|
|
|
 hen Caltech scientist Charles F. Richter devised his magnitude scale for comparing earthquakes in 1935, he used the printed record from a seismograph.
A seismograph pen traces a zigzag pattern when the ground moves underneath the instrument. Richter rightly assumed that the larger the quake, the greater would be the amplitude – the distance between the top of the trace and its center – of the swing of the pen.
These sensitive instruments can greatly magnify ground motions and can detect strong earthquakes anywhere in the world. For the scale to work, though, seismographs must be carefully calibrated. Accurate readings depend on first determining each seismograph’s distance from the earthquake’s epi-center, using data from a number of instruments in different locations.
Richter knew that earthquakes came in many sizes, so he borrowed a concept astronomers use to compare the widely varying brightness of stars.
“He decided to go up in powers of 10,” says Tom Henyey, USC earth sciences professor and director of the Southern California Earthquake Center. A 6.0 earthquake, therefore, would produce10 times the swing of the pen as a 5.0 quake – that is, the amplitude of its seismograph trace is 10 times as great.
REVERSE FAULT movement not only cracked this multi-span bridge north of the Taiwanese city of Fengyuan, it actually created a new waterfall along the Taichai River. |
So far, so good. But measuring amplitude isn’t enough. The bottom line in science is energy, and energy increases more rapidly than amplitude. For example, while a 6.0 earthquake’s amplitude is 10 times greater than a 5.0 quake’s, the amount of energy it releases is 32 times greater. By the same token, a 7.0 eartquake’s amplitude is 100 times greater than a 5.0 temblor’s, but it releases 1,000 times as much energy.
Even a single decimal point packs a punch in this science.
A 6.1 earthquake may not seem much larger than a 6.0; it is in fact almost one-and-a-half times more powerful.
There’s yet another complication. Scientists have learned not to trust Richter’s scale to assess larger earthquakes – defined as magnitude 6.5 and up. Again, the problem is measuring energy.
“The energy release from an earthquake and the amplitude recorded at a particular seismograph don’t scale the same way for large earthquakes as they do for small ones,” Henyey says.
Very large earthquakes usually occur over large areas. In class, Henyey illustrates the concept with this analogy: imagine a straight line of 100 people holding hands and shouting. Now compare it to a line of 5,000 people holding hands and shouting. “The larger group wouldn’t sound much louder, because you couldn’t hear those at the ends of the line,” he says. Yet the noise output would be vastly different.
Earthquakes can be deceiving in a similar fashion. “The ground motion and subsequent seismograph amplitude do not increase as rapidly for large earthquakes as they do for smaller ones, while the energy keeps going up at the same rate,” says Henyey. As a result, “the Richter scale greatly underestimates very large earthquakes.”
Scientists now determine the magnitude of very large quakes by taking into account the length and the amount of slip in the rupture (how wide, on average, the new break in the earth is). This modified system is known as the Moment Magnitude Scale.
In 1960, an earthquake broke the entire coastline of Chile – about 1,000 miles of violent shaking.
“On the Richter Scale, the Chilean earthquake was measured at 8.6,” says Henyey. “But using the newer techniques, it came out to a 9.5. That’s almost 30 times as much energy.”
|
|
