An isoseismal map shows the Modified Mercalli Intensities (MMI) reported from a particular earthquake. To use the metaphor of a lightbulb; an earthquake has a fixed magnitude (the Watt rating of the bulb), but the intensity varies with distance (the brightness of the light). By plotting intensities, we may learn more about the mechanisms for earthquakes in those areas.

Isoseismal maps are derived from "felt reports" - accounts from people who felt the earthquake. We also ask people to fill in these reports when they are in the earthquake area and did not feel it, as this may reveal something about the local geology.

Below are some isoseismal maps from earthquakes in our area of operation.