As recently concluded in a parallel discussion, see reference below, LIGO is unable to exclude that mirror displacements as observed along their interferometer arms in fact result from much larger mirror displacements of similar profile along the vertical.
This is because mirror suspensions act along the vertical which over a distance of 4 km varies by an angle of about 2 arc min (a nautic mile = 1,852 m by definition corresponds to 1 arc min of angular distance at sea level). So every vertical mirror displacement will exhibit a displacement component about three orders of magnitude smaller along the connecting interferometer tube.
As LIGO is unable to directly measure vertical mirror displacements with adequate sensitivity they cannot distinguish whether horizontal displacements such as assigned to gravitational wave interaction are due to horizontal excitation or to vertical excitation at three orders of magnitude larger amplitudes.
https://www.researchgate.net/post/Am_I_the_only_one_that_is_doubtful_of_LIGOs_detection_of_gravitational_wave_GW150914?