Hazards, disasters and how magnitude is measured

Hazard or disaster?

A hazard is a natural event that threatens people and property. A disaster happens when a hazard seriously disrupts a community, causing deaths, injuries and losses that exceed its ability to cope with its own resources.

The same hazard can cause a disaster in one place but not in another, because of vulnerability and capacity (see the risk equation lesson that follows).

Magnitude scales

Magnitude scales for the hazards in the specification

Skill (1): correlation and Spearman's rank

Plot magnitude against deaths or economic losses on a scattergraph, then calculate Spearman's rank correlation coefficient:

rs = 1 - (6 x sum of d squared) / (n(n squared - 1))

• Rank each variable (1 = highest); give tied values the average rank.
• d = difference in ranks for each event; square and add.
• rs ranges from +1 (perfect positive) through 0 (none) to -1 (perfect negative).
• Compare rs with the critical value for n in the table given in the exam to judge significance.

Usually there is only a weak link between magnitude and deaths, because vulnerability and capacity to cope vary so much. Correlation does not prove causation.

Worked example

Practice data (invented): for 6 earthquakes the sum of d squared between magnitude rank and death-toll rank is 28. Calculate rs.

1. rs = 1 - (6 x 28) / (6 x (36 - 1))
2. = 1 - 168 / 210
3. = 1 - 0.8

Answer: rs = 0.2: a weak positive correlation.

Check yourself

1. Using the USGS figure of about 32 times more energy per whole-number step, how many times more energy does a magnitude 8.0 earthquake release than a magnitude 6.0 earthquake?
   Show answer

   1024. Two steps: 32 x 32 = 1024.

2. How many times larger is the seismogram amplitude of a magnitude 6.5 earthquake than a magnitude 4.5 earthquake?
   Show answer

   100. Two steps of 10: 10 x 10 = 100.

3. Pinatubo (1991) was VEI 6 and Mount St Helens (1980) was VEI 5. Using the USGS approximation, how many times more ash did Pinatubo erupt?
   Show answer

   10. One VEI step is about 10 times the volume: about 10 km3 against about 1 km3.

4. Practice data (invented): n = 10 hazard events and the sum of d squared is 30. Calculate Spearman's rs to 2 decimal places.
   Show answer

   0.82. rs = 1 - 180/990 = 0.82.

5. A river gauge has 49 years of annual peak discharges. The second-highest flood has rank 2. Using recurrence interval = (n + 1) / rank, find its recurrence interval in years.
   Show answer

   25 years. (49 + 1) / 2 = 25 years.

6. Which scale measures the shaking felt and the damage done at a particular place?
   • Moment Magnitude
   • Volcanic Explosivity Index
   • Modified Mercalli Intensity
   • Saffir-Simpson
   Show answer

   Modified Mercalli Intensity. Intensity varies with location; magnitude is one number per earthquake.

7. A student finds rs = 0.15 between earthquake magnitude and deaths. The best explanation is that
   • death tolls depend heavily on vulnerability and capacity to cope, not only magnitude
   • magnitude is measured wrongly
   • large earthquakes always kill fewer people
   • Spearman's rank cannot be used for hazards
   Show answer

   death tolls depend heavily on vulnerability and capacity to cope, not only magnitude. Population density, building quality and preparedness differ between places.

8. Which is a disaster rather than only a hazard?
   • A cyclone over the open ocean
   • A flood that leaves 50,000 people homeless and overwhelms local services
   • An earthquake in an uninhabited desert
   • A volcano erupting on a remote uninhabited island
   Show answer

   A flood that leaves 50,000 people homeless and overwhelms local services. A disaster requires serious harm that exceeds the ability to cope.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Geography specification (Issue 3, September 2025)
• USGS: Earthquake magnitude, energy release and shaking intensity
• USGS Yellowstone Volcano Observatory: The Volcanic Explosivity Index (26 December 2022)
• NOAA National Hurricane Center: Saffir-Simpson Hurricane Wind Scale

Last updated 8 November 2026. Spotted a mistake? Tell us.