Measuring Earthquake Magnitude and Intensity
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 magnitude 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. So, for example, a magnitude 2 earthquake is 31 times more powerful than a magnitude 1 earthquake. The moment magnitude scale is often referred to by the name of its predecessor, the Richter Scale.
Measurements 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.
| Modified Mercalli Scale | Moment Magnitude Scale | |
|---|---|---|
| I | Detected only by sensitive instruments | Up to about 1.6 |
| II | Felt by few persons at rest, especially on upper floors; delicately suspended objects may swing | ~1.6–2.25 |
| III | Felt noticeably indoors, but not always recognized as earthquake; standing autos rock slightly, vibration like passing truck | ~2.25–2.8 |
| IV | Felt indoors by many, outdoors by few, at night some may awaken; dishes, windows, doors disturbed; motor cars rock noticeably | ~2.8–3.5 |
| V | Felt by most people; some breakage of dishes, windows, and plaster; disturbance of tall objects | ~3.5–4.1 |
| VI | Felt by all, many frightened and run outdoors; falling plaster and chimneys, damage small | ~4.1–4.6 |
| VII | Everybody runs outdoors; damage to buildings varies depending on quality of construction; noticed by drivers of automobiles | ~4.6–5.3 |
| VIII | Panel walls thrown out of frames; fall of walls, monuments, chimneys; sand and mud ejected; drivers of autos disturbed | ~5.3–6 |
| IX | Buildings shifted off foundations, cracked, thrown out of plumb; ground cracked; underground pipes broken | ~6–6.5 |
| X | Most masonry and frame structures destroyed; ground cracked, rails bent, landslides | ~6.5–7.1 |
| XI | Few structures remain standing; bridges destroyed, fissures in ground, pipes broken, landslides, rails bent | ~7.1–7.7 |
| XII | Damage total; waves seen on ground surface, lines of sight and level distorted, objects thrown up into air | ~7.7–9+ |
Comparison between measurements on the Modified Mercalli scale and magnitudes on the moment magnitude scale.
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 that occurred before the introduction of the Richter Scale in 1935 are estimated based on reported damage and intensity. Seismologists categorize modern earthquakes by their magnitude, not by their perceived intensity.