Magnitude is an indication of energy at the earthquake's epicenter and is measured using seismographs. These measurements are adjusted for the distance from the epicenter because ground shaking decreases as the distance from the epicenter increases. Magnitude measurements are reported on the Richter scale. For each unit of increase in magnitude, ground displacement increases by a factor of 10 but energy increases by a factor of 30. This means, then, that the energy from an earthquake of M 5 is a thousand times as great as the energy recorded from a M 3 quake.
Intensity is a measure of the effect of a quake on humans and structures.
Although an earthquake only has one magnitude, intensity measurements vary, depending on the distance from the earthquake and local geologic conditions. The earthquake was near the site of two earlier quakes, a M 4.5 event in 1972 that caused minor damage, such as cracked plaster and chimneys, and a 1909 earthquake that was probably at least M 5.0. Only about a dozen small earthquakes have occurred in northern Illinois in the past century, and no larger earthquakes are expected. The Dixon earthquake was not linked directly to any mapped faults in the area. Those faults appear not to have moved measurably since the beginning of the last Ice Age, about 1 million years ago.