Explain how earthquakes are caused by plate movement and describe how their strength and effects are measured

What this dot point is asking

This outcome asks you to explain how plate movement causes earthquakes, to use the key terms (focus, epicentre, seismic waves), and to describe how earthquakes are measured by both their energy and their effects. The central idea is that an earthquake is the sudden release of pressure that has slowly built up where plates are locked together, and that we measure both how strong it is and how much damage it does.

The answer

How earthquakes are caused

Earthquakes happen where plates meet. As the plates try to move past or against each other, their rough edges catch and lock instead of sliding smoothly. Pressure builds up at the locked point over many years. When the pressure becomes too great for the rock to hold, the plates suddenly slip, releasing the stored energy. This energy travels out as seismic waves that shake the ground, and that shaking is the earthquake. Because the plates keep moving, the pressure builds again, so the same area has repeated earthquakes.

Focus and epicentre

Two key terms describe where an earthquake happens:

• The focus is the point underground where the earthquake starts and the energy is first released.
• The epicentre is the point on the Earth's surface directly above the focus.

Shaking is usually strongest at the epicentre and grows weaker with distance from it.

Measuring the energy: the Richter scale

The Richter scale measures the magnitude, or the amount of energy released, using instruments called seismometers that record ground movement. It is a numerical scale, and each whole step up represents a large increase in energy, so a magnitude 7 earthquake is far more powerful than a magnitude 6. The Richter value is the same wherever it is measured for a single earthquake.

Measuring the effects: the Mercalli scale

The Mercalli scale measures the effects and damage an earthquake causes at a place, based on what people observe, such as cracked walls, fallen chimneys or collapsed buildings. Unlike the Richter scale, the Mercalli value varies from place to place for the same earthquake, because places nearer the epicentre, on softer ground, or with weaker buildings suffer more.

Examples in context

Example 1. The 2004 Indian Ocean earthquake. A massive subduction-zone earthquake off Sumatra, measured at over magnitude 9 on the Richter scale, released enormous energy and triggered a tsunami across the Indian Ocean. The huge magnitude reflected the sudden slip of a long stretch of locked plate boundary, and the damage (Mercalli effects) varied greatly between coastlines depending on distance and defences.

Example 2. Why Singapore feels distant tremors but rarely damage. Singapore sits away from plate boundaries, so it does not have its own earthquakes, but tall buildings sometimes sway when a large earthquake strikes nearby Sumatra. The strong magnitude can be felt at distance, yet the Mercalli effects in Singapore are usually minor because the energy weakens far from the epicentre.

Try this

Q1. Define the epicentre of an earthquake. [1 mark]

• Cue. The point on the Earth's surface directly above the focus, where shaking is usually strongest.

Q2. State what the Richter scale measures and how it is recorded. [2 marks]

• Cue. The magnitude, or energy released by an earthquake, recorded by instruments called seismometers.

Q3. Explain why two towns the same distance from the epicentre might suffer different amounts of damage. [2 marks]

• Cue. Differences in building quality (stronger buildings resist shaking) and ground type (soft ground shakes more than solid rock), and how prepared the people are.