---
title: "Magnitude and intensity of earthquake"
subject: "Engineering Geology II"
chapter: 5
key: "semester_4/engineering_geology_ii:earthquake-magnitude-intensity"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/earthquake-magnitude-intensity
site: BCT Engineering
---

# Magnitude and intensity of earthquake

Engineering Geology II · Chapter 5: Geological Hazards

- Asked in: 2080 Chaitra, 6 marks; 2078 Chaitra, 3 marks; 2076 Baisakh, 1 marks; 2076 Bhadra, 3 marks; 2075 Bhadra, 1 marks; 2073 Magh, 4 marks; 2072 Asoj, 2 marks; 2072 Magh, 5 marks; 2071 Magh, 3 marks; 2071 Bhadra, 2 marks; 2070 Magh, 4 marks; 2069 Bhadra, 3 marks (asked 12 times)
- Video: [Relationship of energy, magnitude and intensity of an Earthquake](https://www.youtube.com/watch?v=JyPjl2bdquc) — WIT Solapur - Professional Learning Community
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/earthquake-magnitude-intensity
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-5

## Question

Differentiate between magnitude and intensity of an earthquake. How is the strength of an earthquake measured (calculated)?

## Short answer

Magnitude measures energy released at source, given one value per earthquake, and is calculated using Richter scale (ML = log10 A + 3 log10(8Δt) – 2.92) or moment magnitude (Mw = ⅔ log10 M0 – 6.07). Intensity rates damage on the Modified Mercalli scale, varying with distance and local conditions.

## Answer

#### Difference
| Point | Magnitude | Intensity |
|---|---|---|
| Meaning | Measure of the energy released at the source (focus) | Measure of the effects and damage at a place |
| Basis | Instrument (seismograph) records | Observation of damage, human feeling, objects |
| Value for one earthquake | One value | Varies with distance, soil and structure |
| Scale | Richter, moment magnitude ($M_w$) | Modified Mercalli (I-XII), MSK, EMS |
| Nature | Quantitative, logarithmic | Qualitative, descriptive |
| Example | Gorkha 2015, $M_w$ 7.8 | About VIII-IX near the epicentre, decreasing farther away |

#### How strength is measured
1. **Richter local magnitude (1935):**

$$
M_L = \log_{10} A + 3\log_{10}(8\Delta t) - 2.92
$$

where $A$ is the maximum amplitude on the Wood-Anderson seismograph in mm and $\Delta t$ is the S-P time lag in s. Each unit increase in magnitude means a 10 times larger amplitude and about 32 times more energy.

2. **Moment magnitude** ($M_w$), preferred for large earthquakes:

$$
M_w = \frac{2}{3}\log_{10} M_0 - 6.07
$$

($M_0$ = seismic moment in N m = rigidity $\times$ fault area $\times$ slip.)

3. **Energy relation:** $\log_{10} E = 1.5 M + 4.8$ ($E$ in joules), so the energy ratio per unit of magnitude is $10^{1.5}\approx 31.6$.

4. **Intensity** is assigned on the Modified Mercalli scale from field observation (I: not felt; V: felt by nearly everyone; VII: damage to poorly built buildings; X: most masonry structures destroyed; XII: total destruction).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does earthquake magnitude measure?
   A. Human feeling at a site
   B. Soil type at the site
   C. Energy released at the source
   D. Damage at a place
   Answer: C. Magnitude is an instrument-based measure of energy released at the focus; one value per earthquake. (Key point: Magnitude meaning)

2. Why does intensity vary for a single earthquake?
   A. It is derived from a seismograph at the focus
   B. It is a single logarithmic number
   C. It measures energy only
   D. It depends on distance, soil and structure
   Answer: D. Intensity describes effects at a place and varies with distance, soil and structures. (Key point: Intensity variation)

3. About how much more energy is released per unit increase of magnitude?
   A. About 32 times
   B. About 10 times
   C. About 2 times
   D. About 100 times
   Answer: A. log E = 1.5 M + 4.8 gives 10^1.5, about 31.6 times per unit. (Key point: Energy ratio)

4. What does seismic moment M0 equal?
   A. Mercalli grade x distance
   B. Rigidity x fault area x slip
   C. Amplitude x S-P time
   D. Depth x energy
   Answer: B. M0 in N m is rigidity times fault area times slip; used for Mw, preferred for large earthquakes. (Key point: Moment magnitude)

## Related in Engineering Geology II

- [Seismic waves](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aseismic-waves) — Engineering Geology II, chapter 5, asked 4×

## Related in other subjects

- [Magnitude versus intensity of earthquake](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Amagnitude-versus-intensity-of-earthquake) — Engineering Geology I, chapter 2
- [Earthquake intensity, magnitude, seismic waves](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aearthquake-intensity-magnitude-seismic-waves) — Engineering Geology I, chapter 2
- [Earthquake design philosophy, strength and ductility](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aearthquake-design-philosophy-strength-and-ductility) — Design of RCC Structure, chapter 12
- [Limiting error in calculated power](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Apower-error-limiting-error) — Instrumentation and Measurement, chapter 1