---
title: "Seismic waves"
subject: "Engineering Geology II"
chapter: 5
key: "semester_4/engineering_geology_ii:seismic-waves"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/seismic-waves
site: BCT Engineering
---

# Seismic waves

Engineering Geology II · Chapter 5: Geological Hazards

- Asked in: 2076 Baisakh, 2 marks; 2071 Bhadra, 3 marks; 2069 Bhadra, 2 marks; 2068 Magh, 3 marks (asked 4 times)
- 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_4/engineering_geology_ii/seismic-waves
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-5

## Question

Describe the different types of seismic waves generated during an earthquake (differentiate P-wave and S-wave).

## Short answer

P-waves are compressional, fastest, travel through all media; S-waves are shear, slower, travel only through solids. Surface waves: Love waves (horizontal shear) and Rayleigh waves (elliptical rolling, most destructive). S-P arrival time difference gives epicentral distance.

## Answer

Seismic waves are the elastic waves produced by an earthquake that carry energy through and over the earth. They are of two main groups.

#### A. Body waves (travel through the interior)
1. **P-waves (primary, compressional):** particles vibrate parallel to the direction of travel (push-pull); fastest (about 5-8 km/s in crust); travel through solids, liquids and gases; arrive first.
2. **S-waves (secondary, shear):** particles move at right angles to the direction of travel; slower (about 3-4.5 km/s); travel through solids only, not liquids (so they showed the outer core is liquid); arrive second; more damaging than P-waves.

#### B. Surface waves (travel along the surface)
3. **Love waves (L):** horizontal shear motion, no vertical movement; fast surface wave.
4. **Rayleigh waves (R):** elliptical rolling motion like ocean waves; slowest, but largest amplitude and most destructive.

#### Difference between P- and S-waves
| Point | P-wave | S-wave |
|---|---|---|
| Motion | Compression and dilation (longitudinal) | Shearing (transverse) |
| Speed | Faster | Slower (about 0.6 of P) |
| Medium | Solid, liquid, gas | Solid only |
| Arrival | First | Second |
| Damage | Less | More |

The S-P time lag at a station gives distance to the epicentre, and three stations locate it by triangulation.

## Check yourself

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

1. How do particles move in a P-wave?
   A. Parallel to wave travel (push-pull)
   B. Perpendicular to travel
   C. In ellipses along the surface
   D. Horizontally only
   Answer: A. P-waves are compressional/longitudinal. (Key point: P-wave particle motion)

2. Why are S-waves not transmitted through the outer core?
   A. They travel only on the surface
   B. They travel only through solids, and the outer core is liquid
   C. They are too fast
   D. They are absorbed by air
   Answer: B. S-waves cannot pass through liquids, which showed the outer core is liquid. (Key point: S-wave medium)

3. Which surface wave is slowest but has the largest amplitude?
   A. P-wave
   B. S-wave
   C. Rayleigh wave
   D. Love wave
   Answer: C. Rayleigh waves have rolling elliptical motion and are most destructive among surface waves. (Key point: Most destructive surface wave)

4. How is the epicentre located?
   A. From P-wave colour
   B. By Mercalli grade only
   C. By one station alone
   D. S-P time lag gives distance and three stations triangulate
   Answer: D. S-P lag gives distance; three stations locate the epicentre by triangulation. (Key point: Epicentre location)

## Related in Engineering Geology II

- [Magnitude and intensity of earthquake](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aearthquake-magnitude-intensity) — Engineering Geology II, chapter 5, asked 12×

## Related in other subjects

- [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
- [Seismic waves and earthquake mechanism](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aseismic-waves-and-earthquake-mechanism) — Engineering Geology I, chapter 2
- [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×
- [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