Hazard Magnitude & Frequency (DP IB Geography)

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

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

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)

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

                                    Based on data from USGS

Recurrence of Volcanic Eruptions of Different Magnitudes