When an earthquake strikes, it is assigned a Rating on the Richter Scale. The recent Sea of Japan earthquake was assigned a rating of 7.6, but what does this mean and how much more powerful is an earthquake with a rating beginning with a 7 than one with a six?

The Richter Scale was devised by Charles Richter and Otto Gutenberg in 1935, after the duo collaborated to produce a standardised scale to measure the magnitude of earthquakes. They chose a logarithmic scale to measure the amplitude of seismic waves, information about which was captured by a specific type of seismograph, the Wood-Anderson torsion seismograph. The scale ranges from 1 to 10, although technically there is no upper limit, with each whole number representing a tenfold increase in amplitude and a 31.7 increase in energy release. So, the wave amplitude of an earthquake with a rating of 7 is 100 times greater than a level 5 quake.

While the Richter scale eliminated much of the subjectivity that existed before in comparing earthquakes, the decision to tie it to readings from a specific type of seismograph limited its uptake. Nowadays, however, instruments are carefully correlated with each other and magnitude can be computed from any type of calibrated seismograph. Earthquakes with a magnitude of 2 or below, microearthquakes, are usually only picked up on local seismographs but those in excess of 4.5 are strong enough to be detected by sensitive seismographs the world over.

What the Richter scale cannot do is give an accurate sense of the extent of damage. This is provided by the Mercalli scale, which are expressed in Roman numerals. A low intensity earthquake which produces little in the way of property damage and a sensation of vibrations under the feet would have a Mercalli rating of II, whereas one in which structures are destroyed, the ground is cracked, and other natural disasters are triggered would have a rating of XII.

The appropriate rating on the Mercalli scale, which involves a range of subjective criteria, takes time to establish which is why the more immediately determined Richter scale is more familiar and satisfies the media’s desire to pin a tag on the size of the earthquake.  

So now we know.