How do earthquakes cause hazards, and how is seismic risk measured and reduced?
The causes of earthquakes (elastic rebound, focus and epicentre), seismic waves (P, S and surface waves), the measurement of size (magnitude and intensity), the primary and secondary hazards (ground shaking, liquefaction, landslides, tsunami), and the prediction and mitigation of seismic risk.
A focused WJEC and Eduqas A-Level Geology T1 answer on how earthquakes are generated by elastic rebound, the P, S and surface waves they produce, the measurement of magnitude and intensity, the primary and secondary hazards including ground shaking, liquefaction and tsunami, and how seismic risk is predicted and reduced.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
T1 applies the structural and tectonic content to hazards. This dot point covers earthquakes: how they are generated, the waves and the way size is measured, the primary and secondary hazards, and how risk is predicted and reduced. WJEC wants the clear distinction between magnitude and intensity and a realistic account of mitigation. It builds directly on faults (G2) and plate boundaries (F4).
The answer
How earthquakes are generated
Seismic waves
Three wave types carry the energy:
Measuring size: magnitude and intensity
Magnitude measures the energy released at the source on a logarithmic scale (such as moment magnitude); it is a single number for the whole event. Intensity measures the shaking and effects at a place (such as the Mercalli scale); it varies with distance, ground conditions and construction, so one earthquake has many intensity values.
Hazards
The primary hazard is ground shaking, the direct effect of the passing waves, which collapses structures. Secondary hazards are triggered by the shaking:
- Liquefaction: saturated, loose sediment loses strength and flows, so buildings sink or tilt.
- Landslides and rockfalls on shaken slopes.
- Tsunami: long sea waves from sea-floor displacement, flooding coasts.
- Fire from ruptured gas and power lines.
Prediction and mitigation
Earthquakes cannot yet be reliably predicted, so risk is reduced by mitigation: earthquake-resistant design (flexible frames, cross-bracing, deep foundations, base isolation), building codes, avoiding liquefiable ground, hazard mapping, public education and early-warning systems.
Examples in context
The 2011 Tohoku earthquake off Japan generated a devastating tsunami from sea-floor displacement, the secondary hazard causing most of the loss. The 1989 Loma Prieta and 2011 Christchurch earthquakes showed severe liquefaction of loose, saturated ground, with buildings sinking and tilting. Japan's building codes and early-warning system illustrate effective mitigation, with flexible, base-isolated structures designed to survive strong shaking.
Try this
Q1. Distinguish the focus from the epicentre of an earthquake. [2 marks]
- Cue. The focus is the point of rupture at depth; the epicentre is the point on the surface directly above it.
Q2. Explain the difference between magnitude and intensity. [2 marks]
- Cue. Magnitude is the energy released at the source on a logarithmic scale (one value); intensity is the shaking and effects at a place, varying with distance, ground and construction.
Q3. Describe what happens during liquefaction and why it is hazardous. [2 marks]
- Cue. Saturated, loose sediment loses strength and behaves as a liquid during shaking, so buildings sink, tilt or collapse.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20196 marksDescribe the primary and secondary hazards associated with a large earthquake.Show worked answer →
Separate primary from secondary hazards, because the distinction is the point of the question.
The primary hazard is ground shaking caused directly by the passage of seismic waves. Shaking can collapse buildings, bridges and other structures and is the main cause of death and damage, especially where construction is weak.
Secondary hazards are triggered by the shaking. Liquefaction occurs where saturated, loose sediment loses strength and behaves as a liquid, so buildings sink or tilt and the ground fails. Landslides and rockfalls are shaken loose on slopes. Tsunami are generated where the sea floor is displaced by submarine faulting, sending long waves that flood coasts. Fires follow from ruptured gas and power lines.
So the primary hazard is direct shaking, while liquefaction, landslides, tsunami and fire are secondary hazards triggered by it.
Markers reward ground shaking as the primary hazard, and liquefaction, landslides, tsunami and fire as secondary hazards, each correctly described.
WJEC Eduqas 20215 marksExplain the difference between the magnitude and the intensity of an earthquake, and describe how seismic risk can be reduced.Show worked answer →
Define the two measures and then give mitigation, because both parts are required.
Magnitude is a measure of the energy released at the source, given on a logarithmic scale (such as the moment magnitude scale), so it is a single number for the whole earthquake. Intensity is a measure of the effects and shaking felt at a particular place (such as the Mercalli scale), so it varies with distance, ground type and construction, and one earthquake has many intensity values.
Seismic risk is reduced by engineering and planning rather than prevention: designing buildings to flex and resist shaking (cross-bracing, deep foundations, base isolation), enforcing building codes, avoiding building on liquefiable ground, educating the public and drilling responses, and using early-warning systems and hazard mapping.
Markers reward magnitude as energy at the source on a logarithmic scale, intensity as effects at a place varying with conditions, and named mitigation such as earthquake-resistant design, building codes and land-use planning.
Related dot points
- The control of magma composition and gas content on eruptive style, the volcanic hazards (lava flows, pyroclastic flows, ash falls, lahars, gases, sector collapse), the monitoring of volcanoes (seismicity, ground deformation, gas, thermal), and the prediction and management of volcanic risk.
A focused WJEC and Eduqas A-Level Geology T1 answer on how magma composition and gas content control eruptive style, the range of volcanic hazards from lava flows to pyroclastic flows, lahars and ash, the methods used to monitor volcanoes, and how volcanic risk is predicted and managed.
- The types of mass movement (rockfall, slide, slump, flow), the factors that control slope stability (slope angle, rock type and structure, water, vegetation, undercutting), the ground hazards of subsidence and collapse (mining, dissolution), and the prediction and prevention of these hazards.
A focused WJEC and Eduqas A-Level Geology T1 answer on the types of mass movement, the factors controlling slope stability (slope angle, rock type and structure, water, vegetation and undercutting), the ground hazards of subsidence and collapse from mining and dissolution, and how these hazards are predicted and prevented.
- The classification of faults (normal, reverse, thrust, strike-slip) by the relative movement of the hanging wall and footwall and by the stress regime, the terminology of fault planes (dip, throw, heave, slickensides), and the recognition of faults in the field and on maps.
A focused WJEC and Eduqas A-Level Geology G2 answer on the classification of normal, reverse, thrust and strike-slip faults by hanging-wall and footwall movement and stress regime, the terms used to describe fault planes (dip, throw, heave, slickensides, fault breccia), and how faults are recognised in the field and interpreted on geological maps.
- The three types of plate boundary (constructive, destructive and conservative), the processes and features at each, and the driving forces of plate motion (mantle convection, ridge push and slab pull).
A focused answer to WJEC and Eduqas A-Level Geology F4 on plate boundaries, covering constructive, destructive and conservative margins, the processes and landforms at each (ridges, subduction zones, ocean trenches, volcanic arcs, fold mountains, transform faults), and the driving forces of plate motion.
- The compositional and mechanical layering of the Earth (crust, mantle, outer and inner core; lithosphere and asthenosphere) and the seismic evidence (P and S wave behaviour, shadow zones, discontinuities) used to deduce it.
A focused answer to WJEC and Eduqas A-Level Geology F4 on Earth structure, covering the compositional layers (crust, mantle, outer and inner core) and mechanical layers (lithosphere and asthenosphere), and the seismic evidence (P and S wave behaviour, shadow zones and discontinuities) that reveals them.
Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)