How are earthquakes measured? Get the details on magnitude scales and how today's event stacks up

An earthquake rattled much of the Northeast on Friday morning, with shaking felt across the New York City metropolitan area and in communities from Baltimore and Philadelphia up through New England.

The quake had a magnitude of 4.8, according to the U.S. Geological Survey. No major damage was reported as of midday Friday, officials said. 

Here's how earthquake strength is measured and what different quake magnitudes mean.