What makes earthquakes hazardous, and how are they located, measured and predicted?
Earthquakes are caused by the sudden release of stress along faults, mainly at plate margins; they radiate seismic waves (P-waves and S-waves) whose arrival times locate the epicentre and whose amplitude measures magnitude; the hazards include ground shaking, building collapse, tsunamis, fires and landslides; the risk is reduced by hazard mapping, building design and emergency planning, but precise short-term prediction remains impossible, so forecasting relies on probability from past records.
A focused answer to the Eduqas GCSE Geology statement on earthquake hazards. Covers how earthquakes are caused by stress release on faults, the P-waves and S-waves used to locate the epicentre and measure magnitude, the hazards, and how risk is reduced when precise prediction is impossible.
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What this dot point is asking
Eduqas wants you to explain that earthquakes are the sudden release of stress along faults, mainly at plate margins, that they radiate seismic waves (P-waves and S-waves) used to locate the epicentre and measure magnitude, and to list the hazards (shaking, building collapse, tsunamis, fires, landslides). You also need to explain how risk is reduced (hazard mapping, building design, emergency planning) given that precise short-term prediction is impossible, so forecasting is based on probability from past records. The epicentre calculation is a recurring quantitative question.
The answer
What causes an earthquake
Rocks at a fault are pushed by tectonic forces and gradually store elastic strain, like a bent ruler. Friction locks the fault until the stress overcomes it, and the rocks suddenly slip, releasing the stored energy in a moment. This sudden release is the earthquake. Most earthquakes happen at plate margins, where plates grind past, pull apart or push together, and the point underground where slip begins is the focus; the point on the surface directly above it is the epicentre.
Seismic waves
The released energy travels out as seismic waves, recorded by a seismograph. Two body-wave types matter at GCSE:
- P-waves (primary) are fast and arrive first. They are push-pull (compressional) waves and travel through solids and liquids.
- S-waves (secondary) are slower and arrive second. They shake side to side and travel through solids only.
The difference in their speeds is the key to locating an earthquake.
Locating the epicentre
Because P-waves are faster, the gap between the P-wave and S-wave arrivals grows with distance from the earthquake: the further away the station, the bigger the gap. So the gap at a station gives the distance to the earthquake (read from a travel-time graph), but not the direction. To pin the epicentre you need three stations: each distance draws a circle around its station, and the single point where all three circles intersect is the epicentre.
Measuring magnitude
The magnitude is a measure of the energy released, found from the amplitude (size) of the waves on the seismograph (corrected for distance). The magnitude scale is logarithmic, so each step up represents a large jump in energy. Magnitude is one number for the whole earthquake; the intensity of shaking felt at a place also depends on distance, depth and ground conditions.
The hazards
A large earthquake brings several linked hazards:
- Ground shaking and building collapse (the main cause of deaths).
- Tsunamis if the sea floor is suddenly displaced.
- Fires from ruptured gas and power lines.
- Landslides and liquefaction (saturated ground losing strength and behaving like a liquid).
Reducing the risk when prediction fails
Precise short-term prediction (the exact time and place) is not possible. Forecasting instead uses probability from past records: regions on active margins are assigned a likelihood over decades. Because the timing cannot be known, risk is reduced by preparation:
- Hazard mapping to identify high-risk ground and avoid building on it.
- Earthquake-resistant building design (reinforced, flexible structures), the single biggest life-saver.
- Emergency planning: drills, warning systems, trained rescue services and resilient infrastructure.
So the same magnitude can kill thousands in an unprepared city and few in a well-prepared one.
Examples in context
Example 1. Tsunami from a megathrust. A large undersea earthquake at a subduction zone can suddenly lift the sea floor, displacing a huge volume of water and sending a tsunami across the ocean. The earthquake's shaking and the tsunami are separate hazards from one event.
Example 2. Liquefaction in a city. Where a city is built on soft, water-saturated ground, shaking can turn the ground to a liquid-like state, so buildings tilt and sink even if their structure survives the shaking itself.
Try this
Q1. State the difference between the focus and the epicentre of an earthquake. [1 mark]
- Cue. The focus is the point underground where the slip starts; the epicentre is the point on the surface directly above it.
Q2. Explain why the gap between P-wave and S-wave arrivals can be used to find the distance to an earthquake. [2 marks]
- Cue. P-waves travel faster and arrive first, and the gap to the slower S-wave grows with distance, so the size of the gap indicates how far away the earthquake is.
Q3. Give one way the risk from earthquakes can be reduced. [1 mark]
- Cue. Any one of: earthquake-resistant building design; hazard mapping; emergency planning and drills.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20205 marksThe seismograph at a station records the P-wave arriving and the S-wave arriving 40 seconds later. Explain how the gap between the two arrivals is used to find the distance to the earthquake, and why readings from three stations are needed to locate the epicentre.Show worked answer →
Explain the wave-speed difference, then the three-station method.
Why the gap gives distance. P-waves travel faster than S-waves, so they arrive first. The further the station is from the earthquake, the larger the gap between the P-wave and the S-wave arrivals, because the faster P-wave pulls further ahead over a greater distance. A 40-second gap therefore corresponds to a particular distance, read from a standard travel-time graph.
Why three stations are needed. One station gives only the distance, not the direction, so the epicentre could lie anywhere on a circle of that radius around the station. A second station gives a second circle; the two circles cross at two points. A third station's circle picks out the single point where all three intersect, which is the epicentre.
Markers reward the idea that the P-S gap increases with distance (so it gives the distance to each station) and that three intersecting circles are needed to fix the single epicentre point.
Eduqas 20186 marksTwo cities of similar size experience earthquakes of the same magnitude, but one suffers far more deaths and damage than the other. Suggest and explain three factors that could account for the difference.Show worked answer →
Give three valid factors, each explained in terms of how it changes the impact.
- Building design and quality
- A city with earthquake-resistant buildings (reinforced frames, flexible structures) suffers far less collapse than one with poorly built or unreinforced buildings, which are the main cause of deaths.
- Preparedness and emergency planning
- A city with drills, warning systems, trained rescue services and an organised response saves more lives than one without, where help is slow and disorganised.
- Ground conditions and depth
- Soft, water-saturated ground amplifies shaking and can liquefy, increasing damage, while solid bedrock shakes less. A shallow focus also produces stronger surface shaking than a deep one. Time of day, population density and distance from the epicentre are also valid.
Markers reward three distinct factors (for example building quality, preparedness, ground conditions or focal depth), each explained by how it raises or lowers the death toll and damage for the same magnitude.
Related dot points
- Volcanic activity ranges from gentle effusive eruptions of runny basaltic lava to violent explosive eruptions of viscous silica-rich magma; the hazards include lava flows, ash falls, pyroclastic flows, lahars (mudflows) and toxic gases; volcanoes can be monitored using seismometers (earthquake swarms), ground deformation (tilt and bulging), gas emissions and rising temperatures, so eruptions are more predictable than earthquakes, and risk is reduced by monitoring, hazard mapping, exclusion zones and evacuation.
A focused answer to the Eduqas GCSE Geology statement on volcanic hazards. Covers effusive versus explosive eruptions and what controls them, the hazards (lava, ash, pyroclastic flows, lahars, gases), the warning signs used to monitor and predict eruptions, and how risk is reduced.
- Mass movement is the downslope movement of rock and soil under gravity; it includes slow creep, slides, slumps, flows and rockfalls; failure is triggered when the driving force (gravity, increased by steep slopes, heavy rain, loading and undercutting) exceeds the resisting force (friction and cohesion, reduced by water and weak or weathered rock); the risk is reduced by improving drainage, reducing slope angle, building retaining structures and avoiding building on unstable ground.
A focused answer to the Eduqas GCSE Geology statement on mass movement. Covers the types of mass movement, why slopes fail (the balance of driving and resisting forces and the role of water), the triggers, and how landslide risk is reduced by drainage, regrading, retaining structures and avoidance.
- The Earth's outer layer is divided into tectonic plates that move slowly over the mantle, driven by convection; the evidence for plate tectonics includes the fit of the continents, matching fossils and rock sequences across oceans, and the symmetrical magnetic stripes of the sea floor; plates meet at constructive (divergent), destructive (convergent) and conservative (transform) margins, each with characteristic earthquakes, volcanoes and landforms.
A focused answer to the Eduqas GCSE Geology statement on plate tectonics. Covers tectonic plates and the convection that drives them, the evidence (continental fit, matching fossils and rocks, magnetic stripes and sea-floor spreading), and the three types of plate margin with their earthquakes, volcanoes and landforms.
- Rocks deform when stressed: compression produces folds (anticlines arch upwards, synclines sag downwards) and reverse faults, while tension produces normal faults; the type and orientation of folds and faults are evidence of the direction of past Earth movements and are shown on geological maps and cross-sections.
A focused answer to the Eduqas GCSE Geology statement on folds and faults. Covers how compression produces folds (anticlines and synclines) and reverse faults, how tension produces normal faults, the parts of a fold and fault, and how these structures record the direction of past Earth movements.
- Engineering geology assesses the ground before construction: foundations, tunnels, dams and reservoirs must suit the rock and soil present; geologists check the strength and stability of rock, the presence of faults, the slope stability, the permeability of the ground (for a reservoir to hold water or a tunnel to stay dry), and the hazards of weak, soluble or swelling materials; poor ground investigation can lead to subsidence, leakage, collapse or failure, so a site investigation is carried out first.
A focused answer to the Eduqas GCSE Geology statement on engineering geology. Covers why the ground must be assessed before construction, the factors checked (rock strength, faults, slope stability, permeability, weak or soluble materials), and how poor investigation leads to subsidence, leakage or collapse.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Geology specification (teaching from 2017) — WJEC Eduqas (2017)