Figure B1:

The physical environment monitor (PEM) array at the Livingston detector, as seen on http://pem.ligo.org [10]. Gray dashed lines enclose instrumentation in separate structures: the corner station building located at the vertex of the laser-interferometric detector, the two end stations located at the end of the 4km detector arms, and the ‘vault’, which houses PEM sensors away from all buildings to measure noise due to the external environment. Purple dashed lines indicate rooms within structures, or spaces just outside of structures. For example, the corner station and both end stations have PEM sensors in electronics rooms containing computers that sense and control the detector as well as PEM equipment mounted on a mast on the roof. See [4, 6] for detailed description of the optical layout shown.