Can the LIGO detectors sense continental drift? I know the detectors can sense earthquakes and the movement of the Pacific Ocean, and given the sensitivity of the arms to detect gravitational waves, it makes sense that they should detect continental drift, but I have been unable to find any source that explicitly states they can. I know that GPS is used in LIGO, does this GPS take into account continental drift?
 A: The LIGO detector is extremely sensitive. It can detect the smallest motions and vibrations. Whether these come from close or far away (as we saw it detected merging massive objects many light years away). I just read that it can sense "staff biking alongside detector arms, the force of the Pacific Ocean rumbling" and even "the random motion of the atoms within the mirrors and their housings can be detected". But I'm thinking for continental drift that this may not be the case. The first reason is that continental drift is extremely slow. The second reason is because the LIGO is actually attached to the ground of the continent, so that there is no relative motion between the LIGO and the ground. And I cannot think of any other mechanism where it would detect the motion of other continents - if the continents made a sudden jerk maybe (and sent vibrations through the earth - similar as for the vibrations sent through the earth like what happens when there are earthquakes which can be detected by LIGO)*, but as I said they are moving extremely slowly.

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*You should also take note that LIGO is built such that one arm is at a right angle to the other arm which is a requirement to detect gravitational waves (because of the structure of these waves). The fact that they can detect other vibrations does not means that these vibrations have a similar structure to these waves.

A: Continental drift is about 2 inches per year, which should be easy to detect with a interferometer even much less sensitive than LIGO.  2 inches is about 50 mm or 50,000 microns,  so we can expect a fringe drift rate on the order of 150 fringes per 24 hours.  I used to track one fringe per hour easily, 40 years ago.
