October 1, 2016.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) consists of two facilities, LIGO Hanford Observatory (LHO) and LIGO Livingston Observatory (LLO). Gravitational-wave astronomy began when LIGO observed gravitational-waves for the first time in 2015 during the inaugural observation run (01) of the advanced detector era. Isolation of LIGO detectors from seismic noise is necessary to achieve a desired level of performance. Analysis of seismic noise is essential to make improvements in seismic isolation, and to validate gravitational-wave candidates with a high degree of certainty. Hourly trends of seismic noise in the 0.1 – 0.3 Hz frequency band from Guralp CMG-40T three-axis seismometers located at LHO’s Corner Station (CS), End Station-X (EX), and End Station-Y (EY) were analyzed over a three year period, 2014 through 2016. Seismic noise in the 0.1 – 0.3 Hz band is primarily attributed to the microseismic peak caused by pressure exerted on the ocean floor by ocean waves and deep-sea storms. The 01 50% threshold for this band was determined by the percentage of hours that laser lock in LHO’s gravitational-wave detector was maintained when ground velocity exceeded given thresholds ranging from 0.1 – 2 μm/s during the 01 observation run. No significant differences in ground motion were observed between hours of the day, days of the week, or between stations. Average ground motion was about four times greater during December than during summer months. Greater ground motion occurred in the z direction (vertical) than in the horizontal directions (x and y) at all three stations, with no significant difference between the x and y directions. In winter months, ground motion in the z direction was about 1.4 times the motion in the horizontal axes. Our results suggest that the microseismic peak may contribute to difficulties maintaining laser lock in 40 times as many hours in December as in summer months.
LIGO Hanford Observatory (LIGO)
This material is based upon work supported by the National Science Foundation through the Robert Noyce Teacher Scholarship Program under grant# 1136419. Any opinions, finding, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The research was made possible by the California State University STEM Teacher Researcher Program. LIGO is operated by Caltech and MIT for the National Science Foundation under NSF Cooperative Agreement 0757058.