📏 Richter scale
Based on the height of seismometer waves. Logarithmic. Good for small and medium earthquakes. Not accurate for the very biggest.
Impacts of Tectonic Hazards » Measuring Earthquakes and Eruptions
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 4.3.4
- How the moment magnitude scale and the Richter scale measure the size of an earthquake
- How the Mercalli scale measures the effects of an earthquake at a place
- How the volcanic explosivity index measures the size of an eruption
- Why the size of an event does not tell you how many people will die
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Unlock This CourseWhy we need scales
Words like 'big' or 'small' are not good enough for scientists, planners or the news. Geographers use scales to give an earthquake or an eruption a number, so events can be compared fairly. There are two different ideas to keep apart. One is how big the event is. The other is how strong its effects are in a particular place.
Key terms:
- Magnitude: the size of an earthquake at its source, measured as one number for the whole event.
- Intensity: how strongly an earthquake is felt, and how much damage it does, at one particular place.
- Logarithmic scale: a scale where each step up is a multiple (such as ten times) of the step below, not just one more.
Remember from Earthquakes: Focus, Epicentre and Seismic Waves that a seismometer records the shaking. Magnitude scales use those records.
Measuring magnitude: the moment magnitude and Richter scales
The Richter scale was created in 1935 by Charles Richter. It uses the height of the waves drawn by a seismometer. It is logarithmic. Each whole number up the scale means:
- waves on the seismogram about 10 times bigger (about 10 times more ground shaking);
- about 30 times more energy released.
So a magnitude 7 earthquake has waves 100 times bigger than a magnitude 5 (10 × 10). It releases about 1,000 times more energy (30 × 30 is about 1,000).
The Richter scale works well for small and medium earthquakes. For very large earthquakes it stops showing the real difference in size. Scientists now use the moment magnitude scale (Mw). It measures the total energy released by the earthquake. This comes from how big the fault area that slipped was, how far the rocks moved and how strong the rock was. It is used today for large earthquakes and gives numbers similar to Richter's for smaller ones. It is also logarithmic, and it is the scale usually quoted in the news.
⚡ Moment magnitude scale
Based on the total energy released. Logarithmic. Used today, especially for large earthquakes. The largest recorded, Chile in 1960, was about 9.5.
Measuring intensity: the Mercalli scale
The Mercalli scale does not use instruments. It measures intensity from what people see, feel and what damage is done. It uses Roman numerals from I to XII.
| Mercalli level | What is observed |
|---|---|
| I | Not felt, except by a very few people |
| III to IV | Felt indoors; hanging objects swing; dishes rattle |
| VI to VII | Felt by everyone; furniture moves; weak buildings are damaged |
| IX to X | Many buildings badly damaged or destroyed; cracks in the ground |
| XII | Total destruction |
One earthquake has one magnitude but many intensities. Intensity is highest near the epicentre and falls with distance. It also varies with the type of ground (soft, wet soil shakes more than solid rock), with the depth of the focus and with how well buildings are made. A strong earthquake may be only a level III in a city 300 km away and a level IX at the epicentre.
Measuring eruptions: the volcanic explosivity index
Volcanoes have their own scale. The volcanic explosivity index (VEI) runs from 0 to 8. It is based on the volume of material thrown out (ash, rock and lava) and the height of the ash column. It is also logarithmic: each step up is about 10 times more material erupted.
| VEI | Description | Example |
|---|---|---|
| 0 | Gentle, non-explosive; lava flows quietly | Hawaiian eruptions such as Kilauea |
| 4 | Very large; ash column over 10 km high | Eyjafjallajokull, Iceland, 2010 |
| 5 | Huge; ash column over 25 km | Mount St Helens, USA, 1980 |
| 6 | Colossal; over 10 km3 of material | Mount Pinatubo, Philippines, 1991 |
| 7 | Super-colossal; over 100 km3 | Tambora, Indonesia, 1815 |
| 8 | Mega-colossal; over 1,000 km3; very rare | The ancient Yellowstone and Toba eruptions |
A VEI 7 eruption is about 10 times bigger than a VEI 6, and about 100 times bigger than a VEI 5. Note that the VEI measures the size of the eruption, not how many people are harmed.
Why magnitude does not predict deaths
A big number on a scale does not mean a big death toll. Here are two earthquakes that prove it.
- Alaska, USA, 1964: magnitude 9.2, the second largest recorded with modern instruments. About 130 people died, because few people lived nearby.
- Bam, Iran, 2003: only magnitude 6.6, yet it killed more than 25,000 people. It was shallow, it struck right under a city, and most of the buildings were made from weak materials such as mud brick.
What decides the death toll?
Depth of the focus (shallow earthquakes shake harder at the surface). Distance from people. Population density. Quality of buildings. Time of day. Ground type. Secondary hazards such as tsunamis. Preparation and rescue (an LIC often has fewer resources than an HIC).
Common mistakes
Thinking magnitude 8 is only a bit more than magnitude 7. It is about 10 times more shaking and about 30 times more energy. Mixing up magnitude (one number for the event, Richter or Mw) with intensity (many numbers, Mercalli). Writing that the VEI measures deaths.
Exam-style question
Fig. 1.1 shows made-up data for two earthquakes.
| Fig. 1.1 | Earthquake P | Earthquake Q |
|---|---|---|
| Magnitude (Mw) | 6.4 | 7.5 |
| Depth of focus | 8 km | 60 km |
| Where it struck | Under a large city | Mountains, few villages |
| Buildings | Mostly weak | Mostly weak |
| Deaths | 2,500 | 40 |
(a) State what the Mercalli scale measures. [1 mark]
(b) Calculate how many times bigger the seismometer waves are in a magnitude 7 earthquake than in a magnitude 5 earthquake on the Richter scale. [2 marks]
(c) Explain why Earthquake P killed more people than Earthquake Q. [3 marks]
(d) Discuss how far the magnitude of an earthquake decides how many people will die. [6 marks]
Model answer
(a) The intensity of an earthquake, meaning the effects felt and the damage seen at a place (1).
(b) Each whole number is 10 times bigger (1), so 10 × 10 = 100 times bigger (1).
(c) P was shallow, so the shaking at the surface was stronger (1). It struck under a large city, so many people were close to the shaking (1). Weak buildings collapsed and killed people, while Q hit few villages (1).
(d) Level 3 (5-6 marks): Magnitude does matter, because a larger magnitude means more energy and stronger shaking, which is more likely to destroy buildings. Fig. 1.1 shows Q is bigger than P, so if magnitude alone decided deaths, Q would have killed more. It killed far fewer (40 against 2,500), because other factors matter more here. P was shallow (8 km against 60 km) and struck under a large city, so far more people were exposed. In both, buildings were weak, which made damage worse, but only P had a crowded city to damage. Other real cases show the same: Alaska in 1964 at 9.2 killed about 130 people, but Bam in 2003 at 6.6 killed over 25,000. Time of day, tsunamis and rescue resources also matter. Magnitude is therefore one factor, but not the main one. Where people live, building quality and depth usually decide the death toll more (links made and a judgement given).
Exam tip
In a 'Discuss' answer, give the point for magnitude, then at least two other factors, and finish with a clear judgement. Use figures from the table.