Earthquakes & Tsunamis (Years 10–11) — Geography

Earthquakes release built-up stress at plate boundaries, can trigger tsunamis, and hit poorer countries harder — but risk can be reduced.

On Boxing Day 2004, the sea off Indonesia suddenly drew back from the beaches. Minutes later a wall of water many metres high roared ashore across the Indian Ocean, killing around 230,000 people in fourteen countries. It began with a single violent lurch of the sea floor — an earthquake beneath the waves.

Why this matters — Earthquakes cannot be stopped, but preparation saves lives. The very same magnitude of quake can kill a handful of people in one country and tens of thousands in another. Geography helps explain that gap and how to close it.

How earthquakes happen

At plate boundaries, plates try to move but often lock together through friction. Stress builds up in the rock for years. Eventually the rock can take no more and suddenly snaps free, releasing a burst of energy as seismic waves that shake the ground.

The focus is the point underground where the earthquake starts.
The epicentre is the point on the surface directly above the focus — usually where shaking is strongest.

The focus is underground; the epicentre is the point on the surface directly above it.

Measuring earthquakes

A seismometer records ground shaking as a zig-zag trace. The energy released is given as a magnitude (the Richter idea): each step up the scale means about ten times more shaking and roughly thirty times more energy. So a magnitude 7 is far more than "a bit worse" than a magnitude 6 — it is enormously greater.

Each whole step up the magnitude scale means roughly ten times more shaking — the scale is not evenly spaced.

How tsunamis form

When an underwater earthquake at a destructive boundary jolts the sea floor upwards, it shoves the entire column of water above it. That energy spreads out as fast, low waves in the deep ocean. As they reach shallow coasts they slow and pile up into towering waves — a tsunami.

A sea-floor quake pushes water upward; low fast waves grow tall as they reach shallow water.

Richer vs poorer countries

Factor	Richer country (HIC)	Poorer country (LIC)
Buildings	Quake-resistant, enforced codes	Weak, poorly built
Warning systems	Advanced, tested	Few or none
Emergency services	Fast, well-equipped	Slow, under-resourced
Typical death toll	Usually lower	Often far higher

The same shaking leaves a braced building standing but topples a weakly built one — wealth and codes decide the toll.

Reducing the risk: the three P's

Prediction — monitoring gases, tremors and animal behaviour to guess when and where.
Protection — earthquake-proof buildings with deep foundations, cross-bracing and rubber shock absorbers; sea walls against tsunamis.
Preparation — drills, emergency kits, education and tsunami warning sirens so people know to reach high ground.

Same planet, different outcomes: Japan (a richer country) survives frequent large quakes with relatively few deaths thanks to strict building codes and drills, while a similar-magnitude quake in a poorer country with weak buildings can be catastrophic. Wealth and preparation, not just the quake, decide the toll.

Common mistake: The focus is underground; the epicentre is on the surface above it. Don't swap them. Also, tsunamis are not caused by wind — they are triggered by the sea floor moving.

Memory hook: The three P's for managing quakes: Prediction, Protection, Preparation.