When the British Geological Survey reports a magnitude 3.2 tremor under the Welsh borders, the number sounds like a measurement of shaking. It is not. Magnitude measures what happened at the fault, kilometres underground; what people feel at the surface, and what falls down, is governed by an entirely separate set of factors. Confusing the two is the most common misreading of earthquake news.
The scale in use today is moment magnitude, written Mw, adopted in the late 1970s to fix a flaw in Charles Richter's original 1935 method. Richter calibrated his scale to the amplitude traced by a specific instrument, the Wood-Anderson seismograph, at a standard distance in southern California. It worked well for moderate local quakes but saturates above roughly magnitude 7: the needle's swing stops growing even as the rupture keeps releasing more energy, so Richter's method scores a giant quake and a merely large one almost identically. Moment magnitude instead derives from the seismic moment — the area of the fault that ruptured, multiplied by how far the two sides slipped and by the rigidity of the rock. A magnitude 9 rupture such as Japan's in 2011 tore a fault patch some 500 kilometres long; a magnitude 5 breaks a patch a few kilometres across. The formula was deliberately fitted so the numbers line up with Richter's in the mid range, which is why newsreaders still say "Richter scale" decades after seismologists stopped using it for anything big.
The scale is logarithmic in a way that intuition handles badly. Each whole step represents about ten times the ground-motion amplitude and roughly 32 times the energy, since energy scales as 10 raised to 1.5 times the magnitude. Two steps is a thousandfold energy difference. The 2008 Market Rasen earthquake in Lincolnshire, at magnitude 5.2 the largest UK event in a generation, released around one thirty-thousandth of the energy of the 2023 Kahramanmaras quake in Turkey. Britain's all-time record, the magnitude 6.1 Dogger Bank event of 1931, was offshore — the North Sea absorbed most of it, and mainland damage amounted to toppled chimneys.
Why the same number kills in one place and rattles crockery in another
Depth comes first. Seismic waves lose strength with distance, so a magnitude 6.0 at 10 kilometres' depth delivers violent surface shaking directly above it, while the same rupture 600 kilometres down in a subducting slab may pass almost unnoticed. Then the ground itself intervenes. Soft sediments amplify and prolong shaking; solid bedrock transmits it briefly and moves on. Mexico City's 1985 disaster owed much of its death toll to the drained lakebed the city sits on, which resonated like a bowl of jelly at frequencies that matched the sway of mid-rise buildings, flattening blocks 350 kilometres from the epicentre.
Construction decides the rest. The comparison seismologists reach for is 2010: Haiti's magnitude 7.0 in January killed well over 100,000 people in unreinforced masonry and informal concrete; Chile's magnitude 8.8 in February — releasing about 500 times more energy — killed around 500, because Chile has enforced strict seismic codes since the 1960s. The engineering aphorism holds that earthquakes do not kill people, buildings do.
Because magnitude cannot capture any of this, seismologists run a second scale alongside it. Intensity — measured in Europe on the EMS-98 scale, elsewhere on the Modified Mercalli — grades observed effects from I (not felt) to XII (total destruction), and varies from place to place within a single event. One earthquake has one magnitude but a whole map of intensities, strongest near the epicentre and fading outwards, distorted by geology. The BGS still collects "did you feel it?" reports from the public after UK tremors precisely to draw that map; instruments record the energy, but only people and buildings record the consequence. Next time a headline leads with the magnitude, the more revealing questions sit underneath it: how deep, on what ground, and what was standing on top.
