ± 5 meters beneath the plateau. Notably, the adjacent Chedrang fault slipped as a subvertical tear fault, registering over 10 meters of dramatic down-to-the-west normal-sense slip at the surface.
- Epistemological Shifts in Global Seismology: Oldham’s exhaustive 1899 memoir detailing the earthquake served as a watershed moment in the global field of seismology. He successfully utilized the early instrumental records of this specific event to pioneer the distinct identification and separation of primary (P) compressional waves, secondary (S) shear waves, and surface seismic waves. This breakthrough fundamentally altered how scientists understood the propagation of seismic energy through the Earth’s interior.
- Long-term Seismic Recurrence: Modern reappraisals of the triangulation data from 1897 combined with contemporary GPS velocities show approximately 5 mm/year of shortening across the plateau. This strain accumulation suggests that while magnitude 7 earthquakes may occur roughly once per century in the region, the recurrence interval for a massive, 1897-style event is several thousand years.
Earthquakes • Historic Events
Q.21) The devastating 1934 Bihar-Nepal Earthquake (Mw 8.2) is geologically infamous for generating a massive 300 km zone of:
Ans > An expansive “slump belt” characterized by extensive soil liquefaction
- Epicentral Details and Magnitude: Occurring on January 15, 1934, at approximately 2:13 PM IST, this massive Mw 8.2 earthquake had its epicenter located in eastern Nepal, just south of Mount Everest. The sheer power of the tectonic energy release caused catastrophic destruction across an immense geographic footprint, stretching from Kathmandu down deep into the northern plains of Bihar, India.
- The Formation of the Slump Belt: The 1934 earthquake is seismologically highly notable for generating a catastrophic zone of ground failure formally recognized in literature as the “slump belt”. Stretching over 300 km laterally across the deep Indo-Gangetic alluvium, this belt suffered intense ground subsidence, severely exacerbating the structural damage to the urban centers situated upon it.
- Extensive Soil Liquefaction Dynamics: Because the northern Bihar region sits atop deep, saturated, unconsolidated alluvial sediments, the extreme seismic shaking triggered massive and violent soil liquefaction. Sand and water vents erupted violently across the landscape, completely choking community wells with sand and causing widespread ground fissures. The loss of soil bearing capacity led to bizarre structural failures, where many heavy, solidly built masonry structures literally sank into the ground or floated completely off their foundations in the liquefied soil matrix, while ramshackle (‘kutcha’) buildings simply collapsed from the shaking.
Earthquakes • Historic Events
Q.22) The 1950 Assam-Tibet Earthquake (Mw 8.6) holds the seismological record as the largest known earthquake caused primarily by:
Ans > Continental collision
- The Continental Collision Record: Occurring on August 15, 1950, with a calculated moment magnitude ranging between Mw 8.6 and 8.8, the Assam-Tibet earthquake is globally recognized as the largest onshore earthquake ever recorded that was caused by active continental collision, rather than oceanic plate subduction. It resulted directly from the immense tectonic convergence stresses between the northward-moving Indian plate and the Eurasian plate at the Namche–Barwa syntaxis.
- Complex Subsurface Rupture Mechanics: The epicenter was located deep within the rugged Mishmi Hills near the McMahon Line. Scientific analysis of the seismic waveforms reveals that the event involved highly complex oblique-slip rupture across multiple thrust fault planes. The rupture propagated simultaneously along the northwest-dipping Main Frontal Thrust (MFT) and the northeast-dipping Mishmi Thrust (MT), generating immense slip values averaging between 11 and 17 meters, completely reshaping the local geomorphology.
- Acoustic Anomalies and Fluvial Catastrophes: A highly documented and terrifying anomaly of this earthquake was the generation of extraordinary, deafening acoustic noises originating from the earth, reported extensively by botanist Frank Kingdon-Ward and locals across Assam and Tibet. The colossal ground shaking triggered massive landslides across the Abor and Mishmi hills, which catastrophically blocked major tributaries of the Brahmaputra River, notably the Subansiri. When these massive natural landslide dams subsequently burst days later, they unleashed devastating 7-meter-high flash floods, submerging entire villages and severely compounding the earthquake’s death toll.
Earthquakes • Historic Events
Q.23) The 1967 Koyna Earthquake (Mw 6.3) is globally recognized as the most prominent example of:
Ans > Reservoir-Induced Seismicity (RIS)
- The Paradigm of Reservoir-Induced Seismicity: Striking on December 10, 1967, the devastating Koyna earthquake is universally cited in geophysics as the world’s most significant and extensively studied example of Reservoir-Triggered Seismicity (RTS) or Reservoir-Induced Seismicity (RIS). The magnitude 6.3 event occurred shortly after the impoundment of the massive Koyna Dam reservoir in the Deccan Traps, a region previously presumed to be an entirely stable, aseismic cratonic block.
- Pore Fluid Pressure and Fault Lubrication: Scientific investigations concluded that the primary trigger mechanism was not merely the static weight of the impounded water pushing down on the crust, but rather the deep percolation of reservoir water into ancient basaltic fractures. This hydraulic infiltration fundamentally altered the subsurface pore fluid pressure. The introduction of super-hydrostatic pressure effectively lubricated pre-existing, critically stressed fault planes located at seismogenic depths of 5 to 13 km, dramatically lowering their frictional resistance and triggering massive rupture.
- Kinematics and Engineering Impact: Initial conventional P-wave polarity studies and subsequent advanced broadband waveform inversions determined that the Koyna mainshock resulted predominantly from a strike-slip faulting mechanism, operating within the ambient stress field generated by the northward push of the Indian plate. The disaster, which caused roughly 200 fatalities, shattered the prevailing assumption that large dams could be safely constructed anywhere in Peninsular India. It mandated sweeping global revisions in dam safety protocols, cementing RIS as a critical, non-negotiable factor in civil engineering and large-scale environmental impact assessments.
Earthquakes • Historic Events
Q.24) Which major Himalayan thrust fault was primarily associated with the rupture mechanism of the 1991 Uttarkashi Earthquake?
Ans > The Main Central Thrust (MCT)
- Rupture Along the Main Central Thrust: The 1991 Uttarkashi earthquake (frequently referred to as the Garhwal earthquake) occurred on October 20 with a moment magnitude of 6.8. Extensive instrumental data and fault plane solutions definitively localized this thrust event along the primary, northwest-striking Main Central Thrust (MCT) within the Garhwal region of Uttarakhand.
- Asymmetric Slip and Energy Release: Analysis of robust data from six three-axis accelerometers recorded within 60 km of the epicenter revealed a highly asymmetric distribution of fault slip. The maximum recorded slip of approximately 1.5 meters occurred primarily to the west and southwest of the hypocenter, representing a typical low-angle thrust mechanism characteristic of the Himalayan collision zone. The seismic energy release began slowly before building to a massive crescendo four seconds into the rupture.
- Devastation and Seismotectonic Implications: The seismic event generated an extreme peak ground acceleration of 0.31g and reached a maximum Mercalli intensity of IX (Violent). This immense energy release devastated traditional heavy-roofed stone masonry homes, resulting in up to 2,000 fatalities and the destruction of tens of thousands of structures. Crucially for seismologists, while theoretical models predicted that such ruptures would propagate southward fully to the Main Frontal Thrust (MFT), the 1991 Uttarkashi rupture fell short of reaching the surface trace of the MFT. This anomalous behavior provided vital data regarding the complex locking zones and creeping transitions within the Himalayan flat-ramp-flat décollement system.
Earthquakes • Historic Events
Q.25) The occurrence of the 1999 Chamoli earthquake was notably preceded by a distinct period of:
Ans > Relative seismic quiescence in the region
- Seismic Quiescence and Stress Shadows: The 1999 Chamoli earthquake (Mw 6.6) struck approximately 8 years after, and just 65.5 km southeast of, the 1991 Uttarkashi event. Retrospective statistical analysis of regional micro-seismicity revealed a highly significant 2.0-year period of relative seismic quiescence immediately preceding the main shock. Geophysicists determined that the Coulomb stress changes (ΔCFS) generated by the earlier Uttarkashi earthquake created a temporary “stress shadow”. This stress shadow suppressed local micro-seismicity, allowing elastic strain to quietly accumulate before ultimately loading the adjacent Chamoli fault zone to the point of catastrophic rupture.
- Crustal Shortening and Ground Deformation: Like Uttarkashi, the Chamoli earthquake resulted directly from relentless crustal shortening along the northern edge of the Indian plate. The event occurred via a thrust mechanism associated with the complex sub-thrust network of the Main Central Thrust (MCT). The earthquake unleashed severe topographic alterations, generating deep ground fissures in towns like Gopeshwar, cracking asphalt roads, and triggering massive landslides that completely obliterated sections of the Mandakini and Mandal valleys.
- Topographic Amplification of Damage: The disaster killed roughly 103 people and damaged over 50,000 houses across numerous districts. Notably, the structural destruction was heavily amplified by local topography; houses and commercial properties constructed haphazardly on the steep slopes of the lower town of Chamoli suffered near-total destruction compared to those situated on flatter, more stable terrain, highlighting the critical role of site effects in Himalayan seismicity.
Earthquakes • Zoning
Q.26) The specialized process undertaken by the Indian government to subdivide mega-cities like Delhi and Kolkata into detailed, highly localized seismic risk zones based on soil and geological characteristics is termed:
Ans > Seismic Microzonation
- Moving Beyond Macro-Zonation: While the IS 1893 standard effectively divides the entire Indian landmass into broad macro-zones (II through V), these large-scale classifications fail to account for highly localized, block-by-block variations in soil mechanics and geology. Seismic Microzonation is the advanced scientific process of subdividing a potential earthquake-prone city or region into granular, highly detailed zones to assess specific localized risks.
- Integrating Geotechnical and Hydrological Variables: Microzonation maps are generated by meticulously integrating local geological, seismological, hydrological, and geotechnical site characteristics through extensive fieldwork and borehole sampling. Because deep, soft alluvial soils (like those found in the Indo-Gangetic plain) can violently amplify seismic waves compared to hard bedrock, understanding the exact subterranean layout is critical for predicting surface-level Peak Ground Acceleration (PGA).
- Policy Implementation for Mega-Cities: The Ministry of Earth Sciences, in consortium with premier institutes like IIT Kharagpur, completed landmark microzonation reports for highly vulnerable mega-cities like New Delhi (macro Zone IV) and Kolkata (macro Zone III/IV border). The primary objective of generating this granular data is to inform highly specific urban planning and safe building construction codes. By precisely identifying localized liquefaction threats and amplification hot-spots, civic bodies can mandate targeted, plot-specific structural reinforcements, thereby minimizing future property damage and loss of human life.
Earthquakes • Peninsular Block
Q.27) Which prominent central Indian tectonic feature acts as an ancient active palaeorift separating the northern and southern blocks of the Peninsular shield and generates recurrent intraplate earthquakes?
Ans > The Narmada-Son Lineament
- A Deep Crustal Palaeorift: The Narmada-Son Lineament (NSL), frequently grouped with the Tapti fault, is a massive ENE-WSW trending tectonic feature traversing central India. It represents a highly active, ancient continental palaeorift that fundamentally divides the Indian peninsular shield into the northern (Bundelkhand craton) and southern (Dharwar/Bastar cratons) geological blocks.
- Recurring Intraplate Seismicity: Although located hundreds of kilometers away from the active Himalayan plate boundary, the NSL is characterized by recurrent and highly destructive intraplate seismicity. Notable moderate-to-large continental midplate earthquakes, such as the 1927 Son-valley (Mw 6.4) and the 1997 Jabalpur (Mw 5.8) earthquakes, were triggered by tectonic slippage along the Narmada South Fault (NSF) within this lineament complex.
- Thermo-Tectonic History and Stress Regimes: The elevated seismogenic potential of the NSL is exacerbated by highly specific crustal conditions. High lower-crustal pore pressures and lower frictional coefficients—derived from past episodes of thermo-tectonic processes, including massive Deccan volcanism triggered by the Réunion plume interaction—make the faults highly susceptible to rupture. The regional faults within the lineament respond acutely to the prevailing N-S compressive stress regime generated by the relentless northward motion of the Indian plate. This constant tectonic pressure causes the periodic reactivation of the pre-existing rift faults, definitively invalidating the antiquated geological assumption that the Peninsular Plateau is a completely stable, aseismic entity.
Earthquakes • Himalayan Belt
Q.28) In the structural architecture of the Himalayas, which thrust fault represents the southernmost, youngest, and most active expression of the collision boundary interacting directly with the Indo-Gangetic Plain?
Ans > The Himalayan Frontal Thrust (HFT)
- The Himalayan Frontal Thrust (HFT): The Himalayan Frontal Thrust (HFT), sometimes referred to as the Main Frontal Thrust (MFT), constitutes the southernmost, youngest, and currently most active principal thrust fault in the Himalayan orogenic wedge. It represents the immediate active tectonic boundary where the advancing, folded Himalayan foothills are mechanically overriding the soft Quaternary sediments of the Indo-Gangetic Plain.
- Evolution of the Thrust Belt: The Himalayan thrust architecture features a clear southward progression of activity. Situated further north of the HFT, the Main Boundary Thrust (MBT) is an older parallel fault system that typically separates the lesser Himalayas from the sub-Himalayan Shiwalik formations. Located even deeper in the hinterland is the Main Central Thrust (MCT), which separates the Lesser Himalayas from the High Himalayan crystalline rocks. While earthquakes like Uttarkashi (1991) proved the MCT remains active, the primary locus of current tectonic shortening and deformation has largely migrated southward to the HFT.
- Strain Accumulation and Slip Deficits: Modern GPS geodetic studies reveal a massive slip deficit accumulating across the Himalayan Seismic Belt. The ongoing convergence of the Indian plate is effectively locked along the shallow décollement joining the HFT, MBT, and MCT at depth, generating immense elastic strain within the crust. When this heavily locked zone eventually ruptures in a major earthquake, the energy will propagate directly down to the HFT, posing an existential catastrophic threat to the densely populated, sediment-amplified plains immediately to the south.
Earthquakes • NCR Zoning
Q.29) The heightened seismic hazard in the Delhi National Capital Region (NCR) is primarily attributed to its location over thick alluvial sediments and its proximity to the junction of which two subsurface structures?
Ans > Delhi-Hardwar Ridge and Delhi-Sargodha Ridge
- The Subsurface Ridge Junction: The Delhi National Capital Region (NCR) sits directly above a highly complex and active subsurface tectonic junction. The prominent Delhi-Hardwar Ridge (DHR), striking NE-SW, serves as the western boundary of the Ganga Basin, with its central axis located a mere 400 meters below the surface alluvium. At a crustal depth of approximately 12-13 km beneath the region, the NW-SE trending Delhi-Sargodha Ridge (DSR) bifurcates directly from the DHR.
- Localized Intraplate Stress Concentration: The physical intersection of these rigid cratonic extensions, combined with associated local faults like the Mahendragarh-Dehradun Fault (MDF) and the Moradabad Fault, creates a localized zone of intense crustal stress. This junction acts as an impediment to the regional tectonic stresses, rendering the area a hotbed for shallow, intraplate seismic activity completely independent of the main Himalayan fault lines.
- Amplification by Loose Sediments: While the NCR is located over 200 km south of the primary Himalayan plate boundary, its seismic hazard is severely exacerbated by the overlying geology. The deep, loose quaternary sediments and alluvium of the Indo-Gangetic Plain strongly amplify the seismic waves generated by both distant Himalayan mega-quakes and local ridge-fault slippages. This potent combination of an active local fault junction and massive sedimentary amplification justifies Delhi’s high-risk placement in Seismic Zone IV (Z=0.24). Historical damage, such as the axial fracturing observed on the 15th-century Qutab Minar during the 1803 earthquake, underscores the acute vulnerability of this densely populated region to soil liquefaction and wave resonance.
Earthquakes • IS Codes
Q.30) The proposed IS 1893:2025 standard introduces a paradigm shift in Indian seismic engineering by moving from Deterministic Hazard Assessment to which framework, accompanied by the introduction of Zone VI?
Ans > Probabilistic Earthquake Hazard Assessment (PEHA)
- Shift to Probabilistic Hazard Assessment: The publication of the IS 1893:2025 (Parts 1 and 5) represents a monumental, paradigm-shifting update in Indian civil and structural engineering. It decisively moves the national building code away from the older, rigid Deterministic Earthquake Hazard Assessment (DEHA) toward a highly dynamic, statistically driven Probabilistic Earthquake Hazard Assessment (PEHA) framework. This transition aligns Indian codes more closely with advanced global standards, incorporating probabilistic return-period-based structural importance classifications.
- The Introduction of Seismic Zone VI: In stark recognition of regions exhibiting extreme, outlier seismic risk based on historical data and strain accumulation, the 2025 amendment formally introduces Seismic Zone VI as the highest hazard category. Assigned a staggering zone factor of Z = 0.75, this new classification is specifically targeted at the most volatile regions of the Himalayan belt and North-Eastern states. This represents a massive escalation from the previous maximum of Zone V (Z = 0.36) found in the 2016 code.
- Heightened Engineering Demands and Economic Balancing: From a quantitative perspective, the escalation from Zone V to Zone VI more than doubles the seismic input required in the design base shear formulation for equivalent structures. Comparative software analyses on multi-story RC buildings indicate that the new provisions can yield base shear increases of up to 177% and maximum story drift increases of over 190% relative to the older IS 1893:2016 standard. This paradigm shift forces structural engineers to adopt a mindset that anticipates severe ground motion potential. The new code implements refined engineering checks, including shear-wave-velocity-based soil classification, to ensure global stability and prevent gravity-load collapse in regions now statistically recognized as capable of generating catastrophic seismic energy.
- Assessment Framework: IS 1893:2016 uses Deterministic (DEHA). IS 1893:2025 uses Probabilistic (PEHA).
- Highest Seismic Zone: IS 1893:2016 is Zone V. IS 1893:2025 is Zone VI.
- Maximum Z-Factor: IS 1893:2016 is 0.36. IS 1893:2025 is 0.75.
- Design Spectra Extent: IS 1893:2016 up to 6 seconds. IS 1893:2025 up to 10 seconds.
- Soil Classification: IS 1893:2016 uses Generic Type I, II, III. IS 1893:2025 uses Shear-wave velocity based classification.
📌 Quick Summary — Geography Set 15
🌍 Section 1: Origin and Evolution of the Great Plains
- Foredeep Synclinorium Theory: Proposed by Eduard Suess; a vast depression infilling with detritus.
- Rift Valley Hypothesis: Proposed by Sir Sydney Burrard; an immense structural trough between faults.
- Tethys Sea Remnant: Trough left after the Himalayan orogeny, filled by river rejuvenation.
- Basement Topography: Compartmentalized by subsurface features like the Delhi-Sargodha Ridge.
- Asymmetric Sedimentary Wedge: Sediment thickens progressively from southwest to northeast.
🏞️ Section 2: Physiographic Divisions of the Great Plains
- Bhabar: Narrow, highly porous gravel belt where streams disappear underground.
- Terai: Marshy, waterlogged tract with re-emerging subterranean streams.
- Bhangar: Older, elevated upland alluvium characterized by calcareous ‘Kankar’ nodules.
- Khadar: Newer, highly fertile active floodplains enriched by fresh annual silt.
- Rajasthan Bagar: Semi-arid region drained by seasonal streams creating fertile ‘Rohi’ patches.
- Punjab Plains: Characterized by bluff-like ‘Dhayas’ and erosional ‘Chos’.
- Rarh Plain: Undulating western margin terrain of old lateritic alluvium.
- Delta Plains: Dominated by river deposition micro-reliefs called ‘Chars’ (uplands) and ‘Bills’ (depressions).
- Doabs: Fertile tracts between converging rivers (e.g., Bari Doab between Beas and Ravi).
- Reh or Kollar: Toxic, saline efflorescent crusts formed by capillary action in dry zones.
🌋 Section 3: Earthquakes in India
- Zone V Factor: Assigned Z = 0.36 in the IS 1893:2016 code.
- Seismic Zone I: Completely eliminated and merged into Zone II in the 2002 revision.
- IS 13920: Mandates ductile detailing for reinforced concrete structures.
- Response Reduction Factor (R): Accounts for the structure’s ductile deformation capacity.
- 1897 Assam Earthquake: Identified Chedrang tear fault; Oldham separated seismic wave types.
- 1934 Bihar-Nepal Earthquake: Infamous for generating a massive 300 km “slump belt”.
- 1950 Assam-Tibet Earthquake: Largest known earthquake caused primarily by continental collision.
- 1967 Koyna Earthquake: Most prominent global example of Reservoir-Induced Seismicity (RIS).
- 1991 Uttarkashi Earthquake: Primarily ruptured along the Main Central Thrust (MCT).
- 1999 Chamoli Earthquake: Preceded by a distinct period of seismic quiescence.
- Seismic Microzonation: Highly localized, detailed risk zonation based on soil characteristics.
- Narmada-Son Lineament: Ancient active palaeorift generating recurrent intraplate earthquakes.
- Himalayan Frontal Thrust (HFT): Southernmost, youngest, and most active collision boundary fault.
- Delhi NCR Seismic Hazard: Exacerbated by deep sediments and the subsurface Ridge junction.
- IS 1893:2025 Draft: Moves to Probabilistic Earthquake Hazard Assessment and introduces Zone VI (Z=0.75).
Interactive Practice Quiz: Indian Geography
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