Earthquake hazards, risk and mitigation: the primary and secondary hazards of earthquakes (ground shaking, liquefaction, landslides, tsunami, fire); the distinction between hazard, vulnerability, exposure and risk; the factors controlling the severity of impact; and the prediction, monitoring and mitigation strategies (building design, hazard mapping, early warning and planning).

What this dot point is asking

This is part of the Geohazards theme examined in Component 3. Eduqas wants you to distinguish the primary and secondary hazards of earthquakes, to define and relate hazard, vulnerability, exposure and risk, to explain the controls on the severity of impact, and to describe prediction, monitoring and mitigation strategies. It applies the earthquake science (seismic waves, magnitude and intensity) to the human consequences and how to reduce them.

The answer

Primary and secondary hazards

The primary hazard of an earthquake is the ground shaking produced directly by the seismic waves, which damages and collapses buildings, bridges and infrastructure. The secondary hazards are triggered by that shaking:

• Liquefaction: saturated, loose, fine sediment loses its strength when shaken and behaves like a liquid, so buildings sink, tilt or topple and buried tanks float upwards.
• Landslides and rockfalls on slopes, which can bury settlements and dam rivers.
• Tsunami: if the sea floor is suddenly displaced (by a submarine fault or landslide), a series of long-wavelength ocean waves is generated.
• Fires: ruptured gas mains and power lines ignite fires that are hard to fight when water mains are also broken.

The secondary hazards often cause more deaths than the shaking itself (for example tsunami).

Hazard, vulnerability, exposure and risk

These four terms are precise and frequently tested:

Controls on the severity of impact

For a given magnitude, the damage varies enormously depending on:

• Focal depth: shallow-focus earthquakes do far more surface damage than deep ones.
• Distance from the epicentre: intensity falls with distance.
• Local ground conditions: soft, saturated sediment amplifies shaking and can liquefy, while hard bedrock shakes less.
• Building design and quality: engineered, reinforced structures survive where unreinforced masonry collapses.
• Population density, time of day and preparedness: a dense, unprepared population suffers more casualties.

This is why a large earthquake in a well-prepared region can cause fewer deaths than a smaller one in a poorly built, densely populated region.

Prediction, monitoring and mitigation

Earthquakes cannot yet be predicted precisely in time, but the hazard can be managed:

• Monitoring: seismometers, GPS measurement of crustal strain, and historical records identify active faults and recurrence intervals.
• Hazard mapping and land-use planning: maps of fault zones and liquefaction-prone ground steer development away from the worst sites, reducing exposure.
• Earthquake-resistant design: flexible frames, cross-bracing, base isolation and tying walls to frames reduce vulnerability.
• Early-warning systems: detecting the fast P wave gives seconds of warning before the damaging surface waves, enough to stop trains and shut off gas.
• Education, drills and emergency planning: prepare the population and the response.

Examples in context

Example 1. Liquefaction damage. In earthquakes affecting cities built on reclaimed or river-delta sediment, liquefaction tilts and sinks well-built structures whose foundations were sound, showing that ground conditions can outweigh building quality.

Example 2. Early warning saving seconds. Systems that detect the fast P wave and broadcast an alert before the slower surface waves arrive give enough time to halt high-speed trains and shut off gas, reducing secondary hazards even though the earthquake itself was not predicted.

Try this

Q1. State one primary and two secondary hazards of an earthquake. [3 marks]

• Cue. Primary: ground shaking. Secondary (any two): liquefaction, landslides, tsunami, fire.

Q2. Explain the difference between hazard and risk. [2 marks]

• Cue. The hazard is the natural event with the potential to harm; risk is the likelihood of harmful consequences, which also depends on exposure and vulnerability.

Q3. Describe one mitigation strategy that reduces vulnerability and one that reduces exposure. [2 marks]

• Cue. Reduce vulnerability with earthquake-resistant building design; reduce exposure with hazard mapping and land-use planning that keeps development off the worst ground.