Re: earthquake

 Falloff of seismic intensity depends on rock Q and scattering as well as
ray geometry (i.e., not straight lines), so it's complicated. It's also
different for surface waves and body waves. Practicing seismologists work
with displacements usually measured at the surface. To get earthquake
magnitudes they use measured amplitudes and periods plus empirical fudge
factors.

The bottom line, though, is that an earthquake in Japan almost certainly
would not trigger an earthquake in Arizona without triggering other
earthquakes at much smaller distances. Numerous more local
disturbances--such as from earth tides or small California quakes--would
have larger effects in Arizona than would a big quake in Japan. (But don't
ask me to estimate the magnitudes!)

There are foreshocks as well as aftershocks. The way you tell how one
earthquake is related to another is by comparing their foci in context of
known faults.

Don

  ----- Original Message -----
  From: Dawsonzhu@aol.com<mailto:Dawsonzhu@aol.com>
  To: asa@calvin.edu<mailto:asa@calvin.edu>
  Sent: Sunday, November 28, 2004 3:00 PM
  Subject: Re: earthquake

>During our discussion of the second coming in our
>Sunday School class the ceiling mounted projector
>caused the image on the screen to move just at
>9:41 Arizona time.
>Was it an earthquake?
>Did anyone else notice an earthquake ? Was it
>connected to one in Japan early Monday morning?

  To the first question: no.

  Second: I don't know because I don't live there.

  Third: The intensity of an earthquake from the
  epicenter seems to fall off as a function of
  distance (It is probably something approaching
  1/r^2, but I don't know the exact drop-off rate).
  We felt the quake in Nigata prefecture
  here in the Tokyo area (about 200 km away), but
  it already had lost most of its intensity so it was
  little more than a little shaking around. So even
  _direct_ influence on the other side of the world is
  so unlikely, that I think you'd might as well
  consider it impossible. At any rate, the influence
  must run from the stronger quake to the weaker one
  if it could ever happen at all.

  It was interesting that several earthquakes occurred
  in a relately short time span (in geological time scales).
  Usually, there is just one shaking, about ten minutes
  latter, I think it was an aftershock, but the intensity
  didn't drop off all that much. A half hour later, I'm
  sure that was not an aftershock. That is the very first
  time I've experienced anything like that and it was a
  bit nerve wracking.
Received on Mon Nov 29 12:15:25 2004