MENLO PARK (US): At every corner, the San Francisco area is commemorating the 100th anniversary of the April 18, 1906, earthquake that marked one of the worst natural disasters in US history.

Museums and public halls are holding retrospectives, and construction and insurance companies are promoting their services against future earthquakes.

The city has staged a new ballet titled Earthquake which includes sounds of seismic movements amid a sculpture producers say represents the Richter scale.

Phones are ringing at the offices of the US Geological Survey as the media prepare a hearty diet of earthquake related items. Yet for all the publicity, scientists lament that few in the public understand that the Richter scale so often cited in reports on earthquake sizes, is long gone, replaced by ‘moment magnitude’ scale.

And yes, there is a difference.

For decades, historians recorded the 1906 San Francisco earthquake as an 8.3 on the Richter scale. On the newer scale the disaster was reassessed as a 7.8 or 7.9, knocking it a notch down to a ‘major’ rather than ‘great’ earthquake.

“Fundamentally what I think most people don’t understand is this concept of what moment magnitude is,” said Thomas Hanks, who devised what is now the accepted standard scale with Hiroo Kanamori in 1979.

California seismologist Charles Richter pioneered earthquake magnitude measurement in the 1930s. A man who lived a colourful personal life, Richter achieved wide fame, but even decades later he said the public was still confused.

“Lately there have been complaints that the use of the magnitude scale is confusing, or at least the reporting of magnitudes in the newspapers ‘confuses the public’,” Richter wrote in 1958.

Today, the media regularly misreport earthquakes in Richter magnitudes — a scale that fails to give accurate measurements on bigger earthquakes of six or greater.

“It’s only when you get to these larger earthquakes that there is a difference, and it’s only because the instrument on which (Richter’s) local magnitude is based did not receive enough of the frequency band to get the true size of the earthquake,” said Hanks, who works at the US Geological Survey in Menlo Park, south of San Francisco.

In essence what the numbers mean — such as a ‘4’ being a light quake, a ‘6’ a strong quake — has stayed the same, but the way the numbers are calculated has changed.

The US Geological Survey adopted moment magnitude as its official standard in 2000.

As for public confusion, Tom Hanks — whose name sometimes confuses people because of the actor of the same name — blames poor US scientific literacy, but acknowledges that he may not have spent enough time spreading his insights in public.

In a nutshell, the new scale measures the ‘moment’ of an earthquake — the rigidity of rock along the face of a fault, the area of the fault and how much one side of the fault moves relative to the other. The ‘moment magnitude’ then uses that data to calculate the earthquake’s magnitude in an easy-to-digest, one-digit number.—Reuters