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.
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