I've seen earthquakes stop my clock pendulums. Presumably this is 
because they shifted the ground in the direction the pendulum was 
swinging. A clock at 90 degrees to the one that stalled kept running.

The quake in Japan was too far away to have such an effect, but I'm 
thinking about the kind of motion the earthquake is likely to deliver 
from so far away. Will it be an up and down motion?

Is this a difference between P and S waves? Would it be a vertical P 
wave from Japan, and a lateral S wave from a closer quake that might 
stall a clock pendulum?

-- 

Bryan Mumford
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