Key Idea: Topic 10.2 is about the hazards geophysical events create, and why the same event hits two places so differently. It pulls together two micros: 10.2.1 — earthquakes & tsunamis: energy released along a fault shakes the ground (the primary hazard) and triggers secondary hazards — tsunamis, landslides, liquefaction, fires. Magnitude and frequency vary by plate margin (subduction = the biggest quakes + tsunamis). 10.2.2 — volcanic & mass-movement hazards: lava, pyroclastic flows, lahars and ashfall from volcanoes, and rockfalls, landslides and debris flows moving downslope under gravity — fast, unwarned events kill, slow ones mostly wreck property. This is Option D content, examined on Paper 1. SL students answer two options, HL students answer three — the same questions at both levels. Each option is a data-response read off a hazard map or distribution diagram plus a structured question, ending in a [10] Examine essay marked on markbands.
🌍 10.2.1 — Earthquake and tsunami hazards
An earthquake releases energy along a fault: the focus is the rupture point underground, the epicentre is the surface point directly above it. Magnitude measures the energy released; intensity measures the felt shaking, which falls with distance from the epicentre. The shaking is the primary hazard; it triggers secondary hazards — and a tsunami forms when an offshore quake suddenly displaces the seabed and water column.
Earthquake hazards and how they vary with distance
| Hazard | What it is | With distance from the epicentre |
|---|---|---|
| Ground shaking (primary) | The crust shaking as energy radiates from the focus | Weakens — intensity falls outward |
| Liquefaction | Saturated soft ground loses strength and acts like a liquid | Fades — needs strong shaking |
| Landslides | Slopes fail when shaken hard | Fades — needs strong shaking |
| Fires | Broken gas and power lines ignite | Cluster near the epicentre |
| Tsunami | Sea waves from a displaced seabed | The exception — stays destructive thousands of km away |
Read the key first. Most secondary hazards fade outward like the shaking — but a tsunami does not.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Key terms — earthquakes & tsunamis
- Focus — the point underground where the rock ruptures and the quake begins.
- Epicentre — the surface point directly above the focus; shaking is strongest here.
- Magnitude — the energy released (moment-magnitude scale); each whole number is far more energetic.
- Intensity — how strongly the shaking is felt at a place; it falls with distance from the epicentre.
- Secondary hazard — a knock-on hazard triggered by the quake: tsunami, landslide, liquefaction, fire.
- Liquefaction — saturated soft ground behaving like a liquid when shaken, so buildings sink or tilt.
- Plate margin — subduction (destructive) margins give the largest, most frequent quakes plus tsunamis.
Tip: For a hazard map or event profile, read the key or scale first. Identify the most-affected zone, State an exact value off the key, Estimate a distance on the map scale, then describe the trend with distance. Always quote the units.
🌋 10.2.2 — Volcanic and mass-movement hazards
Volcanic hazards — lava flows, pyroclastic flows (fast, super-hot clouds of gas and ash), lahars (volcanic mudflows) and ashfall (tephra) — and mass movements — rockfalls, landslides, debris flows, slumps — are downslope, gravity-driven threats, often triggered by eruptions or earthquakes. Speed, not size, often decides the death toll: slow hazards (most lava, slumps) wreck property but let people escape; fast, unwarned ones (pyroclastic flows, lahars, sudden landslides) cause the mass casualties.
Why two geophysical events have very different impacts
| Control | Less severe impact | More severe impact |
|---|---|---|
| Speed of the hazard | Slow lava flow — people escape | Pyroclastic flow / lahar — no escape |
| Warning time | Monitored volcano, evacuation in time | Sudden landslide or quake, no warning |
| Population density | Remote slope, few people exposed | Dense settlement in the hazard's path |
| Vulnerability / wealth | Strong buildings, response plan | Weak housing, poor emergency services |
Key terms — volcanic & mass-movement hazards
- Lava flow — molten rock flowing downhill; slow, so it destroys property but rarely kills.
- Pyroclastic flow — a fast, super-hot cloud of gas and ash; the deadliest volcanic hazard.
- Lahar — a volcanic mudflow of ash and water that races down valleys and buries settlements.
- Tephra / ashfall — fragments thrown out by an eruption; collapses roofs and ruins farmland.
- Mass movement — downslope movement of rock, soil or debris under gravity.
- Modal class — on a distribution diagram, the altitude or slope band with the most recorded events.
Example: Nevado del Ruiz, Colombia (1985) — a small eruption melted summit ice and sent lahars down the valleys; Armero was buried and about 23,000 died, despite warnings not acted on. Eyjafjallajokull, Iceland (2010) — a moderate eruption killed nobody, but its ash cloud grounded European flights for days: a huge economic impact with almost no death toll. Haiti (2010) vs Tohoku, Japan (2011) — the smaller Haiti quake killed 200,000+ in a poor, weakly built city, while better-prepared Japan limited its quake-and-landslide deaths: vulnerability, not just geophysics, sets the toll.
✍️ IB-style questions
Outline two secondary hazards of a major earthquake, and explain how the severity of each changes with distance from the epicentre.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Examine how geophysical factors caused two named geophysical events to have very different impacts on people.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
✅ Quick self-check
Tap each card to reveal the answer.
Focus vs epicentre — what's the difference? The focus is the point underground where the rock ruptures; the epicentre is the surface point directly above it. Shaking is strongest at the epicentre and fades outward.
Which secondary hazard stays destructive far from the epicentre? A tsunami — it can cross an ocean and still strike hard. Liquefaction, landslides and fires all need strong shaking, so they fade with distance.
Why is a pyroclastic flow deadlier than a lava flow? A pyroclastic flow is fast and super-hot, so people cannot escape. A lava flow is slow, so it destroys property but gives people time to leave — its impact is mainly economic.
How do you read a hazard distribution diagram? Identify the modal band (most events), estimate by counting events above a threshold, and read values, coordinates or distances off the key or scale — always with units.
What does a top [10] Examine answer need? Two named events, two+ developed geophysical factors with data, a weighing of geophysics vs human vulnerability, and a clear judgement.
🎯 Highest-yield exam reminders
Exam tips
- Paper 1 Option D: SL answers TWO options, HL answers THREE — same questions; each option ends in a [10] Examine markband essay.
- On a hazard map or event profile: read the KEY first, State/Identify exact values, Estimate distances off the scale, then describe the trend with distance.
- Focus = underground rupture; epicentre = surface point above it. Magnitude = energy released; intensity = felt shaking, which falls with distance.
- Secondary hazards (liquefaction, landslide, fire) fade with distance — but a TSUNAMI stays destructive far away.
- Severity = type + speed + warning + vulnerability: fast + unwarned + poor = mass deaths (Nevado del Ruiz, Haiti); slow + monitored + remote = few deaths (Eyjafjallajokull, Japan).
- On the [10] Examine, name TWO events, develop two+ geophysical factors with data, weigh geophysics against vulnerability, and finish with a clear judgement.