Measuring Earthquakes

We have been asked the meaning of the Richter magnitude scale now commonly used to report earthquakes.

Some years ago, before seismographs were widely deployed, the Modified Mercalli intensity scale was used. It measured, not how strong an earthquake was, but rather how strongly it was felt or how much damage it caused. The Mercalli scale ran from I to XII; I being an earthquake felt only by a few persons under favorable circumstances and XII meaning total destruction.

The Richter scale uses seismographs to measure actual ground motion at a specific distance (100 km) from the earthquake epicenter (origin). Earthquakes smaller than Richter scale 3 are not usually felt even by persons near the epicenter. The Richter scale is logarithmic, which means that a Richter scale 4 earthquake has ten times more motion than a scale 3 earthquake; Richter scale 5 means ten times as much motion as scale 4, and so on.

The largest earthquakes have Richter magnitudes 8.0 to 8.6, but great damage has been done by earthquakes of only magnitude 5 or 6.

Alaska has had more than its share of truly large earthquakes--Richter magnitude 8.6 near Yakutat on September 10, 1899; the Lituya Bay earthquake of July 10, 1958 was magnitude 8 and the March 27, 1964 Good Friday earthquake near Anchorage was magnitude 8.5.