Lock-loss Due to Earthquakes at LIGO Hanford Observatory*

In 2015, LIGO made the first direct detection of gravitational waves, confirming a prediction made by Einstein nearly a century earlier. To operate and thus detect gravitational waves, optical cavities in the gravitational-wave detector must be in resonance (locked). Lock can be lost if ground motion is too high. One source of excess ground motion is earthquakes. Data collected for more than fifty earthquakes from May 2023 to August 2023 and the ground motion they produced at LIGO Hanford Observatory indicated thresholds for the response. The instrument easily survives small earthquake-caused ground motions of 500 nanometers per second (nm/s) and below and does not survive large ground motions of 3,000 nm/s and above but some medium-sized ground motions lose lock and others do not. Further investigations of these medium-sized ground motions were made by collecting additional LIGO data obtained during these earthquakes to reveal other factors that could contribute to lock-loss such as location, wind, laser power, details of the seismic waves at the site, the levels of microseismic-caused motion, duration of earthquake-mode, or SEISMON alerts. The results will be presented and patterns found will be verified by comparison to additional earthquakes.