A ShakeMap uses recorded and predicted ground motions to show where and how intensely the ground shook during an earthquake—most crucially, they help emergency managers to identify areas of likely damage within minutes of a significant earthquake. They are also used by engineers, city planners, and scientists, and they can show predicted effects from hypothetical earthquakes as well as real ones.
ShakeMaps are color-coded to show how strongly the ground shook in different places. Each color corresponds to a number on the Modified Mercalli Intensity Scale (MMI) (link or sidebar), which was created to describe an earthquake’s severity in a given place. MMI tells us more about the local effects of an earthquake than magnitude because magnitude only describes the energy released by the earthquake at its epicenter.
For example, if we know that a large earthquake has occurred away from a populated area, our most immediate concern is how strongly that earthquake shook the nearest places where people live or that contain vital infrastructure. ShakeMaps use MMI to answer those questions.
You can read more about ShakeMaps and how they are generated on the USGS website.