Saga of mysterious 15 Hz ground motion at LIGO Livingston Observatory

Since at least February 2015, a mysterious noise at approximately 15 Hz has been present at LIGO Livingston Observatory (LLO)'s Corner Station. This feature is visible in seismometers, accelerometers, microphones, and magnetometers distributed throughout the building and can also be seen in the gravitational-wave channel. The frequency of the feature can change by small (0.05 Hz) and large (> 1 Hz) amounts both suddenly and gradually. Various attempts since 2015 to identify the source of the noise, by turning fans and motors on and off to see if it disappears, had been unsuccessful. A breakthrough occurred in May 2022 when it was realized that the LLO office-air-handling system had never been examined as a potential culprit. Correlations of changes of this system with the feature's frequency changes were found. These motivated on-off tests of the system, confirming it as the source.