While magnitude is a single number, intensity varies by location. After an earthquake, researchers collect data on what people felt and how much damage occurred. They plot these “intensity values” on a map and draw isoseismal lines to connect points with equal values. These lines are rarely perfect circles because shaking is influenced by the direction of the fault and the type of ground. For example, a city on soft clay will have a higher intensity (and thus be inside a more inner isoseismal line) than a nearby town on solid granite, even if they are at the same distance from the epicenter.