- 2061 Baisakh (old course) · 2+2+4 marks
Earthquake intensity, magnitude, seismic waves
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.
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.
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
- Plate motion stresses rocks along a fault.
- Friction locks the fault, so rocks deform and store elastic strain energy.
- When stress exceeds strength, the rock ruptures at the focus and rebounds (elastic rebound theory).
- Stored energy is released as seismic waves.
- 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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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