First to Roger,
I have the rimg program, but have not been able to get it to work on my
computer. Perhaps there is a problem with my unzipper, or could there
be another file we need to get it to work. Would it be possible to provide
the program and accessory files to an ftp site where it could be downloaded
without unzipping?

Next to Don,
Your explanation seemed adequate. I think that after all the work you have
done on aspects of this you have a good deal of insight that I do not. Thanks
for the explanation.

Then to Mark,
There is no particular reason, as far as I see it, that a given quake has to
occur as the seismic waves move through. The waves may just add strain
or jostle the fault enough so that it is that much closer to rupture and may
take several days (even 10) before it actually breaks in an earthquake.

Finally to Cathryn,
Seems to me that Don's explanation is pretty good. I would expect that
a fairly broad area is actually affected by the seismic waves since the
earth is heterogeneous and waves tend to spread out a bit because of
the heterogeneity - thus the torus instead of a specific distance around
the mainshock.
The general effect of the waves is to jostle the area in the rings a bit
more than the area outside the rings because more seismic energy is in
these areas than out. If a large quake is about to occur in an area (whether
or not another large quake has occurred) the stress/strain available
for that earthquake is the same. The effect of the strong seismic wave
on the area where an earthquake is about to happen is merely
1) to cause the earthquake to occur a few days before it would normally have
2) to weaken the fault temporarily so that a larger quake might happen
than would otherwise have occurred.