The big idea: An earthquake is a sudden release of energy in the crust that makes the ground shake. The energy comes from stress building up along a plate margin and releasing along a fault.
The focus is the point underground where the rupture starts; the epicentre is the point on the surface directly above it. Magnitude (the energy released, on the moment-magnitude scale) and intensity (how strongly the shaking is felt at a place) are different measures.
A tsunami is a series of large sea waves triggered when an undersea earthquake (or landslide) suddenly displaces the water column.
Key terms for this micro
- Focus — the point underground where the rock ruptures and the earthquake begins.
- Epicentre — the point on the surface directly above the focus.
- Magnitude — the energy an earthquake releases (moment-magnitude scale); each whole number is far more energetic than the last.
- Intensity — how strongly the shaking is felt at a particular place; it falls with distance from the epicentre.
- Focus depth — shallow-focus quakes do far more surface damage than deep-focus ones of the same magnitude.
- Tsunami — sea waves caused by sudden displacement of the seabed; they speed across the ocean and rear up near the coast.
- Secondary hazard — a knock-on hazard triggered by the quake: tsunamis, landslides, liquefaction, fires.
Primary vs secondary hazards: The primary hazard is the ground shaking itself.
Secondary hazards are triggered by it: tsunamis, landslides, liquefaction (saturated soil behaving like a liquid), and fires from broken gas mains. Many earthquake deaths come from these secondary hazards, not the shaking alone.
How this is tested: Paper 1 Option D opens with a data-response built on a real figure — a hazard-event profile (criteria scored 1-7), a hazard map (intensity zones, displacement flows, tsunami arrival times), or a scatter graph of magnitude against deaths. You State or Identify a value, Estimate a distance or flow off a map, or compare two events. Always quote the units and read the correct axis or row.
| Criterion | Haiti 2010 (Mw 7.0) | Tohoku 2011 (Mw 9.0) |
|---|---|---|
| Magnitude | 5 | 7 |
| Frequency at this place | 3 | 5 |
| Speed of onset | 7 | 7 |
| Areal extent | 4 | 6 |
| Deaths (relative) | 7 | 5 |
| Economic loss (relative) | 5 | 7 |
Compare profiles row by row: To compare two events on a profile, read across one row at a time. The bigger Tohoku quake scores higher on magnitude, extent and economic loss, yet Haiti scores higher on deaths — a much poorer, less-prepared place with weak buildings can suffer more deaths from a smaller quake.
Estimate vs State on a figure: State / Identify = read an exact value or label off the figure (a profile score, a labelled event, the deaths for one point). Estimate = give a sensible read where there is no exact mark — a distance measured on a map's scale, or a displacement flow read between bands.
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The same magnitude earthquake can have very different impacts. Two sets of factors decide how bad it is: geophysical factors (the physical nature of the quake and the ground) and human factors (how exposed and prepared people are). Examiners reward answers that link a named geophysical factor to a real impact.
| Factor | Why it changes the impact |
|---|---|
| Magnitude | More energy released = stronger shaking over a wider area |
| Focus depth | Shallow focus = far more violent surface shaking than a deep focus of equal magnitude |
| Distance from epicentre | Shaking and most secondary hazards weaken with distance from the epicentre |
| Geology / ground | Soft, water-logged sediment amplifies shaking and can liquefy; solid bedrock shakes less |
| Relief & coast | Steep slopes trigger landslides; low, funnel-shaped coasts amplify tsunami run-up |
| Tsunami trigger | A shallow, offshore megathrust quake displaces water and adds a tsunami hazard |
Secondary hazards weaken with distance from the epicentre
- Liquefaction — needs strong shaking, so it is worst close to the epicentre and fades with distance.
- Landslides — triggered by strong shaking on slopes; severity falls as shaking weakens further out.
- Fires — depend on local shaking breaking gas and power lines, so they cluster near the epicentre.
- Tsunamis — the exception: they can travel thousands of km across an ocean and still strike hard far from the epicentre.
Always give the mechanism: Don't just name the hazard - say how it is triggered and why distance matters. Strong shaking -> liquefaction -> fades with distance; offshore rupture -> tsunami -> stays destructive far away.
Same region, very different impacts: Haiti 2010 (Mw 7.0) struck a shallow focus right beneath Port-au-Prince, a densely built, very poor city with weak buildings and almost no preparedness - over 200,000 people died and around 1.5 million were displaced.
Christchurch 2011 (Mw 6.3, New Zealand) was a smaller quake near a wealthy, well-prepared city. Its shallow focus and liquefaction on soft ground caused huge damage, but strong building codes and fast rescue meant far fewer deaths (around 185). Wealth and preparedness turned a comparable quake into a far less deadly one.
Magnitude and frequency vary by plate margin: Earthquakes differ in magnitude and frequency from place to place mainly because of the plate margin they sit on.
Destructive (convergent) margins, where one plate is forced under another (subduction), build huge stress and produce the largest, most frequent quakes - and offshore megathrusts that cause tsunamis. Conservative (transform) margins, where plates grind past each other, give frequent large quakes but no tsunami. Constructive (divergent) margins give frequent but generally smaller, shallow quakes. Human triggers (filling large reservoirs, deep fluid extraction or injection) can add quakes far from any margin.
How this is tested - the [10] Examine essay: Paper 1 Option D ends with a 10-mark Examine essay, marked on markbands. Two recurring versions: why earthquakes vary in magnitude and frequency from place to place (plate-margin types + triggers), and how geophysical factors (boundary type, focus depth, geology, relief) gave two named events very different impacts.
Top band needs: accurate terms, two or more developed factors anchored to named real events, a weighing of their relative importance / interaction, and a clear conclusion.