The method of measuring quakes has been upgraded and improved upon since Charles F. Richter developed a scale in 1935 that has been in common usage for decades and often called the Richter Scale.
'All the formulas that we use originally came from variations and upgrades of Richter's original formulas,' said Don Blakeman, USGS earthquake analyst at Golden, Colorado. Seismologists indicate the size of an earthquake in units of magnitude, a logarithmic measure.
In simple terms, this means that at the same distance from the earthquake, the shaking will be 10 times as large during a magnitude 5 earthquake as during a magnitude 4 earthquake. The total amount of energy released by the earthquake, however, goes up by a factor of 32.
Magnitude can be measured as duration, local, surface wave, moment or body, according to the USGS website.
'What it amounts to is that there are a number of different formulas and they are applied in specific cases to get the best representation of the size of the earthquake,' Blakeman said.
Yesterday's earthquake had a moment magnitude of 9.0, the USGS said.