Whenever there is a notable earthquake, reporters inform us that it had “an estimated magnitude of” a number around seven or eight. Assigning magnitude numbers to earthquakes is a way to compare them to each other: eights are “massive”; sixes “major”; fours are “minor.” To a seismologist, however, the number has a different meaning: it represents the amount of energy released by an earthquake instead of the amount of damage it causes.
Most people have heard of the Richter scale of earthquake magnitude. This scale was developed by a California seismologist (no surprise there) seventy-five years ago. The numbers are familiar: big quakes are around eight and little ones around four. There can be tremors with negative magnitudes, too, so small that only delicate instruments called seismographs notice them. Richter’s method of determining magnitude requires measuring lines on the paper earthquake record made by a seismograph. Many seismograph stations worldwide calculate magnitudes, then the numbers are averaged to produce one number. The disadvantages of Richter magnitude estimation are that it becomes inaccurate at large magnitudes, and it requires measurement of a physical record, which is dependent on the design and condition of the seismograph that made it. The advantage is that Richter magnitude is highly accurate for earthquakes of magnitudes less than about four.
In the late 1970s, seismologists Thomas C. Hanks (not the actor) and Hiroo Kanamori introduced an alternative to Richter’s method. Their scale, called moment magnitude, produces numbers very close to Richter estimates. The two are lmost identical for earthquakes in the range of four to six. Moment magnitude is highly accurate for earthquakes of magnitudes greater than four, but less so below that size. Both scales are logarithmic: a quake of magnitude seven is ten times as powerful as a quake of six, and 100 times as powerful as one of five. If vertical ground movement is one inch in an earthquake of magnitude five, it will be ten inches in a magnitude six quake.
The “moment” in moment magnitude may be confusing since it seems to refer to time. Instead, moment refers to torque – the force that causes rotation, or in earthquakes, slippage. An earthquake’s moment magnitude is derived from the quake’s seismic moment; which is the product of the resistance of the earth’s crust to breakage, the surface area of the rupture on the fault, and the distance rocks slipped on that fault. Once these three values are known, seismic moment can be reduced to a number similar to Richter magnitude. Since fault area and movement distance can take days or weeks to determine, moment magnitude cannot be calculated as quickly as Richter magnitude.
Once moment magnitude is calculated, scientists can estimate total energy released by the earthquake. For instance, tremors of magnitude 7.0 (such as in Haiti in January, 2010) release approximately the same energy as thirty-two million tons of TNT (32 megatons).
Unlike Richter magnitudes based on records of seismic waves, moment magnitude is calculated from measurements of the earth’s movement and the size of the subsurface rupture. This means that moment magnitude is independent of recording instruments. Moment magnitude is directly proportional to the energy released by a given earthquake.
Neither moment magnitude nor Richter magnitude measures physical damage caused by earthquakes, since they are related only to the amount of energy released. The amount of damage is controlled by many variables, and varies from place to place for even the same earthquake. Scientists and structural engineers use both a different word and a different scale to measure damage, which is reported using the Mercalli intensity scale.
The moment magnitude of an earthquake will be similar to its Richter magnitude. To a lay person, the difference is negligible, since an earthquake of magnitude eight – whether Richter or moment – is a big earthquake. To seismologists, however, the distinction is very important. In the interests of maximum accuracy, seismologists now use moment magnitude to describe earthquakes larger than magnitude 3.5, and use the Richter scale for smaller magnitudes.
Sources:University of Nevada (Reno), United States Geological Survey, Pennsylvania State University