# Earthquake questions

Subject: Disaster Management

Exam: M2

Reader: https://deep-learning-study-room.vercel.app/dm/#m2/papers/m2-questions/earthquakes

Canonical: https://deep-learning-study-room.vercel.app/read/dm/m2/papers/m2-questions--earthquakes/

2-mark and 3-mark practice · full answers below each question

### Practice question. Explain the cause of a tectonic earthquake and distinguish focus from epicentre. [3 practice marks]

#### Answer

Tectonic movement builds stress in rocks locked along a fault. When resistance is overcome, sudden slip releases stored elastic energy as seismic waves. The **focus** is the point within the Earth where rupture starts; the **epicentre** is the point on the surface directly above it.

#### Illustrative application · A classroom earthquake illustration

A fault begins to rupture below ground at the focus. The surface point vertically above it is the epicentre. Waves spread outward and shake buildings; the damage also depends on local ground and construction. Use the labelled diagram in the earthquake chapter to connect these terms.

### Practice question. Differentiate between interplate and intraplate earthquakes. [2 practice marks]

#### Answer

**Interplate earthquakes** occur at boundaries where tectonic plates interact. **Intraplate earthquakes** occur within a plate, away from its boundary, often on reactivated faults. Both can be damaging; location within a plate does not imply zero seismic risk.

#### Illustrative application · A classroom earthquake illustration

A fault begins to rupture below ground at the focus. The surface point vertically above it is the epicentre. Waves spread outward and shake buildings; the damage also depends on local ground and construction. Use the labelled diagram in the earthquake chapter to connect these terms.

### Practice question. Compare P waves and S waves. [3 practice marks]

#### Answer

| P waves | S waves |
| --- | --- |
| Faster; arrive first. | Slower; arrive after P. |
| Longitudinal compression and expansion. | Transverse shear motion. |
| Travel through solids and fluids. | Do not travel through liquids as shear waves. |

#### Illustrative application · Reading two arrivals

A station records a P arrival before an S arrival because P waves travel faster. The growing arrival-time gap with distance can help locate the source using travel-time information. A gap alone, without an appropriate velocity model, is not a universal kilometre conversion.

### Practice question. Distinguish earthquake magnitude from earthquake intensity. [3 practice marks]

#### Answer

**Magnitude** expresses the size of an earthquake from instrument-based measurements, for example moment magnitude. **Intensity** expresses its observed effects at a particular site, for example Modified Mercalli I–XII. A given event has an estimated magnitude on a specified scale, but intensities vary among sites because distance, soil and construction differ.

#### Illustrative application · Same magnitude, different local effects

One earthquake has a source magnitude, but a nearby settlement with vulnerable buildings may experience stronger damaging effects than a distant settlement on firm ground. A one-unit magnitude difference means a tenfold amplitude ratio in the stated logarithmic comparison; it does not mean ten times the damage.

### Practice question. Explain soil liquefaction and its effects. [3 practice marks]

#### Answer

Liquefaction is a loss of strength in susceptible water-saturated loose soil during strong shaking. Rising pore-water pressure reduces contact forces between soil grains, causing the soil to behave temporarily like a fluid. It can cause settlement, tilting of buildings, lateral spreading and damage to buried pipes.

#### Illustrative application · Loose saturated ground

Imagine two sites experiencing comparable shaking: one rests on competent rock and the other on loose, saturated sand. Excess pore-water pressure can reduce the sand’s strength, allowing a building to settle or tilt. The hazard depends on ground conditions as well as the earthquake.

### Practice question. Explain three earthquake mitigation measures. [3 practice marks]

#### Answer

1. **Safe construction and retrofitting:** design and strengthen structures to resist earthquake forces and reduce collapse.
2. **Non-structural safety:** anchor shelves and equipment and protect utilities to reduce falling-object and fire hazards.
3. **Planning and drills:** prepare communication, evacuation and response arrangements so people and agencies act safely.

#### Illustrative application · A classroom earthquake illustration

A fault begins to rupture below ground at the focus. The surface point vertically above it is the epicentre. Waves spread outward and shake buildings; the damage also depends on local ground and construction. Use the labelled diagram in the earthquake chapter to connect these terms.