How a white lie has caught up with seismologists
Much to their dismay, people are learning that seismologists typically do not use the Richter scale to judge quake size.
"We're just recovering from decades of telling a white lie, that's all," says seismologist Thomas H. Heaton (president of the Seismological Society of America and a USGS researcher in Pasadena, Calif. )
While seismologists generally do not use the original Richter magnitude scale, the measuring systems currently in vogue represent extensions of the type that Charles Richter developed nearly 60 years ago. "[Richter] introduced it because he was tired of the newsman asking him about the relative size of earthquakes," recalls veteran seismologist Bruce A. Bolt from the University of California, Berkeley. That explains why some seismologists continue to use the term when addressing the press.
Prior to Richter's work, researchers in the United States had no way of judging an earthquake's absolute size, which remains the same no matter where it is measured. Instead, they dealt with a concept called intensity , which describes the strength of shaking at a particular location.
In the early 1930s, Japanese seismologist Yiyoo Wadati devised a method of comparing the sizes of quakes. He would take seismic recordings of various shocks and set them on an equal footing by factoring in the distance between the recording station and the earthquake. But this method was not easily grasped by lay people, especially the reporters of quake-plagued southern California.
In 1935, Richter dressed up the Japanese method to create an earthquake index. Richter defined seismic magnitude in terms of a particular type of recording device, called a Wood-Anderson seismograph, situated at a standard distance of 100 kilometers from an earthquake's epicenter.
Richter appropriated from astronomy the idea of a logarithmic scale based on powers of 10 to accommodate the incredible range of earthquake sizes. (The smallest detectable tremors equal the energy of a brick dropped off a table, while monster quakes surpass the largest nuclear explosions) By Richter's original definition, a shake of magnitude 1.0 would cause the arm of the Wood-Anderson machine to swing one-thousandth of a millimeter. A magnitude 2.0 temblor would make the arm swing 10 times as much, or one-hundredth of a millimeter .
In theory , the scale had no upper limit. But in practice, magnitudes could not top 7.0. "You would never see an earthquake bigger than magnitude 7 [on the original magnitude scale], or at least we hope you never would because everything would be dead," Heaton says.
Of course, scientists rarely had a Wood-Anderson seismograph stationed exactly l00 kilometers
from an earthquake. But by comparing the arrival of slow versus fast seismic waves at a recording station, they could calculate what one of the devices would have detected at the standard distance.
The magnitude index, as originally defined, could only measure southern California earthquakes because Richter calibrated the scale for the crust there. What's more, it only worked for jolts within a few hundred kilometers of a Wood-Anderson seismograph. This original magnitude scale was based on waves with periods of 0.1 to 3.0 seconds became known as ML or local magnitude, when a more general
magnitude measurement, denoted as Ms. was devised by Caltech's Beno Gutenberg and Richter to handle distant earthquakes. MS depends on measurements of surface waves rippling through Earth's crust with a period of about 20 seconds.
Even the new and improved magnitude formula had problems, however, because deep earthquakes do not produce many surface waves. So Gutenberg and Richter invented MB, measured from body waves, which travel through the planet's interior. This yardstick proved helpful in distinguishing nuclear explosions from actual earthquakes.
In the 1970s, seismologists realized that all existing magnitude methods underestimated the energy of truly large earthquakes. To circumvent this limitation, Hiroo Kanamori, a successor of Richter and Gutenberg at Caltech, created a magnitude scale, MW, that quantifies the total amount of seismic wave energy released in an earthquake.
But because such calculations are difficult, scientists usually approximate the energy by computing
a quantity called "seismic moment," determined from long period vibrations. In the case of great earth- quakes, these vibrations have cycles longer than 200 seconds. Seismologists therefore refer to MW as the moment magnitude.
MW differs from all other types of magnitude in that it measures the earthquake source, Kanamori says. The Richter magnitude and most others gauge only the strength of vibrations sensed at Earth's surface. But to calculate moment magnitude, seismologists use the long-period waves to decipher the dimensions of the fault rupture that produced the quake. [seismic moment the length of the fault rupture multiplied by
the amount of rock movement and then again by the stiffness of the rock]
In other words, moment magnitude measures the cause rather than the effect.
Although researchers have developed more than a dozen other ways of calculating earthquake magnitude, moment magnitude remains the figure of choice among seismologists, especially for earthquakes larger than magnitude 6.5.
With ML, MS, MB, MW and a litany of other MS floating around, it's no wonder that many seismologists took the easy way out over the years by giving reporters what they thought the media wanted. When pressed for details, researchers typically simplified the issue by calling any magnitude a Richter magnitude, even though this term applies only to the local (ML) magnitudes determined by Richter's original formulation.
"The problem is that seismologists have used the term 'Richter scale' in a very loose way, and now it's catching up with them. We didn't use it among ourselves because it doesn't mean anything," Heaton
Immediately after an earthquake, the USGS National Earthquake Information Center in Golden, Colo., releases a preliminary measurement, which could be a surface wave magnitude, a body wave magnitude, or even a local magnitude (similar to Richter's original formulation except that modern seismographs have replaced Wood-Anderson ones.) After determining the moment magnitude, they release this number, which may fall above or below the preliminary one.
As for the use of the term "Richter scale," the USGS has dodged any decision. "The question of labeling these magnitudes as 'Richter scale' is a matter of tradition, semantics, and personal perspective.
The USGS has no official scientific position on the use of the term," declares the July statement.
The USGS' Heaton, who works across the street from Richter's old Pasadena office, says he wants to avoid the term entirely. "You probably wouldn't catch us using the term 'Richter magnitude' around here, even though this was the home of Richter."
As journalists get more seismically sophisticated, they may head off some of the confusion. The Associated Press recently retired the term "Richter scale" in favor of the phrases "preliminary magnitude" and "moment magnitude."
But simply tidying the terminology will not, on its own, help people better understand the size of an earthquake. After all, how can one number convey the power of something equivalent to a colossal nuclear explosion?
Even moment magnitude does not suffice, says its inventor. "The problem is everyone thinks that a single number determines everything. It's almost like asking how big you are," says Kanamori. "The question is whether you are asking height, weight, or width. Depending on how you measure a person, the answer can be very different. In the case of earthquakes, it's even more complex.
Excerpted from "Abandoning Richter" by Richard Monastersk in Science News, vol. 146, Oct. 15, 1994
It is true that the Richter magnitude scale is logarithmic, but this does not mean a magnitude 8 quake is 10 times stronger than a magnitude 7. One should estimate seismic energy E released by calculating:
lag E = 11.8 + 1.5 M8
where M8 is the surface wave magnitude. Thus a magnitude 8 earthquake is not 10 times stronger than a magnitude 7, but rather about 30 times stronger.
Bemhardt Saint-Eidukat in Science News p.58, vol.137, No.4, Jan. 27, 1990