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A* revision for hazards: plate boundaries, volcanic and earthquake hazards, tsunami, tropical storms and wildfire, with the Nepal 2015 and Philippines case studies, Park model and multi-level responses for AQA 7037, Edexcel and OCR.
🎯 Learning Objectives
- Explain plate movement mechanisms (slab pull, ridge push, convection) and describe the hazards and landforms of constructive, destructive, conservative and collision boundaries (AQA 3.1.5.1; Edexcel 2A.1; OCR 1.1.a)
- Explain volcanic hazards - pyroclastic flows, lahars, ashfall, gases, lava - and evaluate monitoring and prediction using the VEI and Mount Pinatubo 1991 (AQA 3.1.5.2; Edexcel 2A.2; OCR 1.2.a)
- Describe earthquake waves and compare the Richter, moment magnitude and Mercalli scales; explain liquefaction and secondary hazards (AQA 3.1.5.3; Edexcel 2A.2; OCR 1.2.a)
- Explain tsunami formation, characteristics and warning systems, using the 2004 Indian Ocean and 2011 Tohoku tsunamis (AQA 3.1.5.4; Edexcel 2A.3; OCR 1.2.b)
- Explain tropical storm formation and structure, apply the Saffir-Simpson scale, and assess wildfire causes, conditions and impacts (AQA 3.1.5.5-3.1.5.6; Edexcel 2A.2; OCR 1.2.c)
- Apply hazard perception and the Park model, and evaluate a multi-hazard environment (Philippines) plus local, national and international responses using Nepal 2015 (AQA 3.1.5.1, 3.1.5.7; Edexcel 2A.4-2A.6; OCR 1.3.a)
📋 Key Definitions and Core Concepts
Focus (hypocentre): The point within the crust where an earthquake's energy is released; the epicentre is the point on the surface directly above it, where shaking is usually strongest.
Moment magnitude scale (Mw): Logarithmic scale measuring total energy released, calculated from fault rupture area x slip x rock rigidity; does not saturate, so it is the standard for great earthquakes (Nepal 2015 = Mw 7.8).
Pyroclastic flow (nuee ardente): A superheated (300-800 degrees C), fast-moving (100-300 km/h) avalanche of gas, ash and volcanic rock flowing down a volcano's flanks; usually the deadliest volcanic hazard (Montserrat 1997, Pompeii AD 79).
Lahar: A volcanic mudflow formed when ash and tephra mix with water (rainfall or melted snow/ice), flowing tens of kilometres at 30-60 km/h before setting concrete-hard (Nevado del Ruiz 1985 buried Armero, ~23,000 killed).
Liquefaction: Loss of shear strength in water-saturated fine sediments (reclaimed land, river deltas, valley fills) when shaken, so the ground behaves as a liquid and buildings tilt or sink (Christchurch 2011; Kathmandu 2015).
Tsunami: A series of long-wavelength ocean waves generated mainly by undersea earthquakes of Mw 7+ with vertical seabed displacement (also volcanic flank collapse, landslides); open-ocean speed ~700-800 km/h, growing to 10-30+ m near shore.
Storm surge: A rise in sea level at the coast driven by strong onshore winds and low pressure beneath a tropical storm, flooding low-lying land; usually the deadliest tropical-storm hazard (Haiyan 2013; 1953 North Sea surge).
Saffir-Simpson scale: A 1-5 category scale rating tropical storm (hurricane) intensity by sustained wind speed: Category 1 at 119 km/h or more; Category 5 at 252 km/h or more.
Park model (disaster response curve): A curve plotting quality of life against time through a disaster: pre-disaster, disruption (sharp fall), relief, rehabilitation, reconstruction. Steepness of the fall indicates impact severity; gradient and length of recovery indicate level of development.
Hazard perception: How people judge the risk a hazard poses to them, shaped by experience, wealth, education and personality/worldview; perception determines whether households mitigate, insure, evacuate or remain fatalistic.
Plate tectonics: processes and boundary types (AQA 3.1.5.1; Edexcel 2A.1; OCR 1.1.a)
Plate movement: the lithosphere (crust plus rigid upper mantle) is broken into plates riding the semi-molten asthenosphere at 1-10 cm/yr. Driving forces: slab pull (dense oceanic lithosphere sinking at subduction zones - the dominant force), ridge push (gravity sliding off elevated ridges) and mantle convection. Evidence for seafloor spreading: symmetrical palaeomagnetic stripes either side of ridges and sea-floor age increasing with distance from ridges.
Boundary types and their hazards:
| Boundary | Motion / process | Named example | Typical hazards |
|---|---|---|---|
| Constructive (divergent) | Plates separate; magma upwells, new basaltic crust, sea-floor spreading | Mid-Atlantic Ridge (Iceland); East African Rift | Effusive lava, shield volcanoes, shallow low-magnitude quakes; Eyjafjallajokull 2010 ash cloud closed European airspace ~6 days |
| Destructive (oceanic-continental) | Denser oceanic plate subducts beneath continental plate along a Benioff zone, partially melting to viscous andesitic magma | Nazca plate under South America (Andes) | Explosive composite volcanoes (Pinatubo), deep to shallow quakes - some of the largest recorded (Chile 1960, Mw 9.5) |
| Destructive (oceanic-oceanic) | Denser plate subducts, forming volcanic island arcs | Philippine Sea plate subduction; Japan trench | Island-arc volcanoes, frequent shallow quakes, undersea quakes generating tsunami (Tohoku 2011) |
| Conservative (transform) | Plates slide past each other, grinding and locking - no crust created or destroyed | San Andreas Fault (California) | Shallow-focus, powerful earthquakes; no volcanism (San Francisco 1906) |
| Collision | Two continental plates converge; neither subducts - crust folds and thickens into mountain belts | Indian plate meeting Eurasian plate - Himalayas | Shallow-focus but high-magnitude earthquakes on thrust faults (Nepal 2015) |
Volcano types: lava viscosity depends on silica content - basaltic lava (~50% silica) is runny and degasses easily, giving effusive shield volcanoes (Mauna Loa, Hawaii) that rarely kill; viscous, gas-trapping andesitic/rhyolitic magma gives explosive Plinian eruptions and composite volcanoes (Pinatubo, Fuji) with pyroclastic flows. The silica-viscosity-explosivity chain is the AO2 link examiners reward: it explains why subduction zones (andesitic) kill far more people than hotspots (basaltic).
Volcanic hazards, monitoring and prediction (AQA 3.1.5.2; Edexcel 2A.2; OCR 1.2.a)
Primary hazards: pyroclastic flows (nuees ardentes) - superheated (300-800 degrees C) mixtures of gas, ash and rock collapsing down valleys at 100-300 km/h; the deadliest volcanic hazard (Montserrat 1997 - Plymouth abandoned; Pompeii AD 79). Tephra and ashfall - collapses roofs (especially wet ash), contaminates water, destroys crops and halts aviation (Eyjafjallajokull 2010). Lava flows - destroy property and farmland but usually advance slowly enough to evacuate. Volcanic gases - SO2 converts to sulphuric aerosols that reflect sunlight: Pinatubo injected ~20 million tonnes of SO2, cooling global temperatures ~0.5 degrees C for over a year; gases can pool lethally (Lake Nyos 1986 CO2 cloud, ~1,700 killed). Lahars - volcanic mudflows wherever ash meets water: monsoon rain (Pinatubo's lahars buried towns for years after 1991) or flash-melted snow (Nevado del Ruiz 1985 - Armero buried, ~23,000 killed, despite warnings never acted on). Secondary hazards: tsunami (Krakatoa 1883, ~36,000 deaths), landslides and flank collapse, post-eruption famine and displacement.
Monitoring and prediction: unlike earthquakes, volcanoes give precursors days-weeks ahead, so monitoring works where agencies are funded. Methods: seismometers detecting shallow harmonic tremor and magma-rise quake swarms; tiltmeters/GPS measuring summit inflation; gas spectrometry tracking rising SO2; thermal imaging and satellite radar detecting dome growth - all feeding hazard maps, alert levels and evacuation zones. The VEI (Volcanic Explosivity Index) rates eruptions 0-8 logarithmically by ejected tephra volume - Pinatubo 1991 was VEI 6; it describes size, not impact, so pair it with casualty data in answers. June 1991, after ~600 years dormant: PHIVOLCS and USGS combined monitoring (tremor, inflation, SO2) to forecast the climax eruption, evacuating ~200,000 people before 15 June - credited with saving thousands of lives. The eruption ejected ~10 km3 of tephra and ~20 Mt SO2 (global cooling ~0.5 degrees C); Typhoon Yunya's rain plus ashfall collapsed roofs, and lahars destroyed farmland and infrastructure for years, displacing hundreds of thousands. Classic synthesis evidence: prediction cut primary deaths, yet secondary hazards still devastated livelihoods.Case detail: Mount Pinatubo, Philippines 1991 (VEI 6)
Earthquakes, magnitude scales, liquefaction and tsunami (AQA 3.1.5.3-3.1.5.4; Edexcel 2A.2-2A.3; OCR 1.2.b)
Cause and waves: stress accumulates along locked faults until rock fails - elastic rebound releases stored energy as seismic waves from the focus. P-waves: fastest (~6-8 km/s), compressional (push-pull), travel through solids and liquids, arrive first, weakest shaking. S-waves: ~half P speed, shear (side-to-side), solids only, stronger shaking - cannot cross the outer core, creating the shadow zone. Surface waves (Love and Rayleigh): slowest but most destructive - rolling and sideways ground motion. Shaking is strongest near the epicentre and from shallow foci, amplified by soft sediment - valley cities suffer more than rock foundations.
Measurement - three scales, one comparison point: Richter - logarithmic amplitude-based scale (each whole number = 10x amplitude, ~31x energy), but saturates above ~M7, under-rating great quakes. Moment magnitude (Mw) - from fault area x slip x rigidity; measures total energy, does not saturate; the modern standard (Nepal 2015 = Mw 7.8; Tohoku 2011 = Mw 9.0). Mercalli (I-XII) - subjective observed effects; varies with distance, geology and building quality - maps damage patterns but cannot compare quakes objectively. In essays: magnitude measures energy; impact depends on depth, distance, time of day, geology and - above all - development and preparedness.
Liquefaction and secondary hazards: shaking turns water-saturated fine sediments into a fluid - buildings tilt and sink, buried pipes float up (Christchurch 2011 on reclaimed land; Kathmandu valley sediments in 2015). Other secondary hazards: landslides and avalanches in steep relief (Nepal 2015; Everest Base Camp), fires from fractured gas mains (San Francisco 1906), disease from broken sewers, and tsunami.
Tsunami: generated mainly by undersea quakes of Mw 7+ with vertical seabed displacement (also volcanic flank collapse, large landslides), moving the whole water column. Open ocean: wavelength up to ~200 km, amplitude under 1 m, speed ~700-800 km/h. At shore: friction slows the wave, wavelength shortens, energy concentrates - amplitude grows to 10-30+ m, often as several waves with a strong backwash. Detection: seismic alert networks, DART deep-ocean pressure buoys and tide gauges (Pacific Tsunami Warning Centre). The 2004 Indian Ocean tsunami (Sumatra-Andaman, Mw 9.1; ~230,000 deaths across 14 countries) struck coastlines with no regional warning system - one was established after; the 2011 Tohoku tsunami (Mw 9.0, run-up up to ~40 m) overtopped Japan's sea walls, killed ~18,000 and caused the Fukushima nuclear accident - evidence that protection alone is insufficient without evacuation. Cause: collision-boundary earthquake on the Main Himalayan Thrust (India subducting beneath Eurasia), shallow focus (~8-15 km), epicentre near Gorkha, ~80 km northwest of Kathmandu. Impacts: ~9,000 deaths and ~22,000 injured; ~8 million people affected; ~600,000 homes destroyed and a similar number damaged; ~2.8 million displaced. Unreinforced brick masonry in Kathmandu collapsed en masse and soft valley sediments amplified shaking (liquefaction and resonance); Langtang village was buried by a co-seismic avalanche; the Everest Base Camp avalanche killed 22; the Mw 7.3 aftershock (12 May) added deaths and hindered rescue. Economic loss ~US$7 billion - roughly a third of GDP - with heritage sites (Dharahara Tower) destroyed. Landlocked, so no tsunami; instead steep-relief landslides dammed valleys and cut off villages during the approaching monsoon. Responses: local communities dug survivors out before services arrived; national government declared a state of emergency and deployed the army; international aid flowed (India and China first responders; UN and NGO coordination; UK Disasters Emergency Committee appeal raised ~£33 million). Reconstruction was slow: hundreds of thousands remained in temporary shelter years later and earthquake-resistant building codes were enforced weakly - evidence for Park-model arguments about development and recovery speed.Case study: Nepal (Gorkha) earthquake, 25 April 2015 - Mw 7.8
Tropical storms, wildfire, hazard perception, Park model and responses (AQA 3.1.5.1, 3.1.5.5-3.1.5.7; Edexcel 2A.4-2A.6; OCR 1.2.c, 1.3.a)
Tropical storm formation: they develop over oceans warmer than 26.5 degrees C (to ~50 m depth), between 5-20 degrees latitude (Coriolis force is too weak at the equator to organise rotation), with low vertical wind shear, high humidity and a pre-existing disturbance. Rising moist air condenses, releasing latent heat that fuels further uplift - a self-sustaining heat engine; pressure falls below ~950 mb. Structure: calm, cloud-free eye; eyewall with the strongest winds and rain; spiral rain bands. The Saffir-Simpson scale rates intensity 1-5 by sustained wind speed (Cat 1: 119+ km/h; Cat 5: 252+ km/h) - a hazard-profile tool, not an impact scale. Deadliest impacts come from storm surge plus intense rainfall: Typhoon Haiyan (Philippines, November 2013, Cat 5, gusts ~315 km/h) killed ~6,300 and displaced ~4 million, with 5-7 m surge in Leyte Gulf; surges also dominate mid-latitude floods (1953 North Sea surge, ~300 deaths in England). AO3 point: warmer seas mean higher-intensity storms and possible poleward shifts - intensity trends, not frequency trends, are the climate signal.
Wildfire: ignited by lightning or by people (arson, sparks, slash-and-burn clearance). Conditions: abundant dry fuel (worsened where fire suppression lets it accumulate), drought (ENSO-linked in Australia/Indonesia), low humidity, strong wind. Impacts: deaths, property loss, smoke pollution - the 2019-20 Australian 'Black Summer' burnt ~24 million hectares and killed or displaced ~3 billion animals. Some ecosystems are fire-adapted (serotinous cones need heat to open). Management: satellite hot-spot monitoring, prescribed burns, firebreaks, ember-resistant building codes.
Hazard perception: how people judge risk - shaped by experience, wealth, education and personality/worldview. Perception drives response: poor hazard-prone communities often show fatalism and informal adaptation; wealthy ones fund prediction, zoning and building codes - the perception-development link is a high-value synthesis thread in 20-mark answers.
Park model (disaster response curve): plots quality of life against time through pre-disaster, disruption (sharp fall), relief (immediate rescue, often externally aided), rehabilitation (temporary shelter, partial services) and reconstruction. Read comparatively: fall depth = impact magnitude; recovery gradient and duration = development and preparedness - Japan recovered within years after Tohoku (2011), Nepal's stretched over many; repeated hazards can lower the baseline itself.
Multi-hazard environment - Philippines: typhoon belt (~20 typhoons enter the Philippine Area of Responsibility yearly) plus the Pacific 'Ring of Fire' (Philippine Sea plate subduction: ~20+ active volcanoes, frequent shallow quakes) plus monsoons. Hazards compound: Typhoon Yunya's rain met Pinatubo's ash to trigger lahars (1991); Haiyan (2013) was followed by further typhoons and quakes interrupting recovery. Repeated losses erode development gains - the classic AO3 link between multi-hazard incidence and persistent underdevelopment.
Responses at three levels: local - community preparedness, evacuation drills and warnings (Philippine NDRRMC), hazard maps, hazard-resistant traditional building, community search-and-rescue (Nepal's spontaneous rescue saved most lives). national - monitoring agencies (PHIVOLCS, USGS, JMA), building codes and land-use zoning (Nepal's post-2015 code push), stockpiles, reconstruction programmes. international - UN OCHA coordination, NGOs (Red Cross/Red Crescent), bilateral aid (India and China to Nepal, 2015), the Sendai Framework 2015-2030 (reduced mortality and damage). Effective management runs mitigation-preparedness-response-recovery, matched to perceived risk and development capacity.
💡 Worked Example
Exam-style: 'The impact of tectonic hazards on people and property is determined more by the level of development than by the magnitude of the event.' To what extent do you agree? (20 marks) - AQA-style synoptic question (AQA 3.1.5.7; Edexcel 2A.4; OCR 1.3.a)
Mark-scheme logic: <p><strong>Task:</strong> 'To what extent' demands a supported judgement weighing development against physical factors (magnitude, depth, location). <strong>AO1 (~6 marks):</strong> define magnitude measures (Mw vs Mercalli), hazard vs disaster, and the development-preparedness link (hazard perception, Park model stages). <strong>AO2 (~8 marks):</strong> evidence that development dominates - Nepal 2015 (Mw 7.8): ~9,000 deaths, ~600,000 homes destroyed, weak building codes and unreinforced masonry, liquefaction-amplified shaking, slow reconstruction (Park-model recovery stretched over years); contrast Japan 2011 (Mw 9.0, far larger) with ~18,000 deaths but warning systems, engineered buildings and sea walls limiting losses and enabling recovery within years; Pinatubo 1991 shows monitoring and evacuation (~200,000 people) cutting primary deaths where agencies function. Evidence for magnitude's role: the 2004 Indian Ocean tsunami (Mw 9.1, ~230,000 deaths) would have killed far fewer with warning systems regardless of development; shallow focus and proximity to cities magnify any quake's toll; only Mw 7+ seabed displacement triggers tsunami at all. <strong>AO3 (~6 marks):</strong> judgement - development determines the <em>vulnerability side</em> of risk (risk = hazard x exposure x vulnerability / capacity), so at equal magnitude richer countries suffer fewer deaths; but magnitude sets the ceiling of physical energy, and secondary hazards (tsunami) can overwhelm even well-developed protection. Conclude: largely agree - development is the more decisive factor for deaths and recovery in most tectonic events, though at the highest magnitudes (Mw 9+) physical force limits what any development level can achieve.</p>
❓ Practice Questions & Mark Schemes
Distinguish between the characteristics of P-waves and S-waves. (4 marks)
Answer: P-waves travel fastest (~6-8 km/s) and arrive first; they are compressional (push-pull in the direction of travel) and can pass through solids and liquids, so they cross the outer core. S-waves travel at about half the P-wave speed, arrive second, are shear waves (side-to-side, perpendicular to travel) and can only pass through solids - their failure to reach the antipode creates the shadow zone. One mark each for speed, motion, medium travelled and consequence.
Explain the formation of a tropical storm. (6 marks)
Answer: Levels-based: form over oceans warmer than 26.5 degrees C (to ~50 m depth) between 5-20 degrees latitude, where Coriolis force can organise rotation; low wind shear and a pre-existing disturbance are needed. Warm, moist air rises and condenses, releasing latent heat that warms the air aloft, sustains uplift and lowers surface pressure, drawing in more air (self-reinforcing feedback); inflowing air spirals due to Coriolis, forming the eye and eyewall; the storm intensifies while over warm water and decays over land or cooler seas. Chained causation + correct conditions = 5-6.
Assess the effectiveness of monitoring and prediction in reducing the impacts of volcanic hazards. (9 marks)
Answer: Effective: volcanoes give days-weeks of precursors - seismometers detect harmonic tremor and magma-rise swarms, tiltmeters/GPS detect inflation, gas spectrometry tracks SO2 rises; at Pinatubo 1991 PHIVOLCS-USGS combined these to evacuate ~200,000 people before the VEI 6 eruption, credited with saving thousands of lives. Limits: secondary hazards persisted regardless - monsoon lahars buried towns for years, leaving hundreds of thousands displaced; monitoring requires funding and institutional capacity (Nevado del Ruiz 1985 had a hazard map and hours of warning, but Armero's ~23,000 died because warnings were never relayed); dormant volcanoes (Eyjafjallajokull) erupt with little recorded baseline. Judgement: monitoring/prediction is highly effective at cutting primary deaths where agencies are resourced and warnings are acted upon, but weak against secondary hazards, displacement and economic effects - it must sit within wider preparedness and land-use planning.
Using a case study, assess the impacts of a tectonic hazard in a developing country. (9 marks)
Answer: Nepal 2015 (Mw 7.8, Gorkha, shallow focus ~8-15 km on the Main Himalayan Thrust). Social: ~9,000 deaths, ~22,000 injured, ~8 million affected, ~600,000 homes destroyed and ~2.8 million displaced; unreinforced masonry collapsed in Kathmandu, with soft valley sediments amplifying shaking; Langtang village buried and the Everest Base Camp avalanche killed 22; the Mw 7.3 aftershock (12 May) added losses. Economic: ~US$7 billion damage (about a third of GDP), tourism and heritage sites destroyed, monsoon relief disrupted by landslide-dammed valleys. Assessment: severity stemmed less from magnitude (moderate-large) than from vulnerability - poverty, weak building-code enforcement, difficult terrain and landlocked isolation slowing aid; impacts compounded over years (slow reconstruction), illustrating the development-vulnerability link for Level 3-4 credit.
'Warning systems have removed the main threat from tsunami.' To what extent do you agree? (20 marks)
Answer: AO1: define tsunami (seabed displacement Mw 7+, open-ocean speed ~700-800 km/h, growing to 10-30+ m at shore) and warning infrastructure (seismic alerts, DART buoys, tide gauges, PTWC). AO2: agree - the 2004 Indian Ocean tsunami (~230,000 deaths) hit with no regional warning system; one was built afterwards, and Pacific-system alerts plus education saved lives in 2011 (Japan's warnings and drills enabled evacuation of millions before the waves). Disagree - 2011 Tohoku: warnings existed yet ~18,000 died because run-up (up to ~40 m) exceeded assumed defences and arrival times were minutes, not hours; near-field tsunamis strike before any alert can be issued; buoy coverage gaps and warning fatigue remain (2018 Palu, Indonesia - thousands died despite alerts). AO3: warning systems reduce far-field deaths substantially but cannot remove the threat - near-field arrival times, extreme run-up and non-compliance cap their value; they work best combined with land-use zoning, vertical evacuation structures and education. Finish with the judgement: agree only partially - systems transform but do not eliminate risk.