Size of Earthquakes
There are several ways to measure earthquake size. The commonest is the Richter scale, a measure of how much the ground shakes during a quake. Richter developed a logarithmic scale—a magnitude 2 quake shakes the ground 10 times more than a magnitude 1 quake, and a magnitude 3 quake shakes the ground 10 times more than a magnitude 2 or 100 times more than a magnitude 1. You may think of the number of zeros after the 1: if the ground motion of a magnitude 1 quake is 10 with a single zero (you can choose the units you use, or the distance from the quake at which you measure, to get a motion of 10), then at the same distance from the quake in the same units, a magnitude 2 moves the ground 100 (two zeros), a magnitude 3 moves the ground 1000 (three zeros), a magnitude 0 moves the ground 1 (zero zeros), a magnitude -1 moves the ground 0.1, and so on.
Ground motion can be measured with special instruments called seismographs. Scientists usually look at either P-waves or surface waves to get the size of the quake. The motion must be corrected for distance from the quake; the farther away from the quake your seismograph is, the smaller the ground motion you will measure. The distance can be calculated from the difference in arrival time between the first P-wave and the first S-wave from the quake to reach the instrument, using the difference in speed between P- and S-waves, or by timing the arrival of the earthquake waves at three or more stations, and determining where the quake must have been so that the waves arrived earlier at this station than at that one.
A Richter magnitude 1 quake is just big enough to feel if you are standing on the ground very near where the quake occurs. Magnitude 3 or 4 quakes are usually strong enough to convince some people to call the police (although it is not obvious what these people want the police to do), and magnitude 5 quakes usually cause some damage. The largest known quakes, around 9, each release about 10,000 times the energy of the first atomic bombs.
Small quakes are very common and large quakes rare—one or more years may pass between one magnitude-8 quake and the next one anywhere on the planet. Approximately, each increase in the magnitude of 1 causes a 10-fold decrease in the frequency of occurrence. But, moving the ground 10 times more takes about 30 times more energy, so most of the energy released and the damage is by the few big quakes rather than by the many little ones.