Hazard Magnitude & Frequency (SL IB Geography)

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

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

Modified Mercalli Intensity Scale

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)

Volcanic Explosivity Index

• 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
6	51
5	166
4	421
3	868
2	3,477
1	Many
0	Many