What Causes Earthquakes?
Tectonic plates are always moving, but friction between plates stops them sliding smoothly. Stress builds up at the boundary. When the stress is finally released — suddenly — the rocks snap and move, sending out vibrations called seismic waves. This is an earthquake.
Key Terms
| Term | Definition |
|---|---|
| Focus (hypocentre) | The point inside Earth where the earthquake originates |
| Epicentre | The point on the surface directly above the focus — usually the most damaged area |
| Seismic waves | Energy waves that travel outward from the focus |
| Seismometer | Instrument that measures and records seismic waves |
The Richter and Moment Magnitude Scales
The Richter scale (and the more modern moment magnitude scale, Mw) measures earthquake size:
- Each whole number increase = 10× more ground shaking, and ~32× more energy
- Magnitude 2–3: barely felt
- Magnitude 5: moderate damage
- Magnitude 7+: major destruction possible
- Magnitude 9+: catastrophic (e.g. Japan 2011: 9.0; Nepal 2015: 7.8)
Most earthquakes happen along plate boundaries — particularly transform faults (like the San Andreas Fault in California) and subduction zones (like Japan, Chile, and the Pacific Ring of Fire). However, intraplate earthquakes also occur far from boundaries.
Primary and Secondary Effects
Primary (direct, immediate):
- Ground shaking — buildings collapse
- Surface rupture — cracks appear in the ground
- Tsunamis — seabed movement displaces huge volumes of water
Secondary (follow-on):
- Landslides
- Fires from broken gas pipes
- Disease from contaminated water
- Long-term economic disruption
Why Are Some Countries More Affected?
Two key differences:
- Physical: proximity to plate boundaries — Japan, Nepal, Turkey, Chile face high risk
- Economic: wealthier countries can build earthquake-resistant structures, invest in early warning systems, and respond quickly. Haiti's 2010 earthquake (7.0) killed over 200,000; a similar earthquake in richer countries would cause far fewer deaths.
Earthquake Preparation
- Earthquake-resistant building design (flexible frames, base isolators)
- Early warning systems — seconds of warning save lives
- Public education: "Drop, cover, hold on"
- Emergency response drills
Japan uses a national early warning system that automatically stops bullet trains, sounds alarms, and can give 10–60 seconds of warning before shaking arrives. This depends on seismic sensors detecting the first (weaker) P-waves before the destructive S-waves arrive.
Real world: The 2011 Japan earthquake and tsunami
On 11 March 2011, a magnitude 9.0 earthquake struck off the Pacific coast of Japan at a subduction zone where the Pacific Plate dives beneath the Eurasian Plate. The seabed moved by up to 50 metres, generating a devastating tsunami that overwhelmed sea walls and flooded coastal towns. Nearly 16,000 people died, and the tsunami damaged the Fukushima nuclear power plant, causing a nuclear crisis that affected the whole region.
Real world: The 2023 Turkey–Syria earthquake
On 6 February 2023, a magnitude 7.8 earthquake struck southern Turkey and northern Syria, killing over 50,000 people. The earthquake occurred on a transform fault where the Anatolian Plate slides past the Arabian Plate. Many poorly constructed buildings collapsed instantly. The disaster highlighted how a country's level of development — especially building quality — can determine how many lives an earthquake takes.
Real world: California earthquake drills
Millions of people in California take part in the annual "Great ShakeOut" earthquake drill each October — the world's largest earthquake drill. Schools, offices, and emergency services practice "drop, cover, hold on" procedures. California sits on the San Andreas Fault and seismologists expect a major earthquake (sometimes called "the Big One") in the future — preparation saves lives.
Where do you see this in real life? Earthquake news from Japan, Turkey, or New Zealand reminds us that tectonic plates are constantly moving — and when that movement is sudden, communities need to be prepared.