What are the hazards of an earthquake, and how can the risk be reduced?
Earthquake hazards: the primary and secondary hazards (ground shaking, surface rupture, liquefaction, landslides and tsunamis); the distinction between hazard, vulnerability, exposure and risk; the factors that determine the impact of an earthquake (magnitude, depth, ground conditions, population density, building design and preparedness); monitoring and mitigation (building codes, land-use planning, early-warning systems and education); the limits of earthquake prediction.
A focused answer to the OCR H414 dot point on earthquake hazards. Covers primary and secondary hazards (shaking, surface rupture, liquefaction, landslides, tsunamis), the distinction between hazard, vulnerability, exposure and risk, the factors controlling impact, monitoring and mitigation, and the limits of earthquake prediction.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
OCR wants you to describe the primary and secondary hazards of earthquakes, to distinguish hazard, vulnerability, exposure and risk, to explain the factors that determine the impact of an earthquake, to describe monitoring and mitigation, and to recognise the limits of earthquake prediction.
The answer
Primary and secondary hazards
- Primary hazards come directly from the shaking and fault movement:
- Ground shaking, which damages and collapses buildings (the main cause of deaths).
- Surface rupture, where the fault breaks the ground surface, offsetting roads, pipes and buildings.
- Secondary hazards are triggered by the shaking:
- Liquefaction. Strong shaking of water-saturated, loose sediment makes it behave like a liquid, so buildings sink, tilt or topple.
- Landslides on unstable slopes.
- Tsunamis, where a submarine quake displaces the sea floor and so a large volume of water.
- Fires, from ruptured gas and power lines.
Hazard, vulnerability, exposure and risk
These four terms are distinct, and OCR rewards using them precisely:
This is why two earthquakes of equal magnitude can have very different impacts: the hazard may be similar, but a region with high vulnerability (weak buildings, little preparedness) and high exposure (dense population) faces far greater risk.
Factors controlling impact
The impact of an earthquake depends on:
- Magnitude and depth (larger and shallower quakes shake the surface more).
- Ground conditions (soft, water-saturated sediment amplifies shaking and may liquefy).
- Population density and exposure (more people and property at risk).
- Building design (earthquake-resistant design greatly reduces collapse).
- Preparedness (drills, warnings, emergency planning).
Monitoring, mitigation and prediction
- Monitoring uses seismometers, GPS and strain meters to track stress and detect foreshocks, and feeds early-warning systems that give seconds of warning after a quake starts but before the strong shaking arrives at distant cities.
- Mitigation reduces vulnerability: building codes and retrofitting, land-use planning (avoiding building on weak ground or fault lines), and education and drills.
- Prediction of the exact time, place and size of an earthquake is not currently possible; geologists can only assess long-term probability (hazard mapping), so mitigation, not prediction, is the main defence.
Examples in context
Example 1. Liquefaction damage on reclaimed land. Buildings on loose, water-saturated reclaimed ground have repeatedly sunk or tilted during earthquakes because of liquefaction, while nearby buildings on bedrock survived, showing how ground conditions control impact.
Example 2. Building codes reducing deaths. Countries that enforce strict earthquake-resistant building codes suffer far fewer deaths from large earthquakes than countries with weak construction, demonstrating mitigation reducing vulnerability and risk.
Try this
Q1. Distinguish a hazard from a risk. [2 marks]
- Cue. A hazard is a natural process that could cause harm (the earthquake); risk is the likelihood of harm, combining the hazard with exposure and vulnerability.
Q2. Describe one secondary hazard of an earthquake and how it causes damage. [2 marks]
- Cue. For example liquefaction: shaking of water-saturated loose sediment makes it behave like a liquid, so buildings sink, tilt or collapse.
Q3. Explain why earthquakes cannot be reliably prevented from causing harm by prediction alone. [2 marks]
- Cue. The exact time, place and size cannot be predicted, only long-term probability, so reducing vulnerability through mitigation (building codes, planning, warning) is the main defence.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/02 20204 marksTwo earthquakes of similar magnitude occur, one beneath a wealthy city with strict building codes and one beneath a poor rural region with weak buildings. Explain, using the terms hazard, vulnerability and risk, why the impacts differ greatly.Show worked answer →
Define the terms and apply them to the contrast.
- Hazard
- The hazard (the ground shaking from a similar-magnitude earthquake) is broadly the same in both cases, so the difference is not in the hazard itself.
- Vulnerability
- Vulnerability is how susceptible people and structures are to harm. The wealthy city has strict building codes, so its buildings resist shaking and its population is less vulnerable. The poor region has weak buildings and fewer resources, so it is much more vulnerable.
- Risk
- Risk is the likelihood of harm, combining the hazard with vulnerability and exposure. Because the vulnerability is far higher in the poor region (similar hazard, weaker buildings, less preparedness), the risk and so the impact (deaths and damage) is much greater there, even for an earthquake of the same magnitude.
Markers reward the same hazard, contrasting vulnerability (building quality and resources), and risk as hazard combined with vulnerability and exposure explaining the different impacts.
OCR H414/01 20194 marksDescribe two secondary hazards of an earthquake and explain how each causes damage.Show worked answer →
Choose two secondary hazards and explain the mechanism of each.
Liquefaction. Strong shaking of water-saturated, loose sediment causes the grains to lose contact and the material to behave temporarily like a liquid. Buildings then sink, tilt or collapse because the ground loses its strength and can no longer support them.
Tsunami. A submarine earthquake that displaces the sea floor (for example by movement on a thrust fault) suddenly displaces a large volume of water, generating a tsunami. The waves travel across the ocean and, on reaching shallow coastal water, build into high, fast waves that flood and destroy coastal areas.
(Other valid secondary hazards: landslides triggered on slopes, and fires from ruptured gas and power lines.) Markers reward two secondary hazards with a correct mechanism for each (loss of ground strength for liquefaction; sea-floor displacement for tsunami).
Related dot points
- Earthquakes: focus and epicentre; the elastic rebound mechanism; the types of seismic wave (P, S and surface waves) and their properties; magnitude (the logarithmic Richter and moment magnitude scales) versus intensity (Modified Mercalli); the use of P and S wave arrival times and travel-time graphs to locate an epicentre by triangulation.
A focused answer to the OCR H414 dot point on earthquakes. Covers focus and epicentre, elastic rebound, P, S and surface waves, the difference between magnitude (Richter and moment magnitude) and intensity (Modified Mercalli), and how P and S wave travel times and triangulation locate an epicentre.
- Volcanic hazards: the hazards of an eruption (lava flows, pyroclastic flows, ash falls, lahars, volcanic gases and sector collapse) and how they relate to magma type and the Volcanic Explosivity Index; the methods of monitoring a volcano (seismicity, ground deformation, gas emissions and thermal anomalies); the use of hazard maps, exclusion zones and evacuation to mitigate risk; the comparison with earthquakes in terms of predictability.
A focused answer to the OCR H414 dot point on volcanic hazards. Covers the hazards of an eruption (lava, pyroclastic flows, ash, lahars, gases, sector collapse) and their link to magma type and explosivity, the monitoring methods (seismicity, ground deformation, gas, thermal), hazard maps, exclusion zones and evacuation, and how volcanoes compare with earthquakes for predictability.
- Mass movement: the types of slope failure (rockfall, translational and rotational slides, slumps and debris flows); the balance of driving and resisting forces on a slope; the factors that trigger failure (slope angle, rock and soil type, water content, discontinuities, weathering, earthquakes and human activity); the recognition of warning signs; the engineering methods used to stabilise slopes and reduce risk.
A focused answer to the OCR H414 dot point on mass movement. Covers the types of slope failure (rockfall, translational and rotational slides, slumps, debris flows), the balance of driving and resisting forces, the triggers of failure (slope angle, rock type, water, discontinuities, earthquakes, human activity), warning signs, and the engineering methods used to stabilise slopes.
- Engineering geology: the engineering properties of rocks and soils (strength, jointing and discontinuities, weathering and the behaviour of clays, sands and gravels); the purpose and methods of site investigation (desk study, boreholes, trial pits and core logging); the ground conditions that cause problems for foundations (weak or compressible soils, swelling clays, solution cavities in limestone, made ground and high groundwater); the role of foundations and the ground model.
A focused answer to the OCR H414 dot point on engineering geology. Covers the engineering properties of rocks and soils, the purpose and methods of site investigation (desk study, boreholes, trial pits, core logging), the ground conditions that cause foundation problems (weak or swelling soils, solution cavities, made ground, groundwater), and the role of foundations and the ground model.
- Plate margins: the processes and features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-continent, ocean-ocean and continent-continent collision); the Benioff zone and subduction; the characteristic rocks, structures, earthquakes and volcanoes produced at each margin type.
A focused answer to the OCR H414 dot point on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-continent, ocean-ocean and continent-continent sub-types, the Benioff zone and subduction, and the characteristic rocks, structures, earthquakes and volcanoes of each.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)