Science / Medicine : The Richter Scale : Caltech Seismologists Say Temblor Rating Has a Magnitude of Problems

That well-known phrase to Southern Californians, “the quake measured 3.5 on the Richter scale,” is fast leaving the vocabulary of seismologists.

The Richter scale, synonymous with earthquake measurements for most of this century, is on its way out, although habit may keep it in news reports awhile longer, according to seismologists, geologists and other earthquake experts.

It is being replaced by a sort of composite magnitude drawn from a variety of earthquake measurements.

And in a few years more than one magnitude may be given for an earthquake, depending on what kind of measurement is desired.

Not Just One Way

“It’s like measuring a table,” explains Caltech seismologist Hiroo Kanamori, whose energy scale is one of the alternatives now being used. “You need three different numbers at least and usually more than that to convey its dimensions.

“An earthquake is even more complex,” he says. “We need more than one number for magnitude, otherwise for the public it’s really useless information. On the other hand, we don’t want to give 10 numbers.”

“There are as many as 20 different magnitude scales,” explains Thomas Heaton, head of the U.S. Geological Survey’s Southern California field office in Pasadena. “Often in news reports they are all referred to as Richter magnitudes, even though only one of them actually is.”

“The different magnitudes lead to different readings, which is part of the problem. In fact, there are many thousands of problems in giving a precise size for an earthquake.”

Different Waves

The most common alternative scales--surface wave magnitude, seismic moment and energy magnitudes and body wave magnitude--measure different kinds of earthquake waves that emanate from the epicenter to arrive at seismographic stations at different intervals. These waves are of varying lengths and give sometimes conflicting information on the type and strength of a quake.

- Surface wave magnitudes are derived by measuring comparatively long period waves, 20 seconds in duration. Surface waves are detectable at long distances from the epicenter, as contrasted to waves of 0.1 to two seconds in length that have been used to compute the Richter scale. Richter scale waves are local waves detected at shorter distances, usually less than 300 miles.

- Seismic moment magnitudes are set by measuring very long period waves, some of which are 200 to 300 seconds. They also are measurable at a long distance from the quake.

- Energy wave magnitudes use the seismic moment and other measurements to estimate the total energy produced in an earthquake. Unlike the Richter and other scales that rise by a power factor of 10 for each point on the scale, the amount of earthquake energy increases by 30 times for each 1-point rise in magnitude.

- Body wave magnitudes are derived from measurements of other earthquake waves ranging from 1 to 10 seconds long.

All of the scales, taken individually, have certain shortcomings.

Faulty Comparisons

For instance, in some parts of the globe earthquakes generate bigger surface waves than in others, or send out bigger waves in one direction than others. This sometimes makes it difficult to accurately compare the relative strengths of earthquakes using surface waves alone.

Indications of local strength often are misleading. Energy wave magnitudes can be very high when a quake ruptures a fault line over a long distance, for example, even though the actual shaking at any given point might remain comparatively low. For a sharp local quake, the Richter scale, which is based on local waves of short duration, may convey a more accurate impression of size than the energy scale.

The problem of accurately stating earthquake magnitudes has received more attention in recent years. In last fall’s Whittier Narrows and Imperial Valley earthquakes, initial readings were revised weeks afterward by the U.S. Geological Survey, Caltech and other authorities. Such revisions were required as more data was collected and careful calculations were made.

The Whittier Narrows quake of October, originally put at 6.1, was later revised downward by the Geological Survey and Caltech to 5.9. The November quakes in the Imperial Valley, first estimated at 6.0 and 6.3, were later said to be 6.2 and 6.6.

These differences were actually quite substantial. On the logarithmic scale, a 6.6 earthquake is more than twice as powerful as a 6.3.

Composite Calculations

The new magnitudes for all three earthquakes were derived from a kind of composite surface wave and energy wave calculation. Often, the first estimates had been derived from the local waves used to calculate the Richter scale.

Using the new calculations on old earthquakes can change historical perceptions of them.

For instance, in light of what is known today, scientists believe the San Francisco earthquake of 1906 and the Alaskan earthquake of 1964 should not be considered to have been approximately the same size, as they were earlier.

According to Kanamori’s energy scale, for example, the Alaskan quake should be upgraded to a 9.2, while the San Francisco quake is downgraded to a 7.8 or 7.9.

Heaton remarks, “In terms of total energy released, the Alaskan quake was far bigger than San Francisco, probably 100 times greater, despite the fact that both were said at one time to be 8.25 or 8.3 on the Richter scale.”

Worst Shake in Chile

According to these kinds of calculations, the most powerful quake of the 20th Century took place in Chile in 1960. Its magnitude has now been set at 9.5.

Actually, as Heaton notes, it is always a misnomer to refer to a Richter reading of 8.3, the number often used in forecasts of the “big one” widely expected some day on the southern San Andreas fault. The Richter scale, as it is calculated, simply does not meaningfully go that high. Such numbers have always represented interpretations and extrapolations of other scales.

The Richter scale, developed by the late Caltech seismologist Charles F. Richter in 1935, was designed to give the size of an earthquake at its source, or epicenter.

Sometimes also known as the local magnitude scale, it measured the size of local earthquake waves on the Wood-Anderson seismograph at a distance of 100 kilometers from the quake epicenter. A distance correction factor had to be applied, since the seismographs were almost never precisely that far away from the epicenter.

Going Up by Tens

For every increase of one number of magnitude under Richter’s logarithmic scale, an earthquake is 10 times larger in amplitude. So a 4.0 quake on the scale is 10 times larger than a 3.0, a 5.0 quake 100 times larger, a 6.0 quake 1,000 times larger, etc.

But the Richter scale, as calculated, is incapable of measuring the power of earthquakes above about 6 magnitude. It levels out, even as other scales measuring magnitude continue to rise. That’s because local waves measured by the Richter scale do not become more intense as the earthquake becomes larger.

Except for the very largest quakes, energy magnitude readings are about the equivalent of surface wave magnitudes, and frequently when all the factors are analyzed the final magnitude assigned an earthquake reflects these two magnitudes more than the local magnitudes reflected in the Richter scale.

At a final established 5.9 magnitude, however, the Oct. 1 earthquake was not big enough to have a markedly different reading on the Richter scale than on the energy magnitude scale.

Misleading Impressions

The actual shaking, by the way, in earthquakes above 6.0 does not increase very much, certainly not by the amount one might feel indicated by logarithmic measurement. After 6, a quake becomes more powerful because of such factors as length in time, the area it affects and the extent of ground rupture--all criteria measured more accurately by the surface wave, seismic moment and energy wave magnitudes.

A major practical difficulty not only with the Richter but with other scales is that misleading impressions are created as to the violence of shaking at the upper levels.

“This is one of the big sources of confusion about earthquakes,” Heaton observes. “The shaking one feels in an 8-point quake would be maybe three times what was felt in a 6-point quake, not 100 times as some people think is indicated by the scales.”

Or, to put it another way, the shaking motion in an 8.0 quake is definitely not 100,000 times stronger than a 3.0, despite logarithmic readings showing it that much more powerful, or 24 million times more violent, despite energy readings showing it produced that much more energy.

Surprise for Visitors

Heaton, greeting visitors for a discussion of the magnitude question recently, began with the flat statement: “There is no Richter scale.”

Scientists never use that term among themselves, he said. “It has many problems.”

Among those he mentioned:

- Many seismographs are not of the Wood-Anderson variety.

- The Wood-Anderson seismograph, in any event, cannot detect quakes smaller than 2.5 magnitude, and it distorts any quake over about 5. The figures given through the years for such quakes have been recalculations based on other scales.

- Earthquakes occur all over the world, but the prescribed Richter distance correction is good for only about 300 miles. So the scale really does not apply for quakes at large distances, and even within the 300-mile range, the distance correction factor varies from place to place.

Heaton said he feels the Richter scale has continued to be used because it long ago became popular and the public came to expect it. When it is mentioned in the press, earthquake scientists usually do not object, even though they know the magnitude given does not really represent the Richter scale.

It would be more accurate in the future for the news media to simply refer to a quake as being of a certain magnitude and not mention the Richter unless it was a small to moderate quake within the local area, Heaton said.

Not a Clear Picture

Even though the other scales, particularly when weighed against each other and developed into a composite, convey a better impression of the size of an earthquake, they cannot tell everything about it.

Other factors, such as ground acceleration motions, complicate the picture. Variations in shaking power from place to place frequently add to the puzzle. For instance, the biggest ground motion in the Whittier Narrows quake was recorded more than 30 miles away in Tarzana, in the San Fernando Valley.

Even if, as Heaton suggests, the standard moves toward assessing magnitude more on energy scale calculations than any other kind, it still may not be totally satisfactory, and no one is quicker to acknowledge that than the creator of the energy scale, Caltech’s Kanamori.

“Sometimes, you want the magnitude of shaking where you were standing,” he remarked.

This would be the Mercalli scale, a set of more subjective criteria relating to observed phenomena--movement, damage, personal reactions and so forth--at the location of each observer. There are as many Mercalli scale readings for an earthquake as there are locales where it is felt, or people feeling it.

The point is clear to Kanamori. “In an earthquake, there is really no such thing as a single number (for magnitude) that tells all,” he says.