What does the Richter scale measure?

Earthquakes - specifically the amplitude of ground motion recorded on a seismograph, corrected for distance. The important feature is that it is logarithmic: each whole number is ten times the shaking and about 32 times the energy released. A magnitude 7 is not slightly worse than a 6; it releases as much energy as 32 of them.

Seismologists have largely stopped using it. Charles Richter's 1935 scale was calibrated for a particular instrument in southern California and saturates on very large events - it cannot distinguish an 8.5 from a 9.5, which is the difference between a disaster and a catastrophe. The moment magnitude scale replaced it in the 1970s, derived from the actual physics of the rupture: how much rock slipped, how far, over what area. The numbers are deliberately similar so nobody had to relearn the vocabulary, and news reports still say Richter out of habit.

Magnitude also says less about consequences than people assume. Depth, distance, the local geology and the buildings decide the death toll. The 2010 Haiti earthquake was magnitude 7.0 and killed well over a hundred thousand people; the 2010 Chile earthquake was 8.8, some 500 times more energetic, and killed a few hundred. The difference was building codes.