(a) What are hanging wall and footwall of a fault? Describe the different classes of faults based on relative movement of walls. (20 marks)
(b) (i) What are atmospheric windows in Remote sensing? (ii) What are spectral reflectance curves? Explain how

Question

(a) What are hanging wall and footwall of a fault? Describe the different classes of faults based on relative movement of walls. (20 marks)
(b) (i) What are atmospheric windows in Remote sensing? (ii) What are spectral reflectance curves? Explain how clear water, dry soil and healthy vegetation can be demarcated using spectral reflectance curves with neat sketches. (15 marks)
(c) Define drainage pattern, Drainage texture and Drainage anomaly. Also give their general classification. Discuss the significance of those in geological interpretation of aerial photos and satellite images. (15 marks)

UPSC Answer Check · Accepted Answer

This question demands descriptive-explanatory responses across three distinct domains: structural geology, remote sensing physics, and geomorphological interpretation. Allocate approximately 40% of time/words to part (a) given its 20 marks weightage, with ~30% each to parts (b) and (c). Structure each part with clear definitions first, followed by systematic classification or explanation, and conclude with applications—particularly emphasizing Indian examples like the Siwalik frontal faults, Narmada-Tapti lineaments, or Ganga-Yamuna drainage integration.
- Part (a): Precise definition of hanging wall (block above fault plane) and footwall (block below fault plane) with reference to fault plane geometry; classification of faults into dip-slip (normal, reverse, thrust), strike-slip (dextral, sinistral), and oblique-slip with relative movement descriptions
- Part (a): Distinction between normal faults (extensional, hanging wall down) and reverse/thrust faults (compressional, hanging wall up); mention of Anderson's theory of faulting and stress regimes
- Part (b)(i): Definition of atmospheric windows as specific wavelength bands (0.4-2.5 μm visible-NIR, 3-5 μm and 8-14 μm thermal IR) where atmospheric absorption is minimal; explanation of atmospheric constituents (H2O, CO2, O3) that cause absorption outside these windows
- Part (b)(ii): Definition of spectral reflectance curves showing reflectance vs wavelength; clear water (high absorption in NIR, peak in blue-green), dry soil (gradual increase with wavelength, iron oxide absorption), healthy vegetation (chlorophyll absorption in red, high NIR reflectance due to leaf structure) with characteristic curve shapes
- Part (c): Definitions—drainage pattern (arrangement of streams in plan), drainage texture (stream frequency/density), drainage anomaly (deviation from expected pattern); classifications including dendritic, trellis, radial, rectangular patterns and coarse/medium/fine textures
- Part (c): Significance in geological interpretation: drainage patterns reveal rock type and structure (dendritic on homogeneous rocks, trellis on folded strata, radial on domes/volcanoes); texture indicates permeability and runoff; anomalies (offset, deflection, ponding) indicate active tectonics, buried structures, or lithological boundaries—specifically applied to lineament mapping in Indian cratons like Dharwar and Aravalli

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	25%	12.5	Demonstrates flawless understanding across all three parts: for (a) correctly identifies hanging wall/footwall relative to fault plane dip and applies Anderson's theory; for (b) accurately describes atmospheric window physics and spectral signatures with correct wavelength ranges; for (c) precisely distinguishes between pattern, texture, and anomaly with genetic explanations	Provides generally correct definitions but confuses hanging wall/footwall in specific orientations, omits Anderson's theory, gives imprecise wavelength ranges for windows, or conflates drainage pattern with texture	Fundamental errors such as reversing hanging wall/footwall movement directions, confusing emission with reflectance in thermal windows, or treating drainage pattern and texture as synonymous concepts
Diagram / cross-section	25%	12.5	Produces three distinct, labeled diagrams: fault geometry cross-sections showing hanging wall/footwall and movement arrows for each fault class; spectral reflectance curves with axes labeled (wavelength vs % reflectance), distinct curves for water/soil/vegetation with key absorption features marked; drainage pattern sketches showing dendritic, trellis, radial types with structural controls indicated	Provides diagrams with most labels present but missing critical elements like movement arrows on faults, wavelength values on spectral curves, or structural control lines on drainage patterns; diagrams present but not 'neat' as demanded	Diagrams absent, unlabeled, or conceptually wrong—e.g., normal fault drawn with hanging wall up, spectral curves showing vegetation with highest red reflectance, or drainage patterns without genetic structural context
Field evidence	15%	7.5	Cites specific Indian field examples: for (a) mentions Main Boundary Thrust/Main Central Thrust in Himalayas, Koyna-Warna fault zone, or Aravalli strike-slip faults; for (c) references Son-Narmada lineament drainage deflection, Chambal ravines showing anomalous drainage, or Ganga megafan drainage patterns; demonstrates understanding of how field observations validate remote sensing interpretations	Mentions generic or non-Indian examples, or cites Indian localities without specific structural/drainage details; field evidence mentioned but not integrated with remote sensing applications	No field evidence cited, or completely inappropriate examples that demonstrate misunderstanding of geological setting (e.g., citing volcanic radial drainage for the Gangetic plain)
Quantitative reasoning	15%	7.5	Provides quantitative specifications: fault dip angles (Anderson's 90°, 45°, 0° for σ1 orientations), exact atmospheric window ranges (0.4-0.7 μm VIS, 0.7-1.3 μm NIR, etc.), reflectance percentages or ratios (NDVI = (NIR-Red)/(NIR+Red)), drainage density values (km/km²) for different textures; uses these quantitatively to compare materials	Mentions some numerical ranges but imprecisely (e.g., 'visible to infrared' without μm values), or provides numbers without explaining their significance in interpretation	Entirely qualitative treatment where quantitative parameters are essential; no mention of wavelength values, dip angles, or measurable drainage parameters
Indian / economic relevance	20%	10	Explicitly connects all three parts to Indian applications: (a) seismic hazard assessment along Himalayan frontal faults, Koyna reservoir-triggered seismicity; (b) groundwater exploration using NIR bands in hard rock terrains of Peninsular India, crop health monitoring for Indian agriculture; (c) mineral exploration targeting lineaments in Singhbhum, Dharwar craton gold belts, groundwater prospecting in Rajasthan using drainage anomaly mapping	Mentions Indian relevance for one or two parts but not all; applications stated without specific regional context or economic significance explained	No Indian examples, or purely theoretical treatment ignoring the critical applied aspects that make these topics relevant for civil services (disaster management, water resources, mineral exploration)

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