United States Geological Survey

At least theoretically, there is no upper limit for an earthquake-nor, come to that, a lower limit. The scale is a simple measure of force, but says nothing about damage. A magnitude 7 quake happening deep in the mantle-say, four hundred miles down-might cause no surface damage at all, while a significantly smaller one happening just four miles under the surface could wreak widespread devastation. Much, too, depends on the nature of the subsoil, the quakeБЂ™s duration, the frequency and severity of aftershocks, and the physical setting of the affected area. All this means that the most fearsome quakes are not necessarily the most forceful, though force obviously counts for a lot. The largest earthquake since the scaleБЂ™s invention was (depending on which source you credit) either one centered on Prince William Sound in Alaska in March 1964, which measured 9.2 on the Richter scale, or one in the Pacific Ocean off the coast of Chile in 1960, which was initially logged at 8.6 magnitude but later revised upward by some authorities (including the United States Geological Survey) to a truly grand-scale 9.5 ...»