Earthquakes: How They Happen & How We Measure Them
Understand the earthquake mechanism, the difference between magnitude and intensity, and why the same earthquake kills thousands in one country but few in another.
Focus & Epicentre
The focus is where the earthquake starts underground. The epicentre is directly above on the surface - this is where damage is usually worst.
Grade 8/9 Focus: Explain why a magnitude 7.0 earthquake kills thousands in an LIC but few in an HIC. The answer is vulnerability, not magnitude.
How Earthquakes Happen
Earthquakes occur at plate boundaries where friction locks plates together. As plates try to move, stress builds elastically over years or decades. When the stress exceeds the friction threshold, the plates slip suddenly - releasing all that stored energy in seconds.
The 3-Step Sequence:
- Lock: Friction holds plates together
- Build: Pressure accumulates elastically
- Release: Friction exceeded → sudden slip → seismic waves
The energy travels outward as seismic waves. P-waves arrive first (compression), then S-waves (shaking - causes most damage), then surface waves.
Step 1: Friction locks plates together
Step 2: Pressure builds elastically over years
Step 3: Friction threshold exceeded → sudden release in seconds
Seismic Wave Types
Magnitude vs Intensity
Richter Scale vs Modified Mercalli Scale
| Factor | Magnitude (Richter) | Intensity (Mercalli) |
|---|---|---|
| Measures | Energy released at focus | Damage observed at location |
| Value | Fixed per earthquake | Varies by location |
| Scale | 1-9 (logarithmic) | 1-12 (descriptive) |
| Method | Seismograph readings | Observed effects/damage |
| Depends on | Fault slip amount | Distance, buildings, geology |
Each whole number = 30× more energy
Energy: ~32x a magnitude 5.0 earthquake
Damage to poorly built structures
Key Insight
The same magnitude earthquake causes different intensities at different locations. That's why magnitude is fixed, but intensity varies.
Global Earthquake Distribution
90% of earthquakes occur along the Pacific Ring of Fire - where the Pacific Plate meets surrounding plates. The pattern matches plate boundaries exactly.
Primary & Secondary Hazards
- Ground shaking - violent movement as waves pass
- Ground displacement - permanent shifting along fault
- Liquefaction - saturated soil loses strength, behaves like liquid
- Building collapse - structures fail under stress
- Tsunamis - underwater earthquakes displace water
- Fires - ruptured gas pipes ignite
- Landslides - destabilised slopes collapse
- Disease - contaminated water, overcrowded shelters
Click a primary hazard to see the secondary hazards it causes
Exam Practice
Compare the 2011 Christchurch earthquake (6.3 magnitude, 185 deaths) with the 2016 Kaikoura earthquake (7.8 magnitude, 2 deaths). Both occurred in New Zealand. Explain why the lower magnitude earthquake caused more deaths.
Test Your Knowledge
A magnitude 7.0 earthquake releases how much more energy than a 6.0?
Key Terms
Bottom Line: Why do some earthquakes kill more people? It's not just magnitude - it's vulnerability. Population density, building quality, warning systems, and emergency response determine whether an earthquake is a disaster or not.