Measuring earthquake magnitude and intensity

Sample seismogram

Sample seismogram showing information received from seismometer.

Earthquakes can be measured in two ways. One method is based on magnitude—the amount of energy released at the earthquake source. The other is based on intensity—how much the ground shakes at a specific location. Although several scales have been developed over the years, the two commonly used today in the United States are the moment magnitude scale, which measures magnitude (M), or size, and the Modified Mercalli scale, which measures intensity.

Measurements on the moment magnitude scale are determined using a complex mathematical formula to convert motion recorded with a seismometer into a number that represents the amount of energy released during an earthquake. Energy released for each whole number measurement is about 31 times greater than that released by the whole number before. The smallest earthquakes recorded today on the moment magnitude scale have negative magnitudes (such as M -2.0) because the scale's range is based on that of the Richter scale, developed in the 1930s when monitoring equipment was less sensitive. Scientists are now able to detect earthquakes smaller in magnitude than the "0" used as the Richter scale baseline.

Measurements of intensity on the Modified Mercalli Intensity (MMI) scale range from I to XII and are based solely on damage assessment and eyewitness accounts. Intensity measurements near the source of an earthquake are generally higher than those at a distance. Determining intensity can be difficult in sparsely populated areas with few buildings because intensity is calculated largely based on the effects that tremors have on human-made structures.

Although an earthquake's magnitude and intensity measurements are not precisely comparable, they can, in general, be correlated when intensity measurements nearest the epicenter are used in the comparison. The magnitude of earthquakes occurring before the introduction of the Richter scale are estimated based on reported damage and intensity. Seismologists categorize modern earthquakes by their magnitude, not by their perceived intensity.

Resources

 

Earthquake Highlights newsletter

Earthquakes: Kansas Geological Survey Public Information Circular 3

Induced Seismicity: The Potential for Triggered Earthquakes in Kansas: Kansas Geological Survey Public Information Circular 36

Kansas Earthquakes

Resources on induced seismicity

Seismology and Its Applications in Kansas: Kansas Geological Survey Public Information Circular 37, 2014, 6 p.