Edexcel Specification focus:
‘Magnitude and intensity of tectonic hazards are measured using the Mercalli scale, Moment Magnitude Scale (MMS), and Volcanic Explosivity Index (VEI).’
Understanding tectonic hazards requires tools to measure and compare them. Geographers use three main scales to assess earthquakes and volcanic eruptions.
Measuring Earthquake Magnitude
Moment Magnitude Scale (MMS)
The Moment Magnitude Scale (MMS) is the most widely used modern measure of earthquake magnitude. It provides a scientifically accurate assessment of the total energy released.
Moment Magnitude Scale (MMS): A logarithmic scale used to measure the total energy released by an earthquake, based on seismic moment.
The seismic moment is calculated using fault area, the average amount of slip, and the rigidity of rocks involved. The scale is logarithmic, meaning each whole-number increase represents a tenfold increase in amplitude and about 32 times more energy release.
MMS replaced the outdated Richter Scale, which was less reliable for larger, distant earthquakes.
It is suitable for comparing earthquakes globally.
MMS values typically range from 2 (microearthquakes) to 9 or higher (megaquakes).
Mercalli Intensity Scale
The Mercalli Intensity Scale is a qualitative measure of earthquake effects based on observed impacts on people, buildings, and the natural environment.
Mercalli Intensity Scale: A twelve-point scale (I–XII) measuring the intensity of an earthquake based on its observed effects and damage.
Unlike MMS, which measures energy release, the Mercalli scale reflects local experience and varies with:
Distance from the epicentre – effects weaken with distance.
Population density – more observations enable more accurate ratings.
Building structure – damage depends on construction quality.
Key levels of intensity:
I–III: Weak, felt only by a few.
IV–VI: Moderate, felt widely, minor damage.
VII–IX: Strong, causes damage to poorly built structures.
X–XII: Extreme, widespread destruction and ground deformation.
This scale is useful in hazard mapping and risk assessment, especially for urban planning.
Measuring Volcanic Eruption Magnitude
Volcanic Explosivity Index (VEI)
The Volcanic Explosivity Index (VEI) is a logarithmic scale used to classify the size of volcanic eruptions.
Volcanic Explosivity Index (VEI): A scale ranging from 0 to 8 that quantifies the explosiveness of volcanic eruptions based on factors such as eruption cloud height, volume of material ejected, and duration.
Key VEI indicators:
Eruption column height – higher columns indicate more explosive events.
Volume of ejecta – includes ash, lava, and tephra.
Duration – longer eruptions may be assigned higher VEIs.
Qualitative descriptions – such as gentle, explosive, or cataclysmic.
VEI categories include:
0: Non-explosive (e.g. Hawaiian lava flows).
3–4: Severe explosive eruptions (e.g. Mount St. Helens 1980).
6–8: Cataclysmic or super-eruptions (e.g. Krakatoa 1883, Toba ~74,000 years ago).
A logarithmic VEI chart where sphere diameters represent the volume of erupted tephra for each index (0 to 7), annotated with eruption examples (e.g., Mount St. Helens, Tambora).Source
Because VEI is partly based on qualitative data and subjective interpretation, it is less precise than MMS but still vital for assessing volcanic hazard potential.
Comparison and Use of Measurement Scales
Key Differences Between Scales
Quantitative vs. qualitative:
MMS and VEI offer quantitative energy-based measures.
Mercalli provides a qualitative assessment of human and structural impact.
Local vs. global comparison:
MMS and VEI allow for global comparison across events.
Mercalli varies locally depending on built environment and population.
Utility:
MMS is ideal for scientific analysis and hazard modelling.
Mercalli is valuable for emergency response and insurance assessment.
VEI informs volcanic risk management, aviation alerts, and geological studies.
Why Multiple Scales Matter
Each scale provides a different but complementary perspective:
Emergency services use Mercalli intensity to assess damage zones.
Seismologists rely on MMS to understand earthquake mechanics.
Volcanologists use VEI to monitor eruption risk and public safety.
Together, these tools help build more accurate hazard profiles and enable effective disaster planning.
Summary of Key Measurement Scales
Moment Magnitude Scale (MMS): Measures total seismic energy released (logarithmic).
Mercalli Intensity Scale: Measures earthquake impact based on observations (descriptive).
Volcanic Explosivity Index (VEI): Measures eruption magnitude using eruption height and volume (logarithmic and descriptive).
By using these scales, geographers and hazard managers can evaluate tectonic events more accurately and develop strategies to mitigate their impacts.
FAQ
MMS is based on the seismic moment, which considers:
The area of the fault that slipped
The average amount of slip (displacement)
The rigidity of the rocks involved
This means that larger faults and stiffer rocks release more energy. By using these physical parameters, MMS gives a consistent energy-based measurement, unlike earlier scales that relied only on wave amplitude. This makes it particularly useful for comparing earthquakes of different types and in different geological settings.
The Mercalli Intensity Scale measures the effects of shaking on people and structures, not the total energy released.
Factors that influence Mercalli intensity despite identical MMS values include:
Distance from the epicentre
Local ground conditions (e.g. soft sediments amplify shaking)
Building materials and construction methods
Population density and human perception
So, a magnitude 6.0 earthquake near a city with poor infrastructure may score higher on the Mercalli scale than the same magnitude quake in a sparsely populated region.
VEI is primarily applied after an eruption has ended because it requires data such as:
Total volume of ejected material
Maximum height of eruption column
Duration of the eruption
While preliminary VEI estimates may be made during an event, the final VEI classification typically occurs post-eruption. For real-time hazard assessment, volcanologists rely more on satellite monitoring, seismic activity, and gas emissions than VEI alone.
Yes. The Mercalli Intensity Scale ranges from I (not felt) to XII (total destruction).
Intensity I: Not felt except under especially favourable conditions.
Intensity XII: Total destruction, ground visibly moves, structures destroyed.
There are no values below I or above XII. This fixed range makes the scale useful for describing human experience and structural damage, but less effective for measuring very deep or remote earthquakes where effects are minimal.
Although VEI is based on ejecta volume and eruption height, gas content influences explosivity, which indirectly affects VEI.
High gas pressure can cause explosive eruptions with tall plumes.
Low gas content tends to produce effusive eruptions, which have lower VEI values.
Eruption type matters:
Effusive eruptions (e.g. basaltic lava flows) usually score VEI 0–1.
Explosive eruptions (e.g. andesitic or rhyolitic) can score VEI 4–7.
So, gas-rich magma tends to drive higher VEI ratings through more violent eruption dynamics.
Practice Questions
Question 1 (3 marks)
Identify and describe two ways in which the Moment Magnitude Scale (MMS) differs from the Mercalli Intensity Scale.
Mark Scheme:
1 mark for identifying a valid difference between MMS and Mercalli.
1 mark for a second valid difference.
1 mark for a developed description of either difference.
Acceptable Answers:
MMS measures the total energy released by an earthquake, while Mercalli measures observed impacts. (1 mark)
MMS is quantitative and logarithmic, whereas Mercalli is qualitative and based on people's experiences. (1 mark)
Description: The MMS gives a consistent measure worldwide, while Mercalli values can vary depending on location and building quality. (1 mark)
Question 2 (6 marks)
Explain how the Volcanic Explosivity Index (VEI) is used to measure the magnitude of volcanic eruptions. Include reference to the indicators used in the VEI.
Mark Scheme:
1 mark for identifying that VEI is a logarithmic scale used to classify eruptions.
Up to 2 marks for explaining the indicators used (e.g. eruption column height, volume of ejecta, duration).
1–2 marks for linking VEI categories to real-world examples or eruption intensity.
1 mark for clear explanation of how these indicators contribute to determining magnitude.
Acceptable Answers:
The VEI is a logarithmic scale ranging from 0 to 8 that measures the magnitude of volcanic eruptions. (1 mark)
It uses indicators such as the height of the eruption column and the volume of material ejected. (1 mark)
Larger volumes and higher columns indicate a more explosive eruption. (1 mark)
For example, a VEI 3 eruption like Mount St. Helens involved severe explosions, while a VEI 0 is non-explosive. (1 mark)
Duration of the eruption and qualitative descriptions like ‘cataclysmic’ also influence the assigned VEI. (1 mark)
These indicators together help scientists assess and compare the severity of eruptions. (1 mark)
