--- title: "Earthquake intensity, magnitude, seismic waves" subject: "Engineering Geology I" chapter: 2 key: "semester_3/engineering_geology_i:earthquake-intensity-magnitude-seismic-waves" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/earthquake-intensity-magnitude-seismic-waves site: BCT Engineering --- # Earthquake intensity, magnitude, seismic waves Engineering Geology I · Chapter 2: Basic Reviews of the Earth - Asked in: 2061 Baisakh (old course), 2+2+4 marks - Video: [How We Really Measure Earthquakes: The Moment Magnitude Scale](https://www.youtube.com/watch?v=nl9jiubdlXU) — Simple Geography - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/earthquake-intensity-magnitude-seismic-waves - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-2 ## Question Explain intensity and magnitude of earthquake. Write down the mechanism of how an earthquake occurs. Explain the different types of seismic waves generated during the earthquake. ## Short answer Intensity measures effects and damage at a site (Modified Mercalli scale I–XII), while magnitude is the energy released at the source (Richter scale). Earthquakes result from elastic rebound: stress overcomes friction, fault ruptures, releasing seismic waves. Body waves are P (compressional) and S (shear); surface waves, Love and Rayleigh, cause most damage. ## Answer #### Intensity **Intensity** is a measure of the effects of an earthquake on people, structures and ground at a given place. It is judged from observed damage and felt effects. It is expressed on the **Modified Mercalli Intensity (MMI) scale** from I (not felt) to XII (total destruction). It varies from place to place, decreasing with distance from the epicentre. #### Magnitude **Magnitude** is a measure of the energy released at the focus. It is calculated from seismograph amplitudes on the **Richter scale** (logarithmic): each unit increase means 10 times larger amplitude and about 32 times more energy. It has a single value for one earthquake. The 2015 Gorkha earthquake had magnitude about 7.8. #### Mechanism of an earthquake 1. Plate motion stresses rocks along a fault. 2. Friction locks the fault, so rocks deform and store elastic strain energy. 3. When stress exceeds strength, the rock ruptures at the focus and rebounds (elastic rebound theory). 4. Stored energy is released as seismic waves. 5. Aftershocks follow as the crust readjusts. #### Seismic waves - **P-waves**: compressional, fastest, travel through solid, liquid and gas; first to arrive. - **S-waves**: shear, slower, travel only through solids; second to arrive. - **Love waves**: surface waves, horizontal shaking. - **Rayleigh waves**: surface waves, rolling elliptical motion. P and S are body waves; Love and Rayleigh are surface waves and cause the greatest damage. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which statement about intensity is correct? A. It is a single value for the whole earthquake B. It is calculated from seismograph amplitudes on the Richter scale C. It measures effects at a place on the Modified Mercalli scale and decreases with distance from the epicentre D. It measures energy released at the focus Answer: C. Intensity is a descriptive, place-dependent measure of effects. (Key point: Intensity) 2. On the Richter scale, one unit increase in magnitude means about: A. 2 times larger amplitude and 10 times more energy B. 10 times larger amplitude and about 32 times more energy C. 10 times larger amplitude and 10 times more energy D. 32 times larger amplitude and 10 times more energy Answer: B. The scale is logarithmic: 10x amplitude and about 32x energy per unit. (Key point: Richter scale) 3. In the earthquake mechanism, what follows the main rupture as the crust readjusts? A. Foreshocks B. Isoseismal lines C. Aftershocks D. Tectonic locking Answer: C. After the main shock, readjustment causes aftershocks. (Key point: Mechanism) 4. Which wave arrives first and which group causes the greatest damage? A. S-waves arrive first; P-waves cause greatest damage B. Love waves arrive first; body waves cause greatest damage C. Rayleigh waves arrive first; P-waves cause greatest damage D. P-waves arrive first; surface waves (Love and Rayleigh) cause greatest damage Answer: D. P-waves are fastest; surface waves are slow but large and most destructive. 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