Same ZIP code. Three homes. Three experiences. After an earthquake, a USGS Did You Feel It? map colours one ZIP-code area with Intensity VI. A student points at the colour and says, “Then every person and every building in this ZIP code experienced Intensity VI shaking.”
The map does not say that. A Community Internet Intensity Map is a summary built from reports submitted by people who experienced — or sometimes did not feel — the earthquake. USGS groups and processes those responses to describe the intensity of shaking reported in an area. A ZIP-code value is therefore an area summary, not a claim that every location, person or structure experienced exactly the same motion or damage.
Quick answer
No. “DYFI Intensity VI for this ZIP code” should be read as a community-intensity summary for the reporting area, not a uniform blanket of identical shaking. Earthquake intensity can vary from place to place. Reports can differ within the same ZIP code because ground conditions, distance, building behaviour, floor level, human perception and many other factors affect what is experienced and reported.
The exact learner job — and the boundary
This Reality Lab owns one real-world communication object: a Did You Feel It? Community Internet Intensity Map and the temptation to treat one displayed area value as if it were measured identically everywhere inside that area.
It does not replace earthquake physics, seismology, magnitude scales, building engineering, sampling or generic map reading. For the distinction between observation and inference, route to How to Tell Observation, Inference, Prediction and Explanation Apart in PSLE Science. For the separate mistake of confusing local intensity with earthquake magnitude, use PSLE Science Reality Lab Vol No.355 | “ShakeMap Intensity = VI” — Was It a Magnitude 6 Earthquake?.
Original case file: one coloured area, many reports
Imagine an invented earthquake and an invented ZIP-code area. The example is built only to teach the evidence structure; it is not copied from a real USGS event.
Community map area EK-477
Displayed community intensity: VI
Number of submitted questionnaires used in the area summary: 42
Reports came from houses, apartments, shops and an office building
Some people described stronger effects than others
Several people submitted “not felt” or weak-effect reports near the edge of the area
The map can legitimately show one summary colour for the area while the underlying reports vary. Compression is the purpose of a map summary. The error begins when the reader forgets that compression occurred.
A map area is a summary, not a uniform blanket
USGS explains that Did You Feel It? collects reports from the public and produces Community Internet Intensity Maps. Its fact sheet describes intensity values as being averaged for ZIP-code areas in the map product. An average or area summary can represent a group without saying that every member of the group has the summary value.
Think of a class whose average height is 145 cm. The class can honestly have an average of 145 cm even if no pupil is exactly 145 cm. In the same way, one area intensity does not require every report or every location to match the area value exactly.
Observed, reported, summarised and inferred
| Layer | What belongs here? | What can go wrong? |
|---|---|---|
| Physical event | The earthquake produces ground motion that varies across space. | Assuming all places shake identically. |
| Human report | A person answers questions about what they felt or observed at a location. | Treating one report as the whole area. |
| Area summary | USGS combines reports into a mapped community-intensity value. | Treating the summary as a direct instrument reading everywhere. |
| Reader inference | A headline or student describes what happened. | Claiming identical shaking or identical damage across the ZIP code. |
Strong scientific reading keeps these layers separate while still allowing them to connect.
Intensity is local; magnitude belongs to the earthquake
A single earthquake has a magnitude estimate describing the event. Intensity describes shaking effects at a particular place and can vary from location to location. Therefore, “Intensity VI in this ZIP code” is not another way of saying “magnitude 6”.
This matters because a community-intensity map is designed to show spatial variation. If intensity were automatically identical everywhere, there would be little reason to map different areas separately.
Why two houses in the same ZIP code can report different effects
Several factors can make experiences differ over short distances:
- distance from the earthquake source;
- local soil or rock conditions;
- building design and condition;
- which floor a person occupied;
- orientation and contents of a room;
- individual perception;
- whether the person was moving, asleep or focused on another task;
- small-scale variation in ground motion.
These possibilities do not make public reports useless. They are reasons to collect many reports and summarise them carefully.
The response-count check
Suppose one ZIP code has 300 usable reports and another has only three. A colour on each map area may look equally solid, but the evidence support is not necessarily equally rich. USGS notes that more submitted questionnaires generally improve the reliability of the community-intensity estimate.
A good learner therefore asks: How many reports contributed to this area? The displayed intensity value is important, but the amount of underlying evidence matters too.
Why “not felt” reports matter
A person who did not feel an earthquake may think they have nothing useful to report. In fact, USGS encourages “not felt” responses because they help define the spatial limits of felt shaking. Absence of a sensation can be informative when the reporting method records it deliberately.
This is similar to complete checklists in ecology: a structured non-detection can carry information that an ordinary silence does not.
Worked case 1: one ZIP code, two neighbourhoods
An invented ZIP code includes a neighbourhood built on firm rock and another on softer sediments. Residents in the second neighbourhood tend to report stronger shaking effects. The map nevertheless assigns one community-intensity summary to the whole ZIP code.
A student says, “The map is wrong because the reports are not identical.” That misunderstands a summary. A summary is allowed to represent a distribution with variation. The correct question is whether the method combines the reports appropriately for the purpose of the map.
Worked case 2: the high-rise and the ground-floor shop
Two reports come from buildings only 150 m apart. One observer is on a high floor of a flexible building; the other is in a ground-floor shop. Their experiences differ.
The disagreement is not automatically evidence that one person lied. Building response and observer context can change what is felt. Alternative explanations should be considered before rejecting either report.
Worked case 3: one dramatic photograph
A broken shelf in one home is photographed and shared with the caption, “Intensity VI damage throughout the district.” The photograph can be genuine evidence about that home. It is not automatically representative evidence about every building in the district.
The stronger the geographic claim, the broader the evidence base must be.
Worked case 4: the map changes after more reports arrive
At 10:05, an area appears as intensity V. At 11:00, after many more people submit questionnaires, the map displays VI. Did the earthquake become stronger after it ended?
No. The physical event occurred earlier. The map changed because the evidence set and resulting summary changed. USGS notes that DYFI maps can update as additional reports arrive. This is a powerful evidence lesson: a representation can change because knowledge changes, not because the past event changed.
Worked case 5: the ZIP-code boundary
A street crosses from one ZIP code into another. The map colours one side V and the other side VI. A learner imagines a sudden physical jump in shaking exactly at the postal boundary.
The boundary is an aggregation boundary, not a wall in the Earth. Real ground motion can vary continuously while the map summarises reports into administrative areas. Abrupt colour changes can therefore be partly a feature of representation.
Worked case 6: the unreported block
One corner of the ZIP code has no submitted questionnaires. The area still receives a displayed ZIP-code intensity based on reports elsewhere in the same aggregation area. A headline says, “This block experienced intensity VI.”
That statement travels farther than the evidence. The map may support an area summary, but it does not transform an unreported block into a directly observed location.
Worked case 7: intensity does not equal damage grade
Two buildings in the same intensity area show very different damage. One is newer and well maintained; the other has vulnerable contents and older construction. A student says, “If both are intensity VI, they must have the same damage.”
Intensity descriptions can include observed effects, but building damage also depends on vulnerability and construction. USGS cautions that an intensity area should not be read as a guarantee that every location has the same damage.
Representation check: colour blocks hide distributions
A choropleth-style map fills an area with one colour. The colour is visually uniform even when the evidence underneath is not. This is helpful for rapid communication but can create three false impressions:
- every point inside the polygon has the same value;
- the boundary is a physical change in the phenomenon;
- the displayed category is a direct measurement at every point.
The cure is not to distrust colour maps. The cure is to remember what the polygon and colour summarise.
Baseline and comparison check
When comparing two DYFI areas or two earthquakes, ask:
- How many reports contributed?
- Were the reports collected over comparable periods?
- Are the geographic units comparable?
- Did the map update between screenshots?
- Are you comparing community intensity with instrument-based ShakeMap values?
- Are you confusing intensity with magnitude?
- Are population density and internet participation very different?
Two colours can be comparable only when you understand what generated them.
Method check: public reports are evidence with a method
Did You Feel It? is not a free-form social-media poll. It uses a structured questionnaire and processing system. That structure is what makes thousands of public reports scientifically useful. But structured reports still depend on who responds, what they experienced and where reports are available.
Scientific evidence does not become worthless because humans contribute it. It becomes interpretable when the collection and summarisation method is understood.
Alternative explanations for a surprising intensity patch
- Real local ground-motion variation.
- Soil or geological differences.
- Different building response.
- A small number of reports.
- Uneven participation.
- A cluster of observers in one part of the ZIP code.
- Map updates based on later submissions.
A surprising map patch is a question to investigate, not permission to choose the most dramatic explanation immediately.
What strengthens a claim about local shaking?
- Many consistent DYFI reports from the local area.
- Instrumental ground-motion data nearby.
- Comparable reports from neighbouring areas.
- Clear location metadata.
- Stable map values after substantial reporting.
- Independent observations consistent with the reported effects.
What weakens a claim that every place had the same intensity?
- Wide variation among reports.
- Few questionnaires.
- A large ZIP code with varied geology and building types.
- Unreported subareas.
- A claim based only on polygon colour.
- Evidence that the map was still preliminary or updating.
How far can the conclusion travel?
From a DYFI ZIP-code intensity, a careful conclusion can say that submitted reports for that area were summarised into the displayed community-intensity value. With enough reports and supporting evidence, you can discuss the broad pattern of felt shaking across areas.
You cannot use the area value alone to claim that every person felt the same shaking, every building had the same damage, or every point inside the ZIP code had exactly the displayed intensity.
Tempting reasoning that fails
- “Intensity VI means magnitude 6.” Intensity and magnitude are different quantities.
- “The whole polygon is one colour, so every location is identical.” The colour is an area summary.
- “One damaged house proves the whole ZIP code was equally damaged.” One observation does not represent every structure.
- “The map changed, so the earthquake changed.” The evidence set can change after the event.
- “No report from this block means nobody felt it.” Lack of reports is not the same as a structured not-felt response.
Original PSLE-style transfer case: the two screenshots
This is original practice, not an examination-board question.
At 09:30 a DYFI screenshot shows ZIP area A at intensity V based on 12 reports. At 12:00 a later screenshot shows the same ZIP area at intensity VI based on 95 reports. A student says, “The ground shook harder at noon than it did at 09:30.”
Question 1: What changed between the screenshots?
Explained answer: The map’s evidence set and area summary changed as more reports were submitted; the earthquake event itself had already occurred.
Question 2: Why does the later value not mean every home experienced intensity VI?
Explained answer: The ZIP-code value is a summary of reports for the area. Individual locations and buildings can experience different effects.
Question 3: Which screenshot has the richer community-report evidence?
Explained answer: The later screenshot has many more reports, although report count alone does not guarantee perfect spatial coverage.
Question 4: What extra evidence could strengthen a claim about one specific block?
Explained answer: Reports or instrumental measurements from that block would be more directly relevant than the ZIP-wide colour alone.
Delayed independent return
- Why can one ZIP-code intensity coexist with different individual experiences?
- What does a larger number of DYFI reports usually add?
- Why are “not felt” reports useful?
- Why can a colour boundary differ from a physical shaking boundary?
- What is the difference between magnitude and intensity?
Self-check: Area values summarise variable reports; more reports can improve the area estimate; structured non-felt responses help locate the limits of felt shaking; ZIP boundaries are aggregation units; magnitude describes the earthquake while intensity describes local shaking effects.
Explained practice: rewrite the headline
- Too strong: “Every home in ZIP 12345 had Intensity VI shaking.” Repair: “DYFI reports for ZIP 12345 were summarised as community intensity VI.”
- Too strong: “The earthquake grew from V to VI after 10:00.” Repair: “The mapped community-intensity summary changed as more reports were processed.”
- Too strong: “No questionnaire means nobody felt it.” Repair: “No questionnaire was received from that location in the evidence shown.”
- Too strong: “Intensity VI proves every building was damaged.” Repair: “Intensity VI summarises reported shaking effects; damage can vary among structures.”
Parent and tutor teaching guide: average a room without flattening it
Give three learners cards showing different room temperatures: 24°C, 26°C and 28°C. Ask them to calculate or simply identify a middle summary such as 26°C. Then ask: did the summary make every card 26°C? Of course not.
Next, draw three small areas on paper and give each several invented “felt report” cards. Assign one summary label to each area. The learner should practise saying, “The area label summarises the reports; it does not erase variation inside the area.”
For a three-student tutorial, use roles: report reader, map summariser and claim limiter. The report reader sees the individual evidence; the summariser assigns a classroom summary; the claim limiter blocks statements that pretend every report was identical.
Authoritative sources and curriculum frame
- SEAB — 2026 PSLE Science syllabus: current assessment includes interpreting and analysing information, evaluating observations, information and methods, and communicating explanations and reasoning.
- MOE — 2023 Primary Science Teaching and Learning Syllabus: supports healthy scepticism, evidence evaluation and understanding how science is communicated in different forms.
- USGS — Did You Feel It?: official overview of the public reporting system and Community Internet Intensity Maps.
- USGS Fact Sheet — Did You Feel It?: explains public questionnaires, ZIP-code intensity averaging and community intensity mapping.
- USGS Hawaiian Volcano Observatory — Felt Earthquakes: explains community intensity, the value of more reports and differences between felt reports and instrument-based maps.
- USGS — Did You Feel It? FAQ: explains reporting, not-felt responses and interpretation of community intensity.
The quiet habit to keep
A map summary is useful because it compresses many observations. Do not punish it for being a summary — and do not mistake the summary for every observation underneath it. When the polygon is one colour, remember to ask what variation the colour had to compress.
