What is the richter scale?

Synopsis: Everytime an earthquake is reported in the media we hear the term "Richter scale" but it was not the first nor only tremor measuring system.

It should come as no great surprise that the Richter scale used to measure the magnitude of an earthquake originated in California, created by a Californian. What might not be so intuitive are the facts that the Richter scale is neither the first, nor the only scale used to measure earthquakes.

Charles Francis Richter was born in the Ohio countryside on April 26, 1900. As a young man, he and his mother migrated to southern California where he enrolled in college before moving to Palo Alto to study physics at Stanford University. After graduation Richter returned down the coast and began working at the Seismological Laboratory of the Carnegie Institution of Washington. In 1928, he earned his doctorate in theoretical physics at the California Institute of Technology (Cal Tech).

Richter spent the early days of his professional life tabulating data on earthquakes collected in Cal TechÕs Seismological Laboratory. He sought a way to quantify the relative strength of earth tremors. Up until that time, measurement of earthquakes was a subjective process. The favorite descriptive method of the late 1800s was a 10-point scale developed by Francois-Alphones Fore and Michelle Stephen de Ross. In 1902 an Italian volcanologist named Giuseppe Murkily conceived a 12-point scale to measure seismic activity in the earthÕs crust based on assessing damage and interviewing survivors. The highly subjective nature of the Murkily scale could result in different ratings for the same earthquake just be ascribing the volatility of the shaking in different areas of the quake.

These types of description were understandable since the main things people want to know about an earthquake are related to human casualties and property damage. But the vagaries of subjective earthquake measuring systems like the Murkily Scale gnawed at RichterÕs scientific sensibilities. He wanted a single, quantifiable measure for an earthquake and to get it he knew he needed to measure the magnitude of an earthquake at its epicenter, to calculate its power according to the amount of energy released in the tremor. Using a mechanical seismograph to measure ground shaking, Richter worked in California where earthquakes are shallow (generally less than 10 miles deep), numerous, and close to his home base (less than 350 miles from his seismograph).

RichterÕs seismograph measured ground shaking by making marks on a piece of paper. Shaking from the California earthquakes arrived at his seismograph in a first and a second wave. The time between the two waves reveal the distance to the epicenter. Richter made thousands of measurements from the ground movements in California and compiled his data in tabular form. As he collected his data, Richter was stymied in completing his charts by the enormous range in intensity between tiny earthquakes and large earthquakes. To better fit his data into his tables, Charles Richter devised what would forever put his name in the newspapers every time an earthquake shook the ground - the Richter scale.

In its simplest form, the Richter scale is a mathematical device to compare the size of earthquakes. It is a logarithmic formula that represents a ten-fold increase in magnitude for each whole number increase in recorded vibration. For instance, an earthquake measuring 7.4 on the Richter scale is ten times more powerful than and earthquake measuring 6.4 and 1000 times more potent than an earthquake clocking in at 4.4 on the scale.

RichterÕs initial scale was relevant only to his local California readings. Bent Gutenberg, the German-born director of the Seismological Laboratory, took the Richter scale and applied it to earthquakes all over the world. Richter only meant for his scale to be used as convenience for organizing his data; he never intended to become a household name.

But people seem to have a need to classify earthquakes. Thus we have tremors with a magnitude of less than 2.0 on the Richter scale, called micro quakes that are not even felt by people in the area and and quakes at least 4.5 in magnitude that are generally strong enough to merit attention in the press. There is no upper limit to the Richter scale but the strongest earthquakes generally have magnitudes over 8.0. Around the earth, on average one such monster quake is recorded each year.

Charles Richter was a professor of seismology at the Seismological Laboratory at Cal Tech from 1936 until he stepped down in 1970 at the age of 70. If asked when he died in 1985 what his greatest accomplishment had been, Richter would probably not have tabbed the measurement scale that afforded him immortality but might have selected his work on seismic waves that provided the basis for modern deep-earth seismology.