Hazard Magnitude & Frequency (DP IB Geography)

Revision Note

Risk Management & Hazard Magnitude

Magnitude and risk management

  • The magnitude is the strength of a hazard and often refers to the amount of energy released 

  • Most hazards are measured using a scale:

    • Earthquakes – Moment Magnitude Scale, Mercalli Scale and Richter Scale

    • Volcanoes – Volcanic Explosivity Scale

  • Higher magnitude events usually result in greater impact 

Earthquake magnitude

  • Magnitude is the most common way to measure an earthquake

  • This is a measure of the energy released at the source (focus):

    • The magnitude can be measured on different scales

    • The most well-known scale is the Richter Scale

    • The one that is increasingly used now and is reported on the news when an earthquake occurs is the Moment Magnitude Scale (MMS)

  • The MMS goes from 1, which is not felt by humans, to 10

  • The MMS is a logarithmic scale, which means that a 6 on the scale is a ten-times increase in amplitude from a 5: 

    • The energy release is 32 times greater

Diagram to show the magnitude of earthquakes

Chart comparing seismic wave energy in earthquakes with energy equivalents like nuclear tests and lightning bolts, with a key detailing earthquake magnitudes.
Magnitude of earthquakes

Earthquake intensity

  • The intensity of an earthquake is its impact on people, as well as the built and natural environments

  • The Modified Mercalli Intensity Scale is used to measure the intensity

  • The scale goes from I to XII

The Modified Mercalli Intensity Scale

Chart depicting earthquake intensity levels from I (Not felt) to X+ (Extreme), with images and descriptions of effects, such as rocking cars, broken windows, and structural damage.
Modified Mercalli Intensity Scale

Examiner Tip

Remember not to confuse magnitude and intensity.

Magnitude is the amount of energy released, whereas intensity is the severity of ground shaking at a particular place based on the effects on humans, buildings and the environment.

Measuring volcanic eruptions

  • Every eruption is unique; some are gentle, others are hugely explosive

  • The Volcanic Explosivity Index (VEI) is used to measure the size of an eruption

  • This can not be measured on a scientific instrument so is calculated based on a series of measurements and observations:

    • These include:

      • Height of material ejected into the atmosphere

      • Volume of material

      • Duration of the eruption

  • This is a logarithmic scale from 0–8 

Diagram to illustrate the Volcanic Explosivity Index (VEI)

Chart depicting volcanic eruptions by VEI and erupted tephra volume. Highlights Mono-Inyo Craters, Mount St. Helens, Pinatubo, Tambora, and Yellowstone Caldera.
Volcanic Explosivity Index

Risk Management & Hazard Frequency/Recurrence

  • The recurrence interval of a hazard event is measured in years

  • It is the expected occurrence of an event of a particular size

  • Large magnitude events have a longer recurrence interval than smaller magnitude events

  • This means there are many small-scale earthquakes, eruptions and landslides and few large-scale ones

Recurrence of Earthquakes of Different Magnitudes

Magnitude

Average number annually

8+

1

7 – 7.9

15

6 – 6.9

134

5 – 5.9

1319

4 – 4.9

13,000

3 – 3.9

130,000

2 – 2.9

1,300,000

                                    Based on data from USGS

Recurrence of Volcanic Eruptions of Different Magnitudes

Magnitude

Occurrences In last 10,000 years

8

0

7

7

51

166

421

868

3,477

1

Many

0

Many

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