FYI: The earthquake off the coast of Sumatra that set off December's devastating tsunami measured about nine on the Richter scale, while the tremor in Indonesia two weeks ago hit 8.7, and a recent quake in Japan reached seven. How does the Richter scale work? The Richter magnitude scale was developed in 1935 by physicist Charles Richter of the California Institute of Technology. It is a mathematical method of comparing the size of - not necessarily the destruction caused by - earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of the strongest waves recorded by seismographs, which are finely tuned scientific instruments that record a zigzag trace demonstrating the amplitude of elastic ground oscillations directly beneath the device, in response to an earthquake. Adjustments are made for variation in the distance between various seismographs stationed around the world and the epicentre of a quake. The Richter scale has a logarithmic basis (base 10), so an increase of one on the scale represents a tenfold increase in measured ground movement. Therefore, a quake measuring nine on the Richter scale creates 10 times more shaking than a quake registering eight, which is 10 times more aggressive than a seven. Using the Richter scale, therefore, the quake that caused the Boxing Day tsunami was more than 100 times (10x10) more physically violent than the one that recently rocked Fukuoka. The largest earthquake of modern times measured 9.5 on the Richter scale and occurred on May 22, 1960, in Chile. It would have been five times more violent than Sumatra's Boxing Day quake. As an estimate of energy released, however, each whole-number jump on the Richter scale corresponds to the release of more than 31 times more energy than the amount associated with the preceding whole number. So a Richter 'nine' quake lets loose nearly 1,000 times (31x31) more energy than a 'seven'. Earthquakes with a Richter magnitude of two or less are usually called micro-earthquakes. They are not generally discernible by people, and are only recorded by seismographs in the immediate vicinity. Events with magnitudes of about 4.5 or greater - there are several thousand such shocks annually - are strong enough to be recorded by sensitive seismographs across the planet. Massive earthquakes, of which Indonesia has now suffered two in the past three months, have magnitudes of eight or higher. Usually, only one earthquake of such size occurs globally each year. And, contrary to popular belief that the Richter scale is limited to a maximum of 10, the model was designed to have no upper limit. However, the Richter scale is never used to express damage. To assess this, some look to a system known as the Mercalli scale.