Quote Originally Posted by DWThomas View Post
Another consideration - the water in the glass is a relatively undamped (no pun intended) system and once set in motion could slosh around as a sort of resonance. The actual driving impulse from the shutter could be a very short and quickly damped pulse within the camera mechanism. As such, the internal camera vibrations could be settled out during a large percentage of the actual exposure; e.g., a half millisecond wiggle during a 1/2 second exposure won't even register, even though the water sloshes a bit. I also seem to recall some discussions about shutter vibration being a bigger problem at 1/25 or so than really slow speeds for that reason, but it may have been with regard to a very specific camera and tripod.
You are probably right. The visible pulse through the water is perhaps just an indication of the actual vibration due to the very different properties of the water and the camera internals. What one sees in the water may be more alarming than what it is really is.