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UPSC MainsGeography (Optional)GeographyPractice question

Earthquake Magnitude, Intensity, and Seismic Zonation

Discuss the methods of measuring the intensity and magnitude of earthquakes. How are seismic zones demarcated?

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How to approach

Begin by defining earthquakes and clarifying the fundamental conceptual distinction between magnitude (energy released) and intensity (perceived and structural damage). In the body, systematically discuss the quantitative instrumental magnitude scales (Richter, body wave, surface wave, moment magnitude) and qualitative intensity scales (MMI, MSK-64), followed by the geological and engineering principles governing seismic zonation, using the Indian Bureau of Indian Standards (BIS) framework as a prime case. Conclude by emphasizing the necessity of microzonation and compliance with international disaster resilience frameworks.

Model answer

669 words

Introduction

An earthquake is a transient, elastic ground vibration triggered by the sudden, rapid release of strain energy along crustal fault planes. Seismological risk assessment and structural design depend upon two distinct parameters: magnitude, which quantitatively reflects the physical energy radiated from the focus, and intensity, which qualitatively measures the local ground shaking and observed damage at specific surface locations.

1. Methods of Measuring Earthquakes

Earthquake measurement is categorized into magnitude scales measuring instrumental energy release and intensity scales measuring observed empirical impacts.

A. Magnitude Scales (Quantitative Energy Released)

  • Local Magnitude (ML / Richter Scale): Formulated by Charles Richter (1935), it calculates the logarithm of the maximum trace amplitude (A) recorded on a standard Wood-Anderson torsion seismograph calibrated to a reference distance: ML = log10(A) - log10(A0). However, it saturates at approximately ML ≈ 6.5 because high-frequency waves do not scale proportionally with very large rupture areas.
  • Body Wave Magnitude (mb): Measures the amplitude of the first compressional P-waves with a period of around 1 second: mb = log10(A/T) + Q(h, Δ). It is effective for deep-focus events but typically saturates above magnitude 6.5.
  • Surface Wave Magnitude (Ms): Based on the amplitude of fundamental Rayleigh surface waves with a period of approximately 20 seconds: Ms = log10(A/T) + 1.66log10(Δ) + 3.3. It saturates near magnitude 8.0.
  • Moment Magnitude Scale (Mw): Developed by Thomas Hanks and Hiroo Kanamori (1979), this scale avoids saturation during major earthquakes by measuring the physical Seismic Moment (M0): Mw = (2/3)log10(M0) - 6.06 (with M0 in N·m). Here, M0 = μAD, where μ is the shear modulus of the faulted rock, A is the rupture surface area, and D is the average fault displacement.

B. Intensity Scales (Qualitative Damage Impact)

  • Modified Mercalli Intensity (MMI) Scale: A closed 12-point scale (I to XII) based on human sensory perception, structural damage, and secondary ground effects, ranging from instrumental detection to total devastation.
  • MSK-64 Scale (Medvedev-Sponheuer-Karnik): A comprehensive 12-degree intensity scale widely utilized in Eurasia and India. It reduces subjective assessment bias by classifying civil structures into vulnerability typologies (such as adobe, unreinforced brick masonry, and reinforced concrete) and categorizing damage severity quantitatively.

2. Demarcation of Seismic Zones

Seismic zonation partitions geographic territory into regulatory zones to guide structural engineering codes and land-use planning. Demarcation utilizes both Probabilistic Seismic Hazard Assessment (PSHA) and Deterministic Seismic Hazard Assessment (DSHA) through the following criteria:

  • Peak Ground Acceleration (PGA): Evaluates the maximum expected horizontal ground acceleration across specific return periods, which defines the regulatory Zone Factor (Z).
  • Seismotectonic Mapping: Delineation of active faults, crustal lineaments, paleoseismic trenching, and historical epicentral distributions.
  • Local Site Conditions and Attenuation: Spatial analysis of shear-wave velocity across the upper 30 meters (Vs30), soil depth, and susceptibility to liquefaction.

The Indian Seismic Zonation Framework

Governed by the Bureau of Indian Standards (BIS) standard IS 1893 (Part-1): 2016, India is classified into four macro-zones. Zone I was merged into Zone II following the 1993 Latur and 1997 Jabalpur intraplate earthquakes, which demonstrated that no region in the Indian craton has zero seismic hazard:

  • Zone V (Zone Factor Z = 0.36; Very High Hazard): Encompasses active tectonic regimes including Northeast India, parts of Jammu & Kashmir, Himachal Pradesh, Uttarakhand, and the Rann of Kutch.
  • Zone IV (Zone Factor Z = 0.24; High Hazard): Encompasses the Indo-Gangetic Foredeep, Delhi-NCR, Jammu and Kashmir margins, and northern Bihar.
  • Zone III (Zone Factor Z = 0.16; Moderate Hazard): Includes the Western Ghats, parts of the Deccan margin, and the Bengal Delta.
  • Zone II (Zone Factor Z = 0.10; Low Hazard): Covers the stable crystalline shield and Peninsular craton.

Conclusion

Transitioning from broad macro-zonation to localized seismic microzonation is essential for climate-resilient urban agglomerations. Incorporating micro-level geotechnical and geomorphological parameters ensures high-integrity building design and aligns national infrastructure development with the priorities of the Sendai Framework for Disaster Risk Reduction.

Key facts to remember

definition
Seismic Moment (M0)

A physical measure of the total work done by an earthquake, calculated as M0 = μAD, where μ is the rock's shear modulus, A is the ruptured fault area, and D is the average slip.

scheme
IS 1893 (Part 1): 2016

The Bureau of Indian Standards seismic design code that categorizes India into Seismic Zones II through V and provides design spectra and zone factors for earthquake-resistant construction.

case study
1993 Latur Earthquake and Zone I Elimination

The 1993 Killari (Latur) intraplate earthquake (Mw 6.2) struck a region previously designated as stable Zone I, prompting the Bureau of Indian Standards to merge Zone I into Zone II in 2002 to eliminate the misconception of zero seismic risk.

Frequently asked questions

Why does the Richter scale saturate during large earthquakes?

The Richter scale relies on high-frequency seismic waves recorded by standard seismographs. In major earthquakes with massive fault slip zones, high-frequency wave amplitudes stop growing proportionally with energy release, causing the scale to plateau around magnitude 6.5.

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