Two measurements answer different questions
Magnitude describes the size of an earthquake at its source. Intensity describes the shaking and effects at a particular place. One earthquake therefore has a magnitude estimate but many local intensities. Distance and local ground conditions help explain why people in different towns can experience the same event differently. See the USGS explanation of magnitude and shaking intensity.
When reading a map, first identify what the displayed number represents. A magnitude label is not a street-level damage estimate, and a marker at an epicentre is not a boundary around all affected places. Do not turn a single event number into a conclusion about a particular building, road or neighbourhood.
Why can the reported magnitude change?
Early estimates can be revised as more observations and analysis become available. A changed number does not necessarily indicate a second earthquake. The USGS discusses this process in How big was that earthquake? Compare the event identity and time before treating two records as separate events.
Different agencies may also publish different estimates because they use different observations or calculation methods. The USGS guide to agency differences explains why those values are not always identical. Keep the provider attached to a value when quoting it, rather than silently combining numbers from separate catalogues.
A practical reading sequence in PocketWorld
Start with the earthquake overview, then open the disaster map and select an event. Read the supplied location, event time, magnitude and depth together. Follow the provider link for its current record. If a field is unavailable, leave it unknown; another event nearby is not a substitute for missing information.
Check the source and freshness information before comparing a saved screenshot with the current map. The timestamp of the earthquake and the time a catalogue was retrieved describe different things. A retained snapshot can remain useful for exploration without being a complete account of what has happened since it was fetched.
What this guide does not establish
If you share an observation, include its provider link and the time you checked it. This lets another reader distinguish the original event record from your interpretation and revisit later revisions.
This is an explanation of map terminology, not a forecast, a building assessment or an emergency alert. An empty map region is not evidence of safety. For decisions during an incident, use the responsible emergency authority and the original monitoring agency. Our methodology explains the difference between provider observations and the interface presenting them.
Data sources
References are linked beside the explanations above. This evergreen guide does not report current incidents.