First they said the recent earthquake near Gilroy was a magnitude 5.2. A few hours later, they insisted it was a magnitude 4.9. Can't those earthquake scientists make up their minds? Well, yes and no.
With today's methods and equipment, it's easier than ever to come up with a magnitude that accurately reflects the punch of an earthquake.
But it may take a while to do that. In the meantime, the public and the media are clamoring for a number to hang on an event that really shook them up.
Complicating matters, researchers often apply modern methods to an old quake and decide to revise its magnitude years after the fact.
To clear up some of this confusion, the U.S. Geological Survey quietly changed its policy on magnitudes in January -- moving even further away from the Richter scale that is so firmly associated, in the mind of the public, with earthquakes.
From now on, the agency will use something called the ``moment magnitude'' to describe the size of earthquakes. This is considered the most accurate way to measure the big earthquakes that worry people most.
If it takes a while to determine the moment magnitude, the survey will release a preliminary magnitude based on the best available information at the time.
The same group of scientists that came up with the new policy also looked back at significant earthquakes starting in 1556, when a quake of about magnitude 8 killed an estimated 830,000 people in the region of Shensi, China, and compiled a list of official magnitudes. It's on the Web at http://earthquake.usgs.gov/docs/020204mag_signeq.html.
But even that list is not written in stone. It will be reviewed once a year by the agency's National Earthquake Information Center in Golden, Colo., and may be revised as new information becomes available.
"This size-of-the-earthquake problem will never go away,'' said Tom Heaton, a seismologist at the California Institute of Technology in Pasadena, who has been embroiled for years in the magnitude debate.
David Oppenheimer, a seismologist with the USGS in Menlo Park and a member of the magnitude group, said, "This has been an ongoing issue for as long as I can remember. We have different ways of computing magnitudes for earthquakes, and if we don't speak in unison, we will confuse the public by issuing these different estimates.''
Richter scale's faults
So what's wrong with the good old Richter scale?
Nothing, really. It was a great tool for its time. But like last year's computer, it's been nudged aside by new technology.
Developed by Charles Richter of Caltech in 1935, this familiar scale allows scientists to compare earthquakes of vastly different sizes.
It isn't straightforward, like a ruler, but logarithmic. In theory, for each increase of one full point on the Richter scale, the ground shakes 10 times harder, and 32 times as much energy is released.
The Richter magnitude is based on the size of the largest waves arriving at a particular type of instrument, the Wood Anderson seismograph. It's known as a local magnitude scale because it considers only waves arriving within 370 miles of the epicenter -- the spot on the ground directly above where the earthquake started. If there are no instruments within this distance, the size is estimated from readings taken closer and farther away.
However, each earthquake is a rich blend of seismic waves. If you imagine the ground as the surface of a sea, these waves range from ripples to choppy whitecaps to long, slow swells.
The Richter scale measures only the chop -- high-frequency waves that travel through the body of the rock, rather than at the surface. These are the ones most damaging to small structures, from homes to buildings a few stories tall.
But the bigger the earthquake, the more long, slow waves it produces. In fact, this turns out to be a key difference between major earthquakes and their more moderate kin. The big ones don't necessarily shake the ground 10 times harder; but they do shake it longer, and they put out more long, slow swells of the type that can resonate with -- and damage -- big structures like high-rises and bridges.
When it came to measuring these big earthquakes, the Richter scale fell short. So scientists came up with other methods.
Until January, the USGS used the surface wave magnitude scale to determine the official magnitudes of earthquakes. It's based on waves that travel on the surface of the ground, with their crests passing a given spot about once every 20 seconds.
These waves are recorded by instruments all over the world, allowing scientists to accurately measure distant quakes.
However, even this method failed to capture the true size of the biggest quakes, which can generate much longer waves.
So more than a decade ago, scientists started to use the moment magnitude scale. It's based on the full range of waves coming from an earthquake, including those that take up to 100 seconds to pass a given spot.
This type of analysis is made possible by a new generation of seismic instruments that capture everything from the chop to the swells; in the past, any given instrument could measure only part of this spectrum.
Old-style measurements
Old-style seismographs recorded the jiggling of an earthquake on a rotating drum. In early models, a needle scratched squiggles on glass or paper that had been coated with soot from a kerosene lantern. Later, the squiggles were etched on light-sensitive paper that could be developed like a photograph.
In either case, someone had to replace the paper or film every day, and the results could take a week or more to gather and analyze.
"Not only was this a pile of paper, but this was a pile of labor, too,'' said Bill Karavas, chief engineer for the Berkeley Digital Seismic Network, pointing to shelves of old seismic records stored at the University of California-Berkeley.
In contrast, modern instruments record data electronically, rather than on a drum, and send it within seconds over telephone lines to a center like the one at Berkeley.
While many old instruments would go off-scale even in moderate earthquakes -- the paper was simply not wide enough to accommodate the biggest squiggles -- the new ones can record anything from a car door slamming to violent shaking. They pick up things subtle enough to rate a minus 2 on the magnitude scale.
"We can see the ground tilt at sunrise as the sun hits the rocks,'' said Douglas Dreger, a seismologist at UC-Berkeley. "It's impressive.''
The new instruments make a world of difference when it comes to analyzing earthquakes -- like listening to your favorite music on a state-of-the-art stereo system, rather than on a scratchy gramophone.
From the symphony of seismic waves they collect, researchers can calculate the moment of an earthquake -- the forces along the fault that would be needed to generate the waves.
From that, they can infer the area of the fault that slipped, the amount that it slipped and the stiffness of the ground. The end result is the moment magnitude.
"Moment magnitude has a more physical basis than most other magnitude scales,'' Oppenheimer said. ``People in the USGS have been using moment magnitude as the best estimate for several years now, but it's been informal.''
The choice of scale can make a big difference when it comes to estimating the size of historic quakes.
Reassessing quakes
For instance, the 1906 San Francisco earthquake and the 1964 Good Friday quake in Alaska both had surface-wave magnitudes of 8.3.
Yet the Alaskan quake involved a much bigger fault and released 100 times as much energy. So when the more accurate moment magnitude scale came along, San Francisco was downgraded to 7.8, and the Alaskan quake was boosted to 9.2.
The 1989 Loma Prieta quake was demoted, too. Its official surface-wave magnitude is 7.1, but on the moment scale it's only a 6.9.
Oppenheimer said he hoped the new magnitude policy would refocus attention away from magnitudes and toward more important issues in the wake of an earthquake.
"Most seismologists don't really care if it's a 4.9 or 5.0,'' he said. ``We'd really like the public to think beyond that in terms of public safety.'' |
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