Go to:    Introduction Scientists use two measurements to determine the strength of earthquakes. Magnitude is the amount of energy released, as measured by an instrument called a seismograph. Magnitude is sometimes said to be measured on the "Richter scale," which was developed in the 1930s by Charles Richter. That term is not completely accurate -- today's measurements of magnitude are an extension of Richter's original scale but are not the same. Intensity is the degree to which a quake damages people, structures and terrain and is a subjective measurement based on observation. Intensity is pegged to a specific location -- intensity levels from the same quake will vary, usually lower for areas farther from the epicenter. Click the links below for the categories of magnitude and intensity seismologists use. Source: U.S. Geological Survey