rĭkˈtər, measure of the magnitude of seismic waves from an earthquake, devised in 1935 by the American seismologist Charles F. Richter (1900–1985). The scale is logarithmic; that is, the amplitude of the waves increases by powers of 10 in relation to the Richter magnitude numbers. The energy released in an earthquake can easily be approximated by an equation that includes this magnitude and the distance from the seismograph to the earthquake's epicenter. Numbers for the Richter scale range from 0 to 9, though no real upper limit exists. An earthquake whose magnitude is greater than 4.5 on this scale can cause damage to buildings and other structures; severe earthquakes have magnitudes greater than 7. The famous San Francisco earthquake of 1906 was 7.8 on the Richter scale; the Alaskan earthquake of 1964 was 8.4; the Kobe, Japan, quake of 1995 was 6.9; and the Izmit, Turkey, earthquake of 1999 was 7.4. Like ripples formed when a pebble is dropped into water, earthquake waves travel outward in all directions, gradually losing energy, with the intensity of earth movement and ground damage generally decreasing at greater distances from the earthquake focus. In addition, the nature of the underlying rock or soil affects ground movements. In order to give a rating to the effects of an earthquake in a particular place, the Mercalli scale, developed by the Italian seismologist Giuseppe Mercalli, is often used. It measures the severity of an earthquake in terms of its effects on the inhabitants of an area, e.g., how much damage it causes to buildings.

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