Brooke Clarke wrote: > Hi Steve: > > The Ricter scale was developed based on pendulum seismometers and that > the "P" wave arrives first then the "S". The time delay between the P > and "S" wave gives the distance to the epicenter (time nuts connection). > The magnitude of the "S" wave (adjusted for the distance) gives a Ricter > magnitude. But this system does not relate to the energy contained in > the quake. That's why they use "moment magnitude" these days, rather than straight Richter (which after all is the log of the deflection reported on a particular kind of seismograph). > http://www.prc68.com/I/Seismometer.shtml > > At the time of the Loma Prieta quake I was standing in front of the > company where I worked and watched the windows "oil can" with the > magnitude of the in-out motion increasing with each cycle. If the quake > had lasted a few seconds longer they would have exploded, either sending > glass into or out of the building. We were all starting to lay flat on > the ground in case the glass was coming out. Then the quake ended. > > The subject locations are all on or adjacent to the Pacific plate as is > California. Most of California is on the North American Plate. The San Andreas fault is roughly the dividing line. Los Angeles is on the Pacific Plate. Half of San Francisco is on each side(!).. Berkeley, Oakland, etc, are all on the NA plate. >