S5 Geography | Natural Hazards: Earthquakes, Volcanoes, Tsunamis and Floods

Natural hazards is the chapter that joins a physical process to human risk: the same earthquake kills far more people in a dense city than in an empty mountain valley. The exam does not ask you to name disasters; it asks you to put numbers on scale, frequency and consequence.

What this chapter tests

• Energy and magnitude: one Richter unit is about ten times the amplitude and 31.6 times the energy, while wind pressure goes as the square of wind speed.
• Time differences: the P to S arrival lag fixes the distance to an epicentre, and tsunami speed is roughly the square root of g times water depth.
• Frequency and probability: a return period is a long-run average, so a fifty year flood has about a two per cent chance every year.
• Risk and loss: risk combines hazard, exposure and vulnerability, which is why defences, planning and insurance all matter.

Three threads

The first is internal processes: plate boundaries build stress that is released as earthquakes and eruptions, and depth, fault type and soil control how much shaking is amplified. The second is atmospheric and hydrological processes: a tropical cyclone brings wind, rain and storm surge, and intense rain becomes landslides and floods inland. The third is the human response: warnings, land-use zoning, building codes and insurance all subtract from the same risk equation.

Three habits earn marks: unify units first (millimetres to metres, square kilometres to square metres, km/h to m/s), separate a multiple from a percentage rise, and sanity-check the order of magnitude, because a tsunami crosses an ocean in hours, not days.

Where marks are lost

• Writing that one magnitude unit means ten times the energy, when it is ten times the amplitude and about 31.6 times the energy.
• Using total reservoir capacity instead of usable storage when counting drought days.
• Treating a return period as a fixed cycle, so that a recent flood is assumed to make the next years safe.
• Comparing a sea wall and beach nourishment by total cost rather than cost per metre per year.

Practice papers

Four papers build up in difficulty, ten items each, with answers and a one-line explanation; every figure is computed and checked by script, so they work as independent practice or as tutor preparation.

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Frequently asked

Why is one magnitude unit not ten times the energy?
Magnitude measures ground amplitude, which rises about tenfold per unit; energy scales with amplitude to the power 1.5, so it rises about 31.6 times, and two units is about a thousand times.

Does a one in a hundred year flood happen once every hundred years?
No. It is a long-run average of about one per cent a year, independent each year, so the chance of at least one in thirty years is about 26 per cent.