Introduction:  I investigated variations in seismic band levels with month of the year, time of day, direction (X, Y, and Z), and station (CS, EX, and EY). 

Methods:  Hourly trends from seismometers at the Corner Station (CS), End Station X (EX), and End Station Y (EY) were exported from DataViewer, and were analyzed using the R statistical computing and graphics environment.  Three years of hourly maxima were analyzed, 2013 through 2016. 

Results: 

0.1 – 0.3 Hz
This band is dominated by the microseismic peak caused by pressure exerted on the ocean floor by waves and deep-sea storms.  No significant differences in ground motion were observed in this band between hours of the day or days of the week, which is expected for the microseismic peak.  Average ground motion in December was about 4 times what occurred in summer months.  Based on the 50% in lock level for O1, ground motion in this band contributes to difficulties maintaining lock in up to 40 times as many hours in December as in summer months.  This difference is attributed to an increase in ocean storms in the Pacific during the winter months.  Hourly trends showed that ground motion in this band was about 10% lower on average at EY than CS or EX, whereas monthly trends showed EY between 10% to 15% lower than the other stations from December through April.    

0.3 – 1 Hz
Ground motion in this band was about 60% to 80% higher during winter months than summer months.  This annual pattern is much less pronounced than in the 0.1 – 0.3 Hz band, possibly due to the influence of wind that increases in spring and summer [1].  Between 9 a.m. and 5 p.m. local time, ground motion in this band increased by between 25% to 40%.  This diurnal pattern is much less pronounced than the 1 – 3 Hz band.   Hourly and monthly trends showed ground motion at EX was about 25% to 50% higher than EY and between 5% and 20% higher than CS, which may be due to its closer proximity to the vitrification plant construction at the Hanford site.  

1 – 3 Hz
This band is dominated by human activity, which can explain a clear diurnal pattern.  Hourly trends showed that ground motion in this band increased by 60% to 75% between 8 a.m. and 5 p.m. local time.  There may also be evidence of a swing shift.  The higher ground motion at EX compared to EY can be explained by its closer proximity to Hanford site activities, such as the ongoing vitrification plant construction.  

3 – 10 Hz
Hourly and monthly trends in this band showed that ground motion at EY was between 30% to 3 times higher than ground motion at EX, which is likely due to its closer proximity to traffic on Hwy 240.  Ground motion at EY was also about 30% higher in the y direction than the x and z directions, because Hwy 240 is in the y direction.  Ground motion at EY decreased by about 30% in January, which may be due to a decrease in transportation of agricultural products during winter months.  Ground motion at EY was higher than at CS throughout the year, reaching up to about 75% higher than CS in June.  While truck traffic on Hwy 240 is still the largest contributor in this band, it is a factor of 2 lower than iLIGO because of the resurfacing of Hwy 240.  We should be able to watch it increase with time.  A diurnal pattern consistent with human activity can also be clearly seen in hourly trends, especially at CS.  Hourly trends showed that ground motion at CS between 7 a.m. and 11 a.m was up to 1.5 times higher than EX and about 15% higher than EY.   

10 – 30 Hz
Hourly trends showed that ground motion in this band highest at CS and EY overall, however, CS was up to 1.5 times higher than EY between 7 a.m. and 3 p.m. local time.  Hourly and monthly trends showed EY was between 25% to 30% higher than EX throughout the year, except in January.  Monthly variations were very small, however, an increase of up to 30% was observed at CS in this band in March and April; EX and EY both saw a similar increase in June.  Ground motion in the z direction (vertical) was about four times as high as the horizontal directions (x and y) at CS, which is explained in more detail here (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=15819).

References:
[1] Vidrio, Margarita, Schofield, R. Statistics for 8 years of wind at LHO. Aug 2014.  https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=12996