Explain how earthquakes occur and describe the focus, epicentre and how earthquakes are measured

What this dot point is asking

SEAB wants you to explain how earthquakes occur, to define the focus and epicentre, to know how earthquakes are measured, and to understand what makes some earthquakes far more damaging than others. The central insight is that an earthquake is the sudden release of stress that has slowly built up where plates are locked, and that the harm it causes depends not just on its size but on depth, location and human factors.

The answer

How an earthquake is caused

Earthquakes happen mostly at plate boundaries, where plates move and interact:

1. As plates try to move past or against each other, friction locks them together so they cannot slide smoothly.
2. Stress builds up in the locked rocks over time as the plates keep trying to move.
3. When the stress finally overcomes the friction, the rocks suddenly break or slip, releasing the stored energy in moments.
4. This energy travels outward as seismic waves, shaking the ground, which is the earthquake.

Focus and epicentre

Two key terms describe where an earthquake happens:

• The focus: the point inside the Earth where the earthquake starts, where the rocks first break and release energy.
• The epicentre: the point on the Earth's surface directly above the focus, usually where shaking is strongest.

The depth of focus matters: a shallow focus means the energy reaches the surface with little spreading, so shaking is more intense; a deep focus spreads the energy more, so surface shaking is weaker.

How earthquakes are measured

Earthquakes are recorded by seismometers, which detect the seismic waves. Their magnitude (the energy released) is given on a scale such as the moment magnitude scale, where each step up represents a large jump in energy. The intensity (how strongly shaking is felt and the damage caused) is described separately, since the same magnitude can do very different damage in different places.

What makes an earthquake damaging

The damage from an earthquake depends on far more than its magnitude:

• Depth of focus: shallow earthquakes shake the surface harder.
• Distance from the epicentre and population: an epicentre in a crowded city causes far more harm than one in a remote area.
• Building quality and preparedness: strong, earthquake-resistant buildings and good planning save lives; poor construction and little preparation cost them.
• Ground conditions: soft, loose ground shakes more and can fail, worsening damage.

Examples in context

Example 1. The 2011 Tohoku earthquake, Japan. The powerful magnitude-9 earthquake off northeastern Japan in 2011 was caused by sudden slip at the subduction zone where the Pacific Plate descends beneath Japan. Despite Japan's strong, earthquake-resistant buildings limiting collapse, the immense energy and the tsunami it triggered caused massive damage, showing both how subduction-zone stress release produces great earthquakes and how preparation reduces, but cannot eliminate, the toll.

Example 2. The 2010 Haiti earthquake. A magnitude-7 earthquake near the capital Port-au-Prince caused catastrophic loss of life, far more than its magnitude alone would suggest. A shallow focus close to a densely populated city, widespread poorly built structures and little preparedness combined to make it one of the deadliest earthquakes on record. It illustrates how human factors, not just magnitude, determine an earthquake's impact.

Try this

Q1. Define the terms "focus" and "epicentre". [2 marks]

• Cue. The focus is the point inside the Earth where the earthquake originates and energy is first released; the epicentre is the point on the Earth's surface directly above the focus, usually where shaking is strongest.

Q2. Explain how stress builds up and is released to cause an earthquake. [3 marks]

• Cue. Friction locks the moving plates together so they cannot slide smoothly, and stress builds up in the rocks over time; when the stress overcomes the friction, the rocks suddenly break or slip, releasing the stored energy as seismic waves that shake the ground.

Q3. Suggest two reasons why a shallow earthquake in a city may be more damaging than a deep one in the countryside. [2 marks]

• Cue. A shallow focus delivers intense shaking to the surface with little spreading of energy, and a city has far more people and buildings exposed than the countryside, so both the shaking and the exposure are greater.