Reality Lab ID: PSLE-SCI-REALITY-0502
Wait, what? An earthquake map colours one location orange and labels it PGA = 30% g. One learner says, “The ground must have been accelerating at thirty percent of gravity for the whole earthquake.” Another reads the percentage as a damage score: “So thirty percent of the buildings were damaged.” A third thinks the map shows how far the ground moved. None of those conclusions follows from the label.
This PSLE Science Reality Lab examines a real scientific communication object used in earthquake reporting: a map of peak ground acceleration, or PGA. Primary 5 and Primary 6 learners do not need to become seismologists. They do need a powerful evidence habit: when a map gives one number for a changing event, identify exactly what was measured or estimated, what “peak” means, what the unit means, how the value was produced at that location and what other outcomes the number does not directly measure.
The 2026 PSLE Science assessment framework includes interpreting and analysing information, evaluating observations, information and methods, and communicating explanations and reasoning. The 2023 Primary Science syllabus also emphasises healthy scepticism, assumptions, uncertainty and understanding how Science is communicated in different forms and media. A ShakeMap-style PGA map is therefore useful evidence practice: it compresses a complicated time-varying event into a spatial representation that is informative only when its measurement meaning is preserved.
Quick Answer
PGA = 30% g means that the peak ground acceleration represented for that site or map location reached about 0.30 times the standard acceleration due to gravity under the product’s definition. It does not mean the ground maintained that acceleration throughout the earthquake. “Peak” means the largest relevant acceleration reached during the shaking record or estimated for the location.
The percentage also is not a percentage of damage, a percentage of time, a probability of an earthquake, the distance the ground moved or the speed of the ground. To evaluate a PGA map properly, check whether the value came from a nearby station or from a mapped estimate, the time and earthquake represented, the spatial resolution, the product uncertainty and whether another quantity—such as velocity, intensity or building response—is required for the claim being made.
The Owned Learner Job
This article owns one learner job: how to read a PGA percentage on an earthquake communication map without turning a peak acceleration into constant shaking, a damage percentage, a displacement value or a guaranteed building outcome.
It does not re-teach gravity, forces, acceleration, earthquakes, seismic waves, graph reading, interpolation or uncertainty as standalone topics. Existing Science owners keep those jobs. Reality Lab applies them to one communication object and then routes outward.
The Map Detective: Five Questions Before You Interpret the Colour
- Quantity: Is the map showing peak ground acceleration, peak ground velocity, intensity, probability or something else?
- Unit: Is acceleration shown in %g, g, m/s² or another unit?
- Provenance: Is this location supported by a seismic station measurement, a model estimate, or a combination?
- Time: Which earthquake and processing update does the map represent?
- Claim: Are we trying to describe shaking, damage, movement distance, building response or risk?
If the map answers one of these questions, do not silently use it to answer all five.
Original Composite Case: The Four Squares of Cedar Valley
Imagine an original fictional earthquake map for Cedar Valley. Four square map cells show the following PGA values:
| Map cell | PGA | Nearby seismic station? | Map note |
|---|---|---|---|
| A | 12% g | Yes, 2 km away | Strong observational constraint |
| B | 30% g | No station inside cell | Estimated from surrounding data and model |
| C | 22% g | Yes, inside cell | Station contributes directly |
| D | 18% g | No nearby station | Higher mapping uncertainty |
A learner points to B and says, “The instrument in Square B measured exactly 30% g.” That statement is stronger than the map supports. The map itself says there was no station inside B. The mapped value may be scientifically useful, but its provenance is different: it is an estimate constrained by observations and a model rather than a direct reading from an instrument sitting in that exact square.
This is the first Reality Lab habit: a number on a map is not automatically a number measured at that exact pixel or cell.
What Does “Peak” Mean?
During earthquake shaking, ground acceleration changes from moment to moment. It can increase, decrease and change direction. PGA keeps one extreme from that changing record: the largest acceleration magnitude relevant to the product definition during the event.
That means a PGA value behaves more like the highest point reached on a changing acceleration record than like a flat line maintained for the entire earthquake. A peak can last briefly. It tells us that the motion reached that acceleration; it does not tell us how long the ground stayed close to that level.
Peak Is Not Average
Suppose a fictional station records a simplified sequence of acceleration magnitudes during several moments of shaking: 4% g, 10% g, 18% g, 30% g, 16% g, 8% g. The PGA is 30% g because that is the highest value in the sequence. It is not the average of the values, and it does not imply every point in the sequence equals 30% g.
When a communication object uses words such as peak, maximum, minimum, mean or median, that word is part of the scientific quantity. Dropping it changes the meaning.
What Does “% g” Mean?
Earthquake products often express acceleration relative to the standard acceleration due to gravity, commonly represented by g. Thus 30% g means 0.30 g, not “thirty percent gravity remaining” and not “a thirty percent chance of falling”. It is simply a convenient reference scale for acceleration.
Two percentages can look alike while answering completely different questions. “30% g” is a ratio of acceleration to a reference acceleration. “30% of buildings damaged” would be a fraction of buildings. “30% probability” would be a probability. A percent sign does not tell you the denominator; the scientific label does.
PGA Is Not Ground Speed
Acceleration describes how velocity changes. Velocity describes how quickly position changes. They are related, but they are not the same quantity. A map labelled PGA cannot be read as a map of ground speed unless a separate calculation or product supplies that information.
This matters because two earthquake records can have similar peak acceleration but different velocity histories. The communication object must keep its quantity.
PGA Is Not Ground Displacement
Displacement asks how far something moves from a reference position. Acceleration asks how quickly velocity changes. Therefore PGA = 30% g does not mean the ground moved 30 centimetres, 30 percent of a metre, or any other distance.
Whenever a number seems to invite a familiar interpretation, check the unit. A correct unit can stop a wrong inference before it starts.
PGA Is Not a Damage Percentage
A building’s response to earthquake shaking depends on more than one mapped ground-motion number. Construction, height, structural system, foundation, local ground conditions, duration and frequency content can matter. Therefore a PGA of 30% g does not mean 30% of buildings will be damaged, nor does it guarantee a particular building outcome.
This article stays on the evidence side: PGA describes a feature of ground motion. Damage is a different outcome requiring different evidence.
PGA Is Not Earthquake Magnitude
Earthquake magnitude describes the size of the earthquake source using a different measurement system. PGA varies from place to place for the same earthquake because distance, geology, wave propagation and local site effects can change shaking. One earthquake has a magnitude value, while a ground-motion map contains many local PGA values.
If a news graphic puts “Magnitude 6.8” beside “PGA 35% g”, do not merge them. They are different claim objects.
Station Dot Versus Coloured Map Cell
Modern earthquake maps combine observations and scientific modelling. Close to an instrument station, the map may be strongly constrained by measured ground motion. Between stations, the map often estimates shaking using spatial interpolation, ground-motion models, site information and surrounding observations.
That makes the map useful over a broad area, but it also creates an important evidence distinction: measured at this point is not the same statement as estimated for this map location from the available evidence.
Representation Check: Colour Boundaries Are Not Walls in Nature
Suppose a legend colours 20–30% g yellow and 30–40% g orange. The border between yellow and orange can look like a sharp physical boundary. Usually the real ground-motion field changes continuously and the colour bins are a way of communicating ranges.
A road crossing from yellow to orange does not necessarily cross an invisible wall where acceleration suddenly jumps. The colour boundary belongs partly to the representation.
Another Representation Check: One Cell Is Not One Point
A gridded map assigns values to locations according to its spatial representation. Depending on the product, a displayed pixel or cell can represent an area or a sampled grid location. You should not assume every centimetre inside a coloured square was directly measured and had exactly the printed value.
For the general distinction between point measurements and mapped grid values, route to the existing Science owners rather than relearning it here.
Time Check: Which Version of the Map?
Rapid earthquake products can be updated as more observations arrive or processing improves. A map posted soon after an event may not be identical to a later version. A scientific learner records the product time or version before comparing screenshots.
“The map changed” does not automatically mean the earthquake changed after it happened. It may mean the estimate was revised as evidence improved.
Uncertainty Check: Exact-Looking Numbers Can Still Be Estimates
A map cell labelled 29.7% g can look more certain than a cell labelled “about 30% g”. But display precision and scientific certainty are different. Mapped ground motion can carry uncertainty from station spacing, ground-motion models, local conditions and processing.
A strong reader asks whether the product supplies uncertainty, standard error, station markers or quality information rather than assuming extra decimal places create extra certainty.
Method Check: Local Ground Conditions Can Matter
Earthquake shaking can vary over relatively short distances because different ground materials and site conditions affect wave motion. A broad regional map may include site corrections or models, but it still cannot turn one cell into a perfect description of every building foundation inside it.
The correct learner move is not to memorise geology. It is to recognise a scale boundary: regional evidence and site-specific evidence answer different questions.
Alternative Explanations for Different PGA Values
If two places have different PGA values during the same earthquake, distance from the source may matter, but it is not the only possible factor. Wave direction, local ground conditions, basin effects, station placement and mapping method can also contribute. Therefore “Site X is closer, so distance alone caused the difference” may be plausible but requires evidence.
Evidence That Strengthens a PGA Map Claim
- The map clearly identifies PGA rather than another shaking metric.
- The legend defines the units, such as %g.
- Station locations are shown or available.
- The product explains how values between stations are estimated.
- Map uncertainty or standard-error information is available.
- The event and product version or update time are clear.
- The claim matches the map scale and does not pretend every building has identical motion.
- Other ground-motion measures are consulted when the scientific question requires them.
Evidence That Weakens an Overconfident Claim
- The percentage is treated as a damage score.
- A gridded estimate is described as a direct instrument reading at every location.
- The word “peak” is dropped and the value is treated as constant throughout the event.
- The map colour is used as proof of exact movement distance.
- A single PGA value is used to predict every building outcome.
- A screenshot has no legend, event time or version.
- A sharp colour boundary is described as a sharp physical boundary in the ground.
- Map uncertainty is ignored where observations are sparse.
How Far Can the Conclusion Travel?
If a trusted product reports PGA = 30% g at a stated location, you can say that the represented peak ground acceleration there was about 0.30 g under that product’s data and methods. If the location is supported by a station, you can describe the observational basis. If it is a mapped estimate, say so.
You cannot jump from that value to “the ground stayed at 0.30 g”, “the ground moved 30 cm”, “the earthquake was 30% stronger”, “30% of buildings were damaged”, “every building experienced the same motion” or “the map value is exact with no uncertainty”. Those are different claims.
Worked Case 1: Peak Versus Constant
A simplified acceleration trace has values of 5, 12, 24, 30, 17 and 9% g at successive moments. What is the PGA?
30% g. That is the highest value in the simplified record. It does not mean all six moments were 30% g.
Worked Case 2: Map Cell Without a Station
A cell says PGA = 26% g but the nearest station is 15 km away. Can a learner write, “The sensor in this cell recorded 26% g”?
No. First check the product provenance. The cell may be an estimate informed by surrounding stations and a ground-motion model. A correct sentence would preserve that distinction.
Worked Case 3: Same PGA, Different Time Histories
Station R and Station S both have PGA = 20% g. R reaches 20% g briefly and otherwise remains much lower. S repeatedly approaches 20% g. Are the shaking records identical?
No. PGA preserves one peak, not the full duration or shape of the record.
Worked Case 4: The Damage Trap
A map shows 40% g near a town. A caption says, “40% of buildings will be damaged.” Is the caption supported by PGA alone?
No. PGA is a ground-motion quantity. Building damage depends on additional structural and site evidence.
Worked Case 5: Two Map Versions
An early map shows 18% g at a village. A later reviewed map shows 22% g after new station data are included. Did the ground shake twice?
Not from that evidence. The underlying earthquake event is the same; the mapped estimate may have been updated as evidence changed.
Worked Case 6: Percent Signs With Different Denominators
A report says PGA = 25% g and separately says 5% of surveyed buildings showed visible exterior damage. Are these percentages directly comparable?
No. The first compares acceleration with the reference acceleration g. The second compares a building count with a surveyed total. The percent signs hide different denominators.
Tempting but Invalid Reasoning
- “30% g means thirty percent damage.” Wrong denominator and wrong quantity.
- “PGA is how fast the ground moved.” Speed and acceleration are different quantities.
- “PGA is how far the ground moved.” Displacement is different again.
- “Peak means the average.” A peak is an extreme, not the mean.
- “Every map pixel was directly measured.” Many mapped locations are estimated between observations.
- “Same PGA means same shaking.” Full time histories can differ.
- “A precise colour means zero uncertainty.” Representation precision does not remove scientific uncertainty.
PSLE-Style Transfer Case
A fictional earthquake information page shows Station K with PGA = 28% g. A nearby map cell without a station is coloured in the 25–30% g range. The page also shows a different map of reported shaking intensity from public observations.
A learner writes: “Every place in the coloured map cell was directly measured at about 28% g, so everyone there must have experienced exactly the same shaking as Station K.” Evaluate the statement.
Strong answer: Station K provides a measurement at its location, but the nearby grid cell can be a mapped estimate based on observations and modelling. A cell range does not mean every point was directly measured or had exactly the station value. Local ground conditions and distance can change shaking, and reported intensity is a different evidence object from instrumental PGA. The claim should therefore be limited to what each product actually represents.
Practice 1: Decode the Unit
What does 20% g compare?
Answer: The reported acceleration with the standard acceleration due to gravity, g.
Practice 2: Peak Versus Duration
Can PGA alone tell you whether strong shaking lasted two seconds or twenty seconds?
Answer: No. PGA gives the peak acceleration, not the full duration history.
Practice 3: Point Versus Map
Why should you look for station markers on a ShakeMap?
Answer: They help distinguish locations with direct observational constraints from areas mainly estimated between observations.
Practice 4: Damage Claim
What extra evidence would be needed to discuss building damage?
Answer: Information about structures, local site conditions, other shaking characteristics and observed damage, not PGA alone.
Practice 5: Version Check
Why might two screenshots of the “same” event show slightly different PGA values?
Answer: They may be different processing versions created as more data or improved estimates became available.
Practice 6: Colour Boundary
Does a map changing from yellow to orange prove a sharp physical boundary exists at that exact line?
Answer: Not necessarily. The colour boundary may be a classification threshold imposed on a continuously changing field.
Practice 7: Same Peak, Same Record?
Can two acceleration records have the same PGA but different shapes?
Answer: Yes. They only need to share the same peak; their duration and other values may differ.
Practice 8: Rewrite the Headline
Rewrite “Ground accelerated at 30% g throughout the quake” when the evidence is PGA = 30% g.
Answer: “At the stated location, the represented peak ground acceleration reached about 30% of g during the earthquake.”
Delayed Independent Return
On another day, draw a six-point acceleration-time sketch with one obvious maximum. Label the maximum PGA. Then draw a coloured four-cell map and place a station marker in only one cell. Explain aloud the difference between the peak in the time record and the estimate shown across space. If you can keep time, space, unit and provenance separate, you have learned the real job.
Parent and Tutor Teaching Guide
Do not begin with earthquake equations. Begin with the word peak. Ask a child to jump three times and record three approximate jump heights. The highest is the peak; it is not the height maintained for the whole activity. Then transfer the idea to a changing acceleration record.
Next, use a paper map with three dots representing instruments. Colour the spaces between them. Ask: “Did we place a sensor in every coloured square?” The child should see that a scientific map can combine direct observations with estimation. Finally, show two percentage statements with different denominators—“30% g” and “30% of buildings”—and ask what each percent is of.
The teaching target is a four-part sentence: quantity, place, provenance, limit. For example: “This map estimates peak ground acceleration at this location using observations and modelling; it does not directly predict the damage to every building.”
Routes to Existing PSLE Science Owners
- How Scientific Evidence Works — for observation, inference and claim structure.
- How to Read Units, Scales and Measurement Resolution Before Using PSLE Science Data — for general unit and scale discipline.
- How to Keep a PSLE Science Claim at the Right Evidence Level — for controlling conclusion scope.
- Reality Lab Vol No.038 — for the separate owner on direct measurement versus mapped/interpolated values.
- Reality Lab Vol No.016 — for the separate owner on model output versus direct observation.
Authoritative Sources and Further Reading
- Singapore Examinations and Assessment Board — 2026 PSLE Science syllabus.
- Ministry of Education, Singapore — 2023 Primary Science syllabus.
- U.S. Geological Survey — Earthquake Hazards 201 technical Q&A, including definitions of peak ground acceleration and related shaking quantities.
- U.S. Geological Survey ShakeMap Manual — Products, describing PGA maps, units and uncertainty products.
- U.S. Geological Survey ShakeMap Manual — Background, explaining how observations and ground-motion estimates are combined across the map.
The Quiet Return
A map is persuasive because it turns thousands of measurements and model decisions into colours that can be understood at a glance. Scientific reading begins when we keep the hidden definitions attached to those colours.
When you see PGA = 30% g, ask: peak of what, measured or estimated where, in what unit, during which event, and for which claim? Once those questions become automatic, the map stops being merely colourful. It becomes evidence you can use without asking it to say more than it knows.
