CompuServe Messages

Hi, Tony– Welcome to the forum/section! The late Charles Richter, a seismologist at Cal Tech, devised his scale in 1931, basing it on the widest swing in the zig-zag line generated by a seismograph's needle in a quake. It is not very accurate for the quakes above 8; it is based on readings on an obsolete type of seismograph used close to quakes (within 100 miles or so, which is often not possible outside Calif.). Thus, scientists long ago abandoned it, but the public had already grown accustomed to it, so it lives on despite all attempts to "kill" it. Since the early '80s, the most widely used measure has been moment magnitude. It is based on the size of the fault on which an earthquake occurs and the amount the earth slips. The bigger the fault and the larger the slip, the higher the moment magnitude. Seismologists at the USGS in Golden, CO prefer surface-wave magnitude, an analysis of seismic waves crackling around the Earth's surface, and call this Richter magnitude, which it somewhat resembles, though it is not precisely the same. The January 17, 1994 Northridge quake in Los Angeles had a moment magnitude of 6.7. The Loma Prieta quake that hit the San Francisco Bay area in '89 had a moment magnitude of 7.0. The Landers, CA quake of June '92 was called magnitude 7.3 by USGS Pasadena (moment magnitude), while USGS Golden called it 7.6 (surface-wave magnitude). The Alaska quake of '64 had a surface-wave magnitude of 8.4, while its moment magnitude was 9.2. –Doug