---
title: "Seismic waves and earthquake mechanism"
subject: "Engineering Geology I"
chapter: 2
key: "semester_3/engineering_geology_i:seismic-waves-and-earthquake-mechanism"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/seismic-waves-and-earthquake-mechanism
site: BCT Engineering
---

# Seismic waves and earthquake mechanism

Engineering Geology I · Chapter 2: Basic Reviews of the Earth

- Asked in: 2066 Jestha (old course), 3+5 marks
- Video: [Earthquakes and Seismology in Earth’s Interior](https://www.youtube.com/watch?v=Prey5z8WQiU) — Professor Dave Explains
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/seismic-waves-and-earthquake-mechanism
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-2

## Question

Define and describe the types of seismic waves. Write down the mechanism of earthquake origination.

## Short answer

Body waves: compressional P-waves (fastest, through solids/liquids) and shear S-waves (slower, solids only). Surface waves: Love (side-to-side) and Rayleigh (rolling), slowest and most damaging. Earthquakes originate by elastic rebound: stressed rocks rupture, slip, and release stored energy as seismic waves.

## Answer

#### Seismic waves
**Seismic waves** are elastic waves that carry the energy released by an earthquake through and along the Earth. They are of two main groups.

**A. Body waves** (travel through the interior)
- **P-waves (primary, compressional)**: particles vibrate parallel to the direction of travel (push-pull). Fastest (about 6-8 km/s in crust), arrive first, pass through solids, liquids and gases.
- **S-waves (secondary, shear)**: particles vibrate perpendicular to the direction of travel. Slower (about 3.5-4.5 km/s), arrive second, pass only through solids; they stop at the liquid outer core.

**B. Surface waves** (travel along the surface)
- **Love waves**: horizontal side-to-side shear motion.
- **Rayleigh waves**: elliptical rolling motion, like ocean waves.
- Slowest but largest, and cause most damage.

```text
 P: >>><<<>>><<<  (compression)
 S: ~~~~~~~~~~~~  (shear, up-down)
```

#### Mechanism of earthquake origination
Based on the **elastic rebound theory**:
1. Plate movement applies stress on rocks along a fault.
2. Friction prevents slip, so rocks deform elastically and store strain energy.
3. When stress exceeds rock strength, the rock ruptures suddenly at the **focus**.
4. The rock rebounds and the stored energy is released as seismic waves, which reach the surface first at the **epicentre**.
5. Smaller aftershocks follow as the crust adjusts.

Causes are mostly tectonic; others are volcanic activity, landslides, and reservoir loading or explosions.

## Check yourself

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

1. Which description of P-waves is correct?
   A. Shear waves, particles move perpendicular to travel, solids only
   B. Compressional, particles move parallel to travel direction, fastest, pass through solids, liquids and gases
   C. Surface waves with rolling elliptical motion
   D. Slowest waves that cause most damage
   Answer: B. P-waves are primary compressional body waves, the fastest and able to pass through any medium. (Key point: P-waves)

2. Why do S-waves stop at the liquid outer core?
   A. They are too fast to be recorded
   B. They are compressional and are absorbed by gases
   C. They are shear waves and liquids cannot support shear
   D. They are surface waves and cannot go deep
   Answer: C. S-waves vibrate perpendicular to travel (shear) and pass only through solids. (Key point: S-waves)

3. Which pair correctly names the surface waves and their effect?
   A. P and S waves; fastest and least damaging
   B. Love and P waves; arrive first
   C. Love and Rayleigh waves; slowest but largest and most damaging
   D. Rayleigh and S waves; travel only through the core
   Answer: C. Love (horizontal) and Rayleigh (rolling) are surface waves, slow but cause most damage. (Key point: Surface waves)

4. In the elastic rebound mechanism, where does the rock rupture and what is released?
   A. At the epicentre, where heat is released
   B. At the surface, where magma is released
   C. At the focus, where gravitational energy is absorbed
   D. At the focus, where stored strain energy is released as seismic waves
   Answer: D. When stress exceeds strength, rupture begins at the focus and the stored strain energy leaves as seismic waves. (Key point: Origination mechanism)

## Related in Engineering Geology I

- [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 terminology and seismic construction](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aearthquake-terminology-and-seismic-construction) — Engineering Geology I, chapter 2
- [Mechanism of earthquake](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Amechanism-of-earthquake) — Engineering Geology I, chapter 2

## Related in other subjects

- [Seismic waves](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aseismic-waves) — Engineering Geology II, chapter 5, asked 4×
- [Microwaves versus acoustic or seismic waves](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amicrowaves-vs-acoustic-waves) — RF and Microwave Engineering, chapter 1, asked 2×