1.5A - Measuring Magnitude and Intensity
The magnitude and intensity of tectonic hazards is measured using different scales (Mercalli, Moment Magnitude Scale, (MMS) and Volcanic Explosivity Index (VEI))
Magnitude can be seen as the 'size' of a tectonic event. Generally, larger magnitude events have a larger impact, however the relationship is complicated by the vulnerability and capacity to cope parts of the hazard risk equation.
MMS
Earthquake magnitude is measured by the Moment Magnitude Scale, an updated version of the Richter scale. It measures the energy released during an earthquake. This is related to the amount of slip (movement) on the fault lane and the area of movement on the fault plane. MMS uses a logarithmic scale, meaning that a magnitude 6 earthquake has ten times more ground shaking than a magnitude 5.
Mercalli
The Mercalli scale measures earthquake intensity on a scale of I-XII. This older scale measures what people actually feel during an earthquake, i.e. the intensity of the shaking effects not the energy released. It cannot be used to easily compare earthquakes as shaking experienced depends on building type and quality, ground conditions and other factors.
The relationship between magnitude and death toll is a weak one because:
The relationship between magnitude and death toll is a weak one because:
- some earthquakes cause serious secondary impacts, such as landslides and tsunami
- earthquakes hitting urban areas have greater impacts than those in rural areas
- level of development, and level of preparedness, affect death tolls
- isolated, hard to reach places could have a higher death toll because rescue and relief take longer
- a very high magnitude could not cause any deaths in an unoccupied area
VEI (Volcanic Explosivity Index)
This measures that magnitude of a volcanic eruption. It ranges from 0-8 and is a composite index combining eruption height, volume of material (ash, gas, tephra) erupted and duration of eruption.
VEI |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
Eruption height |
<100m |
100m- 1 km |
1-5km |
3-15km |
>10 km |
>10 km |
>20 km |
>20km |
>20 km |
Eruption volume (m or km^3 ) |
<10,000 |
>10,000 |
>0.001 |
>0.01 |
>0.1 |
>1 |
>10 |
>100 |
>1000 |
. |
Effusive |
. |
Explosive |
. |
. |
. |
. |
Colossal |
. |
VEI eruptions from 0-3 are associated with shield volcanoes and basaltic eruptions at constructive plate boundaries and mid-plate hotspots. VEI eruptions from 4-7 occur at destructive plate margins, erupting high viscosity, high gas, high silica andestic magma. No modern human has ever experienced a VEI 8 supervolcano. These are ones whose impacts would be felt globally, because of the worldwide cooling of the Earth's climate, perhaps for up to 5 years. These are rare caldera eruptions such as Yellowstone and Toba (Sumatra, Indonesia).