(a) Discuss the intensity and magnitude scale commonly used to assess seismic damage. Write a note on the global distribution…

(a) Intensity and magnitude scales; global distribution; seismic zones of India

Magnitude measures the size of an earthquake at its source and has one value per event. It is logarithmic.

• Local (Richter) magnitude, ML (1935): based on the maximum trace amplitude on a Wood–Anderson seismograph, corrected for distance. It saturates for large events.
• Body-wave (mb) and surface-wave (Ms) magnitudes: used for distant events.
• Moment magnitude, Mw (Kanamori, 1977): based on seismic moment M₀ = μ × fault area × average slip, and does not saturate. It is the standard for large earthquakes today.
• One unit of magnitude means about 10 times the ground amplitude and about 32 times the energy released.

Intensity measures the effects of shaking at a particular place. It varies from place to place for the same earthquake and is therefore the measure used to assess damage.

• Modified Mercalli (MM) scale, I–XII: from "not felt" (I) to "total destruction" (XII).
• MSK-64 scale, I–XII: used in India, including in IS 1893, and grades damage by building type (A: adobe and rubble, B: brick, C: reinforced concrete) and damage grade.
• EMS-98: the European successor to MSK.
• Isoseismal maps join points of equal intensity. Intensity depends on magnitude, focal depth, distance, local soil (amplification on soft alluvium) and construction quality.

Global distribution

• Circum-Pacific "Ring of Fire": about 80% of the world's earthquakes and most of the largest ones (Chile 1960, Mw 9.5; Alaska 1964; Sumatra 2004; Tohoku 2011). It follows subduction zones, where Wadati–Benioff zones carry shallow, intermediate and deep-focus events down to about 700 km.
• Alpide (Mediterranean–Himalayan) belt: roughly 15–17% of earthquakes, from the Azores through the Alps, Turkey and Iran to the Himalaya and Myanmar, marking continental collision.
• Mid-oceanic ridges and transform faults: shallow, moderate-magnitude events along divergent boundaries.
• Continental rifts and intraplate areas: East African Rift, and stable-continent events such as Latur (1993) and Jabalpur (1997) in India, related to reactivated old faults.

Seismic zones of India. The zonation in force is IS 1893 (Part 1): 2016, with four zones. A 2025 revision that placed the whole Himalayan arc in a new Zone VI was announced in late 2025 but withdrawn by BIS in March 2026, so the 2016 map remains the reference.

• Zone V (very high, zone factor 0.36): all of the North-Eastern states, the Kashmir valley and parts of Himachal Pradesh and Uttarakhand, north Bihar along the Nepal border, the Rann of Kutch, and the Andaman and Nicobar Islands. Examples: Assam 1950 (Mw 8.6), Bhuj 2001 (Mw 7.7).
• Zone IV (high, 0.24): the rest of Jammu and Kashmir, Ladakh, Himachal Pradesh and Uttarakhand; Sikkim; Delhi; the northern parts of Punjab, Haryana, Uttar Pradesh, Bihar and West Bengal; parts of Gujarat; and the Koyna region of Maharashtra (Koyna 1967, M 6.3).
• Zone III (moderate, 0.16): Kerala, Goa, Lakshadweep, and large parts of Uttar Pradesh, Madhya Pradesh, Maharashtra (Latur, 1993), Odisha, Chhattisgarh, Jharkhand, Karnataka, Tamil Nadu and Andhra Pradesh.
• Zone II (low, 0.10): the remaining stable parts of the Peninsula, western Rajasthan and the Deccan interior.

Himalayan seismicity reflects the continuing northward movement of the Indian plate at about 4–5 cm a year, with thrusting on the Main Himalayan Thrust. The Peninsula's moderate hazard reflects intraplate stress on ancient rift and shear zones.

(b) Geomorphic cycle of Davis and Penck

W.M. Davis (1899), the "geographical cycle"

• Landscape is a function of structure, process and stage.
• It assumes rapid uplift of a landmass, followed by a long stillstand during which erosion works down to base level.
• Youth: deep V-shaped valleys, high relief, waterfalls and rapids, and wide undissected interfluves. Maturity: maximum relief early, then graded rivers, wide valleys and rounded divides. Old age: a low undulating peneplain with residual hills (monadnocks) and meandering rivers.
• Slopes decline over time, becoming gentler as the cycle advances. Renewed uplift causes rejuvenation, with knickpoints and incised meanders.
• Criticism: uplift is rarely rapid and followed by long stability, the model is descriptive and time-bound, and it neglects climate and process rates.

Walther Penck (1924), "Die morphologische Analyse"

• Uplift and denudation act simultaneously. Landforms reflect the ratio between the rate of uplift and the rate of erosion, not a sequence of stages in time.
• Waxing development (aufsteigende Entwicklung): uplift accelerates faster than erosion, producing convex slopes.
• Uniform development (gleichförmige Entwicklung): uplift and erosion balance, producing straight slopes.
• Waning development (absteigende Entwicklung): uplift slows, producing concave slopes.
• Slopes evolve by slope replacement, retreating while keeping their angle, leading to an end surface (Endrumpf). He described the Primärrumpf (initial low surface) and Piedmonttreppen (piedmont steps) from intermittently expanding uplift.
• Criticism: his work was published posthumously and was hard to understand in translation. It also lacked field tests of uplift rates.

Comparison: Davis's model is time-dependent, with slope decline. Penck's is time-independent, with slope replacement and parallel retreat, linking endogenetic and exogenetic forces. Modern geomorphology draws on both: Davis for the idea of base level and Penck for process–tectonic interaction and slope analysis.

(c) Contours on a toposheet and identification of landforms

Contours are lines joining points of equal height above mean sea level. On Survey of India toposheets they are drawn in brown, at intervals chosen according to scale (for example 20 m on 1:50,000 sheets). The contour interval is the vertical difference between successive contours, and the horizontal equivalent is the ground distance between them. Slope is the ratio of the two.

Reading landforms from contours

• Slope: closely spaced contours show a steep slope; widely spaced contours a gentle slope; evenly spaced contours a uniform slope. Close contours at the top and wide at the bottom indicate a concave slope, and the reverse a convex slope.
• Hill or conical hill: closed concentric contours with values increasing towards the centre.
• Plateau: closely spaced contours at the edges and widely spaced or no contours on the flat top.
• Depression or crater: closed contours with values decreasing inwards, usually marked with ticks (hachures).
• Valley: V-shaped contours with the apex pointing upstream, towards higher ground. A gorge has closely spaced V-contours.
• Spur: V- or U-shaped contours with the apex pointing downslope, towards lower ground.
• Ridge: elongated closed contours; saddle or col: a low point between two sets of closed contours.
• Cliff or escarpment: contours merge or run very close together.
• Waterfall: contours cross a river at one point, crowded together.
• Terraces: alternating bands of widely and closely spaced contours along a valley side.

Drainage lines, spot heights, benchmarks and triangulation stations help confirm these forms.