---
title: "How an earthquake occurs"
subject: "Engineering Geology II"
chapter: 5
key: "semester_4/engineering_geology_ii:earthquake-mechanism"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/earthquake-mechanism
site: BCT Engineering
---

# How an earthquake occurs

Engineering Geology II · Chapter 5: Geological Hazards

- Asked in: 2080 Chaitra, 2 marks; 2079 Asoj, 2 marks; 2076 Bhadra, 2 marks; 2075 Bhadra, 1 marks; 2073 Magh, 2 marks; 2070 Magh, 2 marks; 2068 Bhadra, 2 marks (asked 7 times)
- Video: [HOW DO EARTHQUAKES OCCUR | Geomorphology| Dr. Krishnanand](https://www.youtube.com/watch?v=JjgJvavhUOo) — TheGeoecologist
- 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-mechanism
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-5

## Question

Define earthquake. How does an earthquake occur? Describe the elastic rebound theory.

## Short answer

Earthquake is shaking from strain release. Elastic rebound theory: rocks on each side of a fault are strained by plate motion; when stress exceeds friction, they rupture and spring back, releasing seismic waves from the focus. The process repeats along active faults.

## Answer

An **earthquake** is the sudden shaking of the ground caused by the release of strain energy stored in the earth's crust, which travels as seismic waves.

#### How an earthquake occurs
1. The crust is divided into tectonic plates which move slowly (cm per year) by mantle convection. India moves toward Eurasia by about 4-5 cm/yr.
2. At plate boundaries and faults, rocks are locked by friction, so strain accumulates and the rocks deform elastically.
3. When the stress exceeds the strength of the rock or the friction on the fault, the rocks suddenly slip.
4. The stored elastic energy is released as seismic waves from the **focus (hypocentre)**; the point on the surface above it is the **epicentre**.

Causes: tectonic (most important), volcanic, reservoir-induced, mining and explosions.

#### Elastic rebound theory (H. F. Reid, 1906, after the San Francisco earthquake)
1. Rocks on both sides of a fault are strained slowly by plate motion; they bend and store elastic energy like a spring.
2. When the strain exceeds the rock strength, rupture occurs at the weakest point.
3. The rocks spring back (rebound) to a less strained position, and the fault moves; the energy is released as seismic waves.
4. The process then repeats, which is why earthquakes recur.

```text
 (a) original   (b) strained   (c) after rupture
  |  |  |         \  \  \          |  |  |
  |  |  |         \  \  \         --|--|--   offset
```

## Check yourself

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

1. What causes an earthquake according to the answer?
   A. Wind pressure on mountains
   B. Sudden release of strain energy stored in the crust
   C. Falling of rain on faults
   D. Heating of the sea surface
   Answer: B. Stored strain energy is released as seismic waves. (Key point: Earthquake definition)

2. What happens in elastic rebound theory when strain exceeds the strength of rock?
   A. Rocks absorb more strain with no rupture
   B. Rocks cool down and shrink
   C. Rupture occurs and rocks spring back, releasing seismic waves
   D. The fault becomes permanently locked forever
   Answer: C. Rocks bend like a spring, rupture at the weakest point, rebound and release energy; the cycle repeats. (Key point: Elastic rebound)

3. What is the epicentre?
   A. The point where energy is released underground
   B. The deepest point of the plate
   C. The seismograph station
   D. The surface point directly above the focus
   Answer: D. The focus (hypocentre) is the source; the epicentre lies above it. (Key point: Focus and epicentre)

4. Elastic rebound theory was proposed after which earthquake?
   A. 1906 San Francisco earthquake
   B. 2015 Gorkha earthquake
   C. 1934 Bihar earthquake
   D. 1988 Udayapur earthquake
   Answer: A. H. F. Reid proposed it in 1906 after the San Francisco earthquake. (Key point: Who proposed)

## Related in other subjects

- [Plastic hinge versus elastic hinge](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Adefine-plastic-hinge-compare-elastic) — Theory of Structures II, chapter 5
- [Elastic vs plastic analysis; plastic hinge](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Aelastic-vs-plastic-analysis-plastic-hinge) — Theory of Structures II, chapter 5
- [Elastic, inelastic, linear and non-linear behaviour](https://www.bctengineer.com/question.md?key=semester_4%2Ftheory_of_structures_i%3Aelastic-inelastic-linear-nonlinear) — Theory of Structures I, chapter 1