PSLE Science Reality Lab 355 | ShakeMap Intensity vs Magnitude

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Wait, What? One Earthquake Can Have One Magnitude and Many Intensity Values

An earthquake map appears online after a large event. Near one town the map is labelled MMI VI. A student reads the Roman numeral and says, “So that was a magnitude 6 earthquake.”

The number looks familiar. The conclusion is not.

Earthquake magnitude and earthquake intensity answer different questions. Magnitude describes the size of the earthquake source using a magnitude scale. Intensity describes the severity of shaking and effects at a particular place. A single earthquake therefore has one reported magnitude but can produce many intensity values across a region.

USGS explains that Modified Mercalli Intensity, or MMI, characterises shaking at a location using effects on people, structures and the landscape. Modern ShakeMaps can also estimate MMI from recorded ground motion. Either way, an intensity label on a map is not a second name for magnitude.

Quick Answer

Ask whether the number is magnitude or intensity.
Magnitude belongs to the earthquake event; intensity belongs to a location affected by the event.
MMI uses Roman numerals and is based on shaking effects or an estimate derived from ground-motion measurements.
Intensity can be different in different places because distance, local ground conditions and wave propagation differ.
“MMI VI” does not mean “magnitude 6”, and it does not mean every place experienced the same shaking.

The Exact Learner Job This Page Owns

This Reality Lab owns one evidence-transfer job: evaluating an earthquake map or headline that shows an intensity value without confusing that local shaking measure with the earthquake’s magnitude.

It does not become the owner of earthquake physics, logarithmic magnitude scales or map-reading skills in general. Those jobs already have homes. This page applies those skills to a real communication object: a ShakeMap or intensity report.

Original Reality Lab Case: One Event, Four Towns

This is an original composite case for learning. A fictional earthquake has a reported magnitude of 6.4. A ShakeMap shows four towns with different estimated intensities.

Place	Map label	Reasonable reading
Town A	MMI IV	Light-to-moderate felt shaking at this location
Town B	MMI VI	Stronger local shaking and effects than Town A
Town C	MMI VII	Still stronger local effects
Town D	MMI V	Different local shaking despite the same earthquake source

The earthquake did not somehow have four magnitudes. The event has one magnitude; the map reports how strongly different locations were affected.

Observed, Reported and Inferred

Layer	What it could contain
Observed	Instrument recordings, reports of felt shaking, movement or damage
Processed	Ground-motion values, location information and mapping procedures
Estimated	Instrumentally derived intensity between and around measurement points
Displayed	A colour band or Roman numeral such as MMI VI
Too-strong claim	“MMI VI means the earthquake was magnitude 6”

The First Check: What Question Does Each Scale Answer?

A useful way to avoid confusion is to attach each number to its scientific job.

Magnitude: How large was the earthquake source?
Intensity: How strong were the shaking and effects at this place?

The same numeral can appear in both systems, but identical-looking numbers do not make the quantities identical. A thermometer reading of 6°C and a wind speed of 6 m/s both contain the number 6; nobody should merge them. The label and quantity matter.

The Location Check: Intensity Belongs Somewhere

Intensity makes sense only when attached to a location or area. Two towns can experience different shaking because they are at different distances from the fault, sit on different ground, or receive seismic waves differently. USGS notes that intensity refers to effects at a particular site and can therefore vary from place to place.

If a headline says only “Intensity VI” without telling you where, an important part of the evidence object is missing.

The Map Check: A Coloured Area Is Not a Wall

ShakeMaps colour broad regions so that people can understand the pattern quickly. But nature does not stop at a sharp painted border. Ground shaking can vary over short distances. A colour boundary is a representation of a changing field, not a physical wall where one side suddenly switches to a completely different state.

USGS also warns that ShakeMap values are approximate and that map products may combine recorded data with estimates in places where stations are sparse. This is not a flaw hidden by the map. It is part of how the representation is constructed.

The Station Check: Which Parts Were Measured and Which Were Estimated?

Near seismic stations, the shaking map can be strongly constrained by recorded data. Between stations, mapping systems use scientifically justified relationships and interpolation. A student should therefore distinguish recorded ground motion from mapped estimate.

This is the same habit used elsewhere in Science: do not erase the difference between observation and modelled extension just because they share one smooth colour surface.

The Damage Check: Same Intensity Does Not Guarantee Same Damage

USGS cautions that locations within the same intensity area do not necessarily experience the same damage. Building design, construction quality, ground motion details and local conditions matter. An intensity category summarises shaking severity; it does not inspect every building one by one.

Therefore, “both neighbourhoods are MMI VII” does not support “every building in both neighbourhoods suffered identical damage”.

The Time Check: Early Maps Can Change

Rapid earthquake products are designed to be useful quickly. As more station data, reports and processing become available, map versions can be updated. A screenshot from the first few minutes and a later revised map may differ.

A careful reader therefore records the map version or timestamp before claiming that two scientific sources disagree.

What Evidence Would Strengthen “Town B Shook More Strongly Than Town A”?

Both places are shown on the same ShakeMap version and legend.
The intensity estimates are clearly different and supported by nearby recordings or reports.
Station data show stronger ground motion near Town B.
The comparison uses the same intensity scale.
The conclusion stays about shaking severity rather than jumping directly to total damage or earthquake magnitude.

What Would Weaken the Claim?

One value is magnitude and the other is MMI.
The two screenshots use different map versions or legends.
The location is far from measurement stations and uncertainty is ignored.
The map colour is compared without reading its scale.
Damage differences are used as if building type and local ground conditions do not matter.

Worked Case 1: M6.4, MMI IV and MMI VII

A magnitude 6.4 earthquake produces MMI IV in a distant town and MMI VII closer to the fault. There is no contradiction. Magnitude describes the event; intensity describes local effects. The stronger intensity near the fault does not mean that part of the earthquake had a larger magnitude.

Worked Case 2: Same Distance, Different Shaking

Two fictional neighbourhoods are equally far from the earthquake source, but one sits on soft sediment and the other on firmer rock. Their recorded shaking differs. Distance matters, but it is not the only condition controlling local ground motion. A one-variable explanation would be too narrow.

Worked Case 3: One Map Pixel, Many Buildings

A map area is labelled MMI VI. A student concludes that every building there had the same experience. The map does not support that level of detail. A regional intensity estimate and an individual building response are different evidence scales.

Worked Case 4: A Revised ShakeMap

An early map estimates MMI V at a town. A later version shows MMI VI after more information is incorporated. The scientific response is not “the first map lied”. A rapid estimate can be revised when the evidence base improves.

Tempting Reasoning That Fails

“MMI VI means magnitude 6.” They are different quantities.
“One earthquake has one intensity.” Intensity varies by location.
“The coloured boundary is exact.” The map summarises a spatially changing field.
“Same intensity means same damage.” Structures and local conditions differ.
“Every colour is directly measured.” ShakeMaps can include estimates between observations.

Model and Measurement Limits

ShakeMap is a scientific representation assembled from available ground-motion recordings, earthquake-source information and mapping relationships. Station coverage is uneven. Shaking can vary rapidly across short distances. Instrumentally estimated intensity is related to, but not identical with, traditional reports of how people and structures experienced the earthquake.

Those limits do not make the map unscientific. They tell us exactly how far its claims can travel.

How Far Can the Conclusion Travel?

An MMI value can support a statement about the severity of shaking and expected effects at a location or mapped area. It cannot, by itself, tell you the earthquake magnitude, guarantee identical damage in every building, or prove that every coloured point was directly measured.

PSLE-Style Transfer Case

A map of one earthquake shows Location P at MMI V and Location Q at MMI VII. The earthquake report gives magnitude 6.2.

Question: A pupil says Location Q experienced a magnitude 7 earthquake while Location P experienced magnitude 5. Explain the error.

Reasoned answer: The magnitude 6.2 describes the earthquake event. MMI V and MMI VII describe the shaking intensity at different locations. The two places experienced different local shaking from the same earthquake; they did not experience separate earthquakes of magnitude 5 and 7.

Explained Practice

Practice A: Two towns have MMI VI but different amounts of building damage. Is that impossible? No. Building type, construction and local ground motion can affect damage within the same broad intensity category.

Practice B: A town’s intensity estimate changes in a later map version. Does the earthquake magnitude have to change too? No. Updated local evidence can alter the intensity map while the event magnitude remains the same.

Practice C: A colour region contains no nearby seismic station. What should you ask? Whether the value is estimated from surrounding data and how uncertain the local map is.

Delayed Independent Return: M-A-P

M — Measure: Is this magnitude or intensity?
A — Address: Which location does the intensity describe?
P — Provenance: Was it observed, recorded or estimated on the map?

Parent and Tutor Teaching Guide

Use one desk tap as the “earthquake”. Put three cups at different distances and on different surfaces. The tap is one event, but each cup may move differently. Then label the event with one “magnitude” card and each cup with a different “intensity” card. The physical analogy makes the ownership difference visible without teaching seismology equations.

After that, show a simple coloured map and ask the learner to point to where the event value belongs and where the location values belong. The target is not memorising Roman numerals. The target is attaching each number to the correct scientific question.

Authoritative Sources

The Quiet Return

One earthquake can shake a whole region, but it does not shake every place equally.

When a map gives you a number, first ask whether it belongs to the event or to the place.

Small Group Tutorials

PSLE Science Reality Lab Vol No.355 | “ShakeMap Intensity = VI” — Was It a Magnitude 6 Earthquake?