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

# Mechanism of earthquake

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

- Asked in: 2066 Bhadra (old course), 5 marks
- Video: [What Causes Earthquakes? Earthquakes Explained | Geology 101](https://www.youtube.com/watch?v=iVx3FBq7WkQ) — Geology in 5
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/mechanism-of-earthquake
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-2

## Question

Describe the mechanism of earthquake.

## Short answer

Tectonic stress bends locked fault rocks, storing elastic strain energy. When stress exceeds strength, rupture occurs and rocks rebound, releasing seismic waves from the focus; the epicenter is the surface point above it.

## Answer

An **earthquake** is a sudden shaking of the ground caused by the release of stored strain energy in the Earth's crust.

#### Elastic rebound theory (H. F. Reid, 1910, after the 1906 San Francisco earthquake)
1. Tectonic forces (plate movements) act slowly on rocks along a fault.
2. Friction locks the fault, so the rocks bend and strain energy is stored, like a stretched spring.
3. When the stress exceeds the strength of the rock or the friction, the rock suddenly breaks or slips.
4. The rocks spring back (rebound) to a less strained position and the stored energy is released as **seismic waves**.
5. The waves travel outward and shake the ground. After the main shock, readjustment causes aftershocks.

```text
 Before        Strain         After slip
 |  |  |       \  \  \        |  |  |
 |  |  |        \  \  \       |__|__|
 straight       bent         rebound,
                             fault slip
```

#### Related terms
- **Focus (hypocentre)**: point of rupture inside the Earth.
- **Epicentre**: point on the surface directly above the focus.

Most earthquakes occur on plate boundaries, such as the Himalayan collision zone, which produced the 1934 Nepal-Bihar and 2015 Gorkha earthquakes.

## Check yourself

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

1. Why does strain energy build up in rocks along a fault before an earthquake?
   A. The fault slips continuously and heats the rock
   B. Magma pressure fills the fault with fluid
   C. Friction locks the fault while plate forces keep acting, so the rocks bend like a stretched spring
   D. Rocks along the fault shrink as they cool
   Answer: C. Tectonic forces act slowly, but friction prevents slip, so the rock deforms elastically and stores energy. (Key point: Strain storage)

2. According to elastic rebound theory, when does the rock suddenly break or slip?
   A. When the plates stop moving
   B. When the stored strain falls to zero
   C. When the stress exceeds the strength of the rock or the friction on the fault
   D. When seismic waves arrive from a distant shock
   Answer: C. Rupture happens once accumulated stress is larger than the rock strength or frictional resistance. (Key point: Trigger of rupture)

3. What happens to the stored energy when the rocks spring back?
   A. It is stored permanently as heat in the fault
   B. It is released as seismic waves that travel outward and shake the ground
   C. It is released only as sound in the air
   D. It moves the epicentre deeper into the Earth
   Answer: B. The rebound to a less strained position releases the energy as seismic waves. (Key point: Energy release)

4. Which statement about focus and epicentre is correct?
   A. The focus is on the surface; the epicentre is underground
   B. Both are the same point
   C. The epicentre is where the waves end
   D. The focus is the rupture point inside the Earth; the epicentre is the point on the surface directly above it
   Answer: D. Focus (hypocentre) is the underground origin; the epicentre lies vertically above on the surface. (Key point: Focus and epicentre)

## Related in Engineering Geology I

- [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
- [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