There is no upper limit defined for the Richter scale, but after a century of seismograph measurements, it appears that rocks in the earth release their stress before building up enough energy to reach magnitude 10. The effect of distance is factored out of the calculation. The distance to the epicenter must also be taken into account because the greater the distance from the earthquake, the smaller the waves get. The maximum amplitude seismic wave – the height of the tallest one – is measured in mm on a seismogram. Two measurements are factored together to determine the Richter magnitude of an earthquake: the amplitude of the largest waves recorded on a seismogram of the earthquake, and the distance to the epicenter of the earthquake. Richter used seismograms of earthquakes that occurred in the San Andreas fault zone to calibrate his magnitude scale. For several decades, earthquake magnitudes were calculated based on a method first developed by Charles Richter, a seismologist based in California. The magnitude of an earthquake is a number that allows earthquakes to be compared with each other in terms of their relative power. Intensity is determined from effects on people, human structures, and the natural environment. Intensity measures the strength of shaking produced by the earthquake at a certain location. Magnitude is determined from measurements on seismographs. Magnitude measures the energy released at the source of the earthquake. Magnitude and Intensity measure different characteristics of earthquakes.