An Instrumental Earthquake Magnitude Scale
Give every earthquake one number — the logarithm of its largest recorded wiggle.
Before 1935 you could say an earthquake felt violent — but not that this one was bigger than that one. Richter turned the shaking into a number.
The idea, unpacked
An earthquake's shaking is strongest near the source and weaker far away, so how hard it shook here tells you as much about where you stood as about the quake. Richter wanted a number for the quake itself. His trick: read it off the squiggle a seismograph draws, then correct for how far the station sat from the source.
The catch is that earthquakes vary enormously — the ground might move a thousandth of a millimetre, or a full metre. To tame that range he used a logarithm: each step up the scale means ten times the ground motion. That is why a magnitude 7 is not a little bigger than a 6 — it is ten times the shaking, and about thirty times the energy.
Where it came from
Richter was a young physicist at Caltech in Pasadena, cataloguing hundreds of small Southern California earthquakes picked up by a new network of identical seismographs. Asked to publish the catalogue, he needed a fair way to size each shock. A few years earlier the Japanese seismologist Kiyoo Wadati had shown that plotting the shaking against distance could rank earthquakes; Richter, working alongside the senior seismologist Beno Gutenberg, turned that idea into a clean, logarithmic, repeatable scale. The word magnitude he borrowed from astronomy.
Why it mattered
For the first time, earthquakes could be compared on one honest axis — across countries, decades, and instruments. That made it possible to count how often quakes of each size occur, to map where the dangerous ones cluster, and to build the science of seismic hazard that decides how we engineer buildings, bridges and dams. A single number became the backbone of earthquake science and of public reporting alike.
Like a star's brightness
Richter took his word from stargazers. Astronomers rate stars by magnitude on a scale where each step is a fixed multiple of brightness, so a dim star and a blazing one fit on the same short ruler. Richter did the same for earthquakes: each step is ten times the shaking, so a barely-felt tremor and a continent-cracking rupture sit just a few numbers apart — close on paper, a million-fold apart in fact.
Where it sits
Richter's scale joined a lineage of people learning to read the Earth from its tremors and its rocks — from James Hutton and Charles Lyell reading deep time in cliffs (both in this Library) to Alfred Wegener and Harry Hess piecing together moving continents. Magnitude gave that science a number. The Richter scale is now usually the moment magnitude, and the public Richter has quietly become Gutenberg's and Kanamori's work too — but the idea of one logarithmic number for one shock is his.
The magnitude of any shock is taken as the logarithm of the maximum trace amplitude, expressed in microns, with which the standard short-period torsion seismometer would register that shock at an epicentral distance of 100 kilometers.