Hi Adam:

You can buy single point Earthquake sensors now.  I got one after the Loma 
Prieta quake.  They work by detecting the P (Primary) wave which arrives ahead 
of the main power containing waves.  The time between the P wave and the start 
of the quake depends on how close you are to the epi center (maybe tens of 
seconds).

I don't see how the system in Japan works.  It's just a receiver.  How do they 
decide when to send a warning?

The USGS now has most earthquake areas of the U.S. covered with seismometers 
that are linked to the Menlo Park, California center.  They now know shortly 
after a quake where it was centered, how big and it's automatically on line. 
The system in Japan may do a similar thing except it's fast enough to broadcast 
a warning as soon as a P wave is confirmed and above some threshold.  It may be 
  that the warning only goes to the area where the quake is located?

The USGS has just published it's latest California Earthquake predictions, see:
http://pubs.usgs.gov/of/2007/1437/
There are two kinds.  One is independent of the current year based only on 
probabilities the other depends on the history of past quakes.  I.e. the longer 
since a quake the more likely one is on a given fault.

My seismometer and Geophone page:
http://www.prc68.com/I/Seismometer.shtml

-- 
Have Fun,

Brooke Clarke
http://www.prc68.com/P/Prod.html  Products I make and sell
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http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam
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