The Richter scale records the magnitude of seismic waves. People usually don’t feel earthquakes of 2.0 or less. Each whole-number increase indicates a tenfold increase in magnitude. A 5.0 is moderate, while a 6.0 is 10 times larger. Great earthquakes, of 8.0 or above, occur somewhere on Earth about once a year. Another scale, called the Mercalli scale, uses Roman numerals to rank earthquakes by how much damage they cause.