Hazardous Environments – Natural Hazard Hazards (Cambridge AS & A Level Geography 9696)
Learning Objective
Develop a comprehensive understanding of the distribution, physical processes, impacts and management of the three major natural hazards covered in the syllabus – tropical cyclones, earthquakes and volcanoes – and be able to evaluate management strategies using contrasting country examples.
1. Tropical Cyclone Hazards
1.1 Distribution
Tropical cyclones (TCs) form over warm oceanic waters (≥ 26.5 °C) between about 5° S and 30° N (or S). Their global pattern is controlled by sea‑surface temperature, the Coriolis effect and prevailing atmospheric circulation.
| Ocean Basin |
Regional Name |
Peak Season (NH) |
Peak Season (SH) |
Average Annual Frequency |
| North Atlantic |
Hurricanes |
June – Nov (peak Sep) |
– |
≈ 12 named storms |
| Eastern Pacific |
Hurricanes |
May – Nov (peak Aug) |
– |
≈ 15 named storms |
| Western Pacific |
Typhoons |
May – Oct (peak Aug) |
– |
≈ 26 named storms |
| North Indian Ocean |
Cyclones |
Apr – Dec (peak Oct) |
– |
≈ 5 named storms |
| South Indian Ocean |
Cyclones |
– |
Nov – Apr (peak Feb) |
≈ 9 named storms |
| Southwest Pacific |
Cyclones |
– |
Nov – Apr (peak Feb) |
≈ 10 named storms |
1.2 Physical Processes of Formation
- Pre‑existing disturbance – tropical wave, monsoon trough or easterly wave.
- Warm sea‑surface temperature (SST) – > 26.5 °C to a depth of ≥ 50 m.
- Low vertical wind shear – < 10 m s⁻¹ between the surface and the upper troposphere.
- Moist mid‑troposphere – relative humidity > 70 %.
- Coriolis force – sufficient to initiate cyclonic rotation; absent within ~5° of the equator.
The Coriolis parameter is calculated as:
$$f = 2\Omega \sin\phi$$
where Ω = 7.2921 × 10⁻⁵ rad s⁻¹ (Earth’s angular velocity) and φ is latitude.
1.2.1 Intensification Mechanism
Latent heat released during condensation lowers central pressure, steepening the pressure‑gradient force (PGF). The gradient‑wind balance is expressed by:
$$\frac{V^{2}}{r}+fV=\frac{1}{\rho}\frac{\partial p}{\partial r}$$
where V = tangential wind speed, r = radius from the centre, f = Coriolis parameter, ρ = air density and ∂p/∂r = radial pressure gradient.
1.2.2 Structure of a Mature Cyclone
- Eye – calm, low‑pressure centre (30–50 km diameter).
- Eyewall – ring of intense convection; contains the strongest winds and heaviest rain.
- Rainbands – spiral bands of showers and thunderstorms extending outward.
1.3 Impacts
Impacts are categorised as primary (direct) and secondary (indirect) hazards.
Primary Hazards
- Wind damage – structural failure, uprooted trees, power outages.
- Storm surge – coastal inundation caused by on‑shore wind stress and low pressure; extreme surges > 5 m.
- Heavy rainfall – flash flooding, landslides, especially on steep terrain.
Secondary Hazards
- Water‑borne disease outbreaks (contaminated water supplies).
- Food insecurity (crop loss, livestock mortality).
- Economic disruption (loss of tourism, damage to transport & utilities).
Case Study Snapshots
- Hurricane Katrina (USA, 2005) – gusts up to 280 km h⁻¹, 5.5 m storm surge in New Orleans, > 1 500 deaths, US$125 billion damage.
- Cyclone Nargis (Bangladesh, 2008) – 5.1 m surge, > 84 000 deaths, > 10 million people affected; illustrates vulnerability of low‑income, deltaic coastlines.
- Typhoon Haiyan (Philippines, 2013) – 315 km h⁻¹ winds, 6 m surge, > 6 300 deaths; highlights the role of inadequate evacuation planning.
1.4 Management
1.4.1 Forecasting & Early Warning
- Satellite imagery (visible, IR, microwave) to monitor convection, eye formation and sea‑surface temperature.
- Numerical weather prediction models (e.g., GFS, ECMWF) for track and intensity forecasts.
- Probabilistic “cone of uncertainty” and colour‑coded public warnings (e.g., the UK Met Office system).
1.4.2 Mitigation Measures
- Structural
- Elevated houses and flood‑proofing in low‑lying zones.
- Wind‑resistant design – reinforced roofs, shutters, roof‑tie‑downs.
- Coastal defences – seawalls, breakwaters, mangrove restoration and dune stabilisation.
- Non‑structural
- Zoning & land‑use planning to restrict development in high‑risk coastal strips.
- Community education on evacuation routes, shelter locations and kit preparation.
- Insurance schemes, catastrophe bonds and disaster‑risk financing.
1.4.3 Emergency Response
- Pre‑positioning of relief supplies, medical teams and portable shelters.
- Timely evacuation orders based on surge and wind‑zone forecasts.
- Coordinated search‑and‑rescue, medical assistance and temporary accommodation.
1.4.4 Long‑Term Adaptation
- Integrating sea‑level rise projections into coastal master plans.
- Promoting flood‑tolerant crops and raised‑bed agriculture in vulnerable floodplains.
- Community‑based DRR programmes that build local capacity for risk assessment and response.
- Periodic review of building codes to reflect evolving climate‑change data.
2. Earthquake Hazards
2.1 Distribution & Tectonic Setting
- Concentrated along plate boundaries:
- Subduction zones – e.g., Pacific “Ring of Fire”, Chile, Japan.
- Transform faults – e.g., San Andreas (USA), Alpine fault (NZ).
- Continental collision zones – e.g., Himalayas, Zagros.
- Intraplate earthquakes occur away from boundaries but are less frequent and generally lower magnitude (e.g., New Madrid, USA).
2.2 Seismic Waves
| Wave Type |
Propagation |
Key Characteristics |
Relevance to Damage |
| P‑waves (Primary) |
Compressional, travel fastest |
First to arrive; felt as a gentle “tap”. |
Used for early‑warning systems. |
| S‑waves (Secondary) |
Shear, slower than P‑waves |
Cause most of the shaking felt. |
Major contributor to structural damage. |
| Surface waves (Rayleigh & Love) |
Travel along the Earth’s surface |
Long‑duration, high amplitude. |
Responsible for severe ground motion and landslides. |
2.3 Magnitude & Intensity Scales
| Scale |
What It Measures |
Typical Use in the Syllabus |
| Richter (ML) |
Amplitude of seismic waves on a Wood‑Anderson seismograph |
Historical, moderate‑size events (M < 7). |
| Moment Magnitude (Mw) |
Seismic moment (fault area × slip × rigidity) |
Modern, all‑size events; replaces ML for M > 7. |
| Modified Mercalli (MMI) |
Observed effects on people, structures and the natural environment |
Assessing damage, informing emergency response. |
2.4 Primary Impacts
- Ground shaking – building collapse, infrastructure damage.
- Surface rupture – offset of the ground along the fault trace.
- Liquefaction – loss of strength in saturated, unconsolidated sediments, leading to ground failure and tilting of structures.
2.5 Secondary Impacts
- Triggered landslides and rockfalls in steep terrain.
- Generation of tsunamis when large seafloor displacement occurs (e.g., subduction events).
- Aftershocks that prolong damage and hinder rescue operations.
- Disruption of water, sanitation and health services → increased disease risk.
2.6 Hazard Mapping & Risk Identification
- GIS‑based seismic‑hazard maps showing expected ground‑motion (PGA) for different return periods.
- Probabilistic Seismic Hazard Assessment (PSHA) – combines earthquake occurrence rates with ground‑motion prediction equations.
- Risk matrices that combine hazard intensity with exposure (population, building stock) and vulnerability (building type, socioeconomic status).
2.7 Case Study Snapshots
- Great East Japan Earthquake (2011, Mw 9.0) – > 19 000 deaths, massive tsunami, Fukushima nuclear disaster; illustrates high‑income response capacity and complex secondary hazards.
- Gorkha Earthquake, Nepal (2015, Mw 7.8) – > 9 000 deaths, widespread collapse of unreinforced masonry in a low‑income, mountainous setting; highlights vulnerability and limited emergency capacity.
- Haiti Earthquake (2010, Mw 7.0) – > 220 000 deaths, severe building‑stock failure, prolonged humanitarian crisis; demonstrates the importance of building‑code enforcement and disaster‑risk financing.
2.8 Management of Earthquake Risk
2.8.1 Prediction & Early Warning
- Short‑term warning systems (e.g., Japan’s J‑Alert, Mexico’s SASMEX) detect P‑waves and issue seconds‑long alerts before damaging S‑waves arrive.
- Long‑term probabilistic assessments (PSHA) inform land‑use planning, building codes and insurance premiums.
2.8.2 Mitigation Measures
- Structural
- Seismic‑resistant design – base isolation, ductile steel frames, reinforced concrete shear walls.
- Retrofitting of vulnerable existing buildings (e.g., “soft‑storey” houses, unreinforced masonry).
- Critical‑infrastructure upgrades (bridges, hospitals, schools).
- Non‑structural
- Strict building‑code enforcement and regular inspections.
- Zoning to avoid construction on active fault traces, liquefaction‑prone soils and steep landslide slopes.
- Public education – “Drop, Cover, Hold‑on” drills, school programmes, community‑based risk workshops.
- Earthquake insurance schemes and catastrophe‑bond markets.
2.8.3 Emergency Response
- Rapid deployment of Urban Search & Rescue (USAR) teams, medical triage centres and temporary shelters.
- Provision of clean water, sanitation kits and disease‑surveillance to prevent secondary health crises.
- Coordination of international humanitarian assistance through UN OCHA and the IFRC.
2.8.4 Evaluation of Management Strategies
- Cost‑benefit analysis of retrofitting programmes (e.g., Japan’s “Seismic Retrofit Subsidy” reduced expected losses by > 70 %).
- Success indicators: reduced casualty rates, faster restoration of essential services, measurable increase in community resilience (post‑event surveys).
- Limitations: funding constraints in low‑income countries, enforcement gaps, and the inherent uncertainty of earthquake prediction.
3. Volcanic Hazards
3.1 Types of Volcanoes & Eruption Styles
| Volcano Type |
Typical Magma Composition |
Eruption Style |
Representative Example |
| Shield |
Low‑silica, basaltic |
Effusive – fluid lava flows |
Mauna Loa (Hawaii) |
| Stratovolcano (Composite) |
Intermediate to high silica (andesitic–rhyolitic) |
Explosive – ash columns, pyroclastic flows |
Mount Pinatubo (Philippines) |
| Caldera |
Varied; often high‑silica |
Catastrophic collapse after massive eruption |
Yellowstone (USA) |
| Cinder cone |
Basaltic to andesitic |
Short‑lived Strombolian eruptions |
Parícutin (Mexico) |
3.2 Physical Processes of Eruption
- Magma generation – partial melting in the mantle (mid‑ocean ridges, hotspots) or crust (subduction zones).
- Magma ascent – buoyancy, fracture propagation, and volatile exsolution (H₂O, CO₂) reduce density.
- Degassing & explosive fragmentation – rapid expansion of gases creates high‑pressure blasts, forming ash columns and pyroclastic density currents.
- Vent opening & conduit development – determines whether an eruption is effusive (lava flows) or explosive (ash, PDCs).
3.3 Primary Hazards
- Lava flows – destroy infrastructure, ignite fires, create new land.
- Pyroclastic density currents (PDCs) – fast, hot mixtures of gas and ash; extremely lethal and can travel > 20 km.
- Volcanic ash fall – roof collapse, respiratory problems, disruption of air traffic, contamination of water supplies.
- Volcanic gases – SO₂, CO₂, H₂S; cause acid rain, crop damage and asphyxiation in low‑lying areas.
3.4 Secondary Hazards
- Lahars (volcanic mudflows) triggered by heavy rain mixing with loose ash and debris.
- Sector collapse leading to tsunamis (e.g., Anak Krakatau 2018).
- Long‑term climate impacts from sulphur aerosols (e.g., “Year Without a Summer” 1816 after Tambora).
- Agricultural loss and food insecurity from ash‑covered fields.
3.5 Monitoring, Early Warning & Hazard Mapping
- Seismic monitoring – increase in volcano‑tectonic earthquakes signals magma movement.
- Ground deformation – GPS, InSAR and tiltmeters detect swelling of the edifice.
- Gas emissions – SO₂ flux measured by spectrometers; sudden spikes can precede eruption.
- Thermal imaging – satellite or drone‑borne IR detects hot spots.
- Hazard maps combine zones of lava flow, ash fall, PDCs and lahars; colour‑coded for public use (e.g., USGS Volcano Hazard Maps).
3.6 Case Study Snapshots
- Mount Pinatubo (Philippines, 1991) – VEI 6 eruption, > 800 km³ tephra, global temperature drop of ~0.5 °C; effective evacuation saved > 70 % of the population.
- Eyjafjallajökull (Iceland, 2010) – Ash plume disrupted European air traffic for 6 weeks; highlighted the economic impact of volcanic ash on aviation.
- Mount Merapi (Indonesia, 2010) – Pyroclastic flows killed > 350 people; lahars caused extensive damage downstream, emphasizing the need for multi‑hazard warning systems.
3.7 Management of Volcanic Risk
3.7.1 Forecasting & Early Warning
- Integrated volcano monitoring networks (seismic, deformation, gas, thermal) provide multi‑parameter alerts.
- Alert levels (e.g., “Advisory”, “Watch”, “Warning”) communicated to authorities and the public.
- Use of probabilistic eruption forecasting models (e.g., Volcano Explosivity Index – VEI) to estimate likely impacts.
3.7.2 Mitigation Measures
- Structural
- Engineering of diversion channels and check‑dams to control lahars.
- Reinforced shelters built on higher ground for ash‑fall protection.
- Installation of ash‑resistant ventilation and water‑filtration systems in critical facilities.
- Non‑structural
- Zoning to restrict permanent settlement within high‑risk zones (e.g., < 5 km of active vent for PDCs).
- Community‑based evacuation plans and regular drills.
- Public education on ash‑clean‑up, respiratory protection and safe water storage.
- Insurance schemes covering volcanic damage and loss of livelihood.
3.7.3 Emergency Response
- Rapid deployment of ash‑clearance crews, distribution of masks and clean‑water supplies.
- Establishment of temporary shelters outside PDC and lahar zones.
- Coordination of health services to monitor respiratory illnesses and mental‑health impacts.
- International assistance for large‑scale evacuations (e.g., UN OCHA support in the Philippines 1991).
3.7.4 Evaluation of Management Strategies
- Effectiveness measured by reduced mortality, successful evacuations, and economic losses relative to predicted hazards.
- Post‑event reviews (e.g., USGS after Pinatubo) highlight strengths (early warning) and weaknesses (long‑term ash‑fall management).
- Challenges include maintaining monitoring equipment in remote areas and ensuring community compliance with evacuation orders.
4. Comparative Evaluation of Management Approaches
When answering exam questions, consider the following analytical framework:
- Hazard characteristics – frequency, intensity, spatial extent, predictability.
- Vulnerability factors – socioeconomic status, building quality, population density, governance.
- Management mix – balance of structural vs. non‑structural measures, early‑warning capacity, community participation.
- Cost‑effectiveness – short‑term emergency costs versus long‑term adaptation investment.
- Residual risk – what hazards remain after mitigation and how they are managed (insurance, contingency planning).
Use contrasting case studies (e.g., high‑income Japan vs. low‑income Bangladesh for cyclones; Japan vs. Haiti for earthquakes; Philippines vs. Iceland for volcanoes) to illustrate how economic resources, governance and cultural factors shape the success of risk‑reduction strategies.