The Sendai, Japan, earthquake is listed as the fifth strongest recorded. The strongest was in Chile in 1960. It registered 9.5. The second was Prince William Sound off Alaska in 1964 at 9.2. The third was off the cost of northern Sumatra in 2004 at 9.1 and the fourth was Kamchatka, Siberia, in 1952 at 9.0. The Sendai quake was recorded at 8.9.
The United States Geological Service website explains what that the Richter Scale is used the measure earthquake intensity. “Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument.
“Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, locations, and magnitude of an earthquake can be determined from the data recorded by seismograph stations.”