[topquark]

LIGO scientist here. The way this is presented can be a little deceptive - the isolation is very frequency dependent. At high frequencies (>10Hz), the pendulums and blade springs in the suspension isolate the mirrors very well, so they are moving by only these small amplitudes (10^-19 meters). But at low frequencies (<1Hz) the isolation ratio is essentially 1, so the amplitude of the mirror motion is roughly the same as that of the ground (about 10^-6 meters).

[idanoeman]

Yes, which makes sense physically. If the earth's rotation slowed by a constant velocity, in order to damp that motion the mirrors would have to displace themselves in the opposite velocity. Of course, there's no room for them to do so, so damping such low frequencies is not possible.