The information ShakeMap presents is different from the earthquake magnitude. Intensity, on the other hand, is a measure of how the ground shook at a particular site. So, while an earthquake has one magnitude and one epicenter, it produces a range of ground shaking levels at sites throughout the region. These different intensities depend on distance from the earthquake, the rock and soil conditions at geographical sites, and variations in the propagation of seismic waves from the earthquake due to complexities in the structure of the Earth's crust. ShakeMap focuses on the ground shaking produced by the earthquake, rather than the characteristics of the earthquake source.
Intensities portray the effects of an earthquake in a particular location.
These effects include potential damage, perception of shaking and permanent changes in topography. Whether there was damage, and the severity of observed damage, is one element of intensity. The impact of the earthquake shaking on the ground, whether, for example, cracks or displacements occurred, or in some cases of severe shaking, landslides, is also a feature of measured intensity. Historically, intensities were derived in the months following an earthquake through questionnaires sent to Post Offices in the impacted area. Postal officials were asked to report the effects of shaking in their district and their observations were combined with those of scientists and engineers.