"Earthquakes occur on geologic faults. However, in nearly all parts of the northeastern U.S., we do not yet know how to tell which fault caused an individual earthquake. Geologic maps show faults that are exposed at the Earth's surface, and maps like this one show the locations of earthquakes, but there is no reliable map of 'earthquake faults' for the northeastern U.S. The most common way to measure an earthquake's size is its magnitude. There are many different magnitude scales, or ways to measure magnitude. Use of different scales can give slightly different values for the magnitude of the same earthquake. Therefore, your favorite earthquake might be shown here with a magnitude slightly different from what you have seen reported elsewhere."
U.S. Geological Survey: http://www.usgs.gov/