The detector is isolated, see here: https://www.ligo.caltech.edu/LA/page/vibration-isolation https://www.ligo.caltech.edu/LA/page/vibration-isolation Essential

The detector is isolated, see here: https://www.ligo.caltech.edu/LA/page/vibration-isolation https://www.ligo.caltech.edu/LA/page/vibration-isolation Essentially a combination of passive/active isolation that effectively completely decouples the system from the environment. I went to a talk by one of the lead scientists in LIGO and they spent a lot of effort on this. I believe the sites are also fairly remote so they don't have trucks driving nearby.