December 2, 2011
by John Vidale
Since last summer, Sound Transit has been tunneling from Husky Stadium towards the future Capitol Hill station. The tunnel boring machines (TBMs) are now 100-200 feet underground near Volunteer Park, and progressing 50-100 feet per day. Behind them, trains run to deliver the tunnel wall sections, which are assembled as the TBM progresses. The CHS Capitol Hill Seattle Blog has lots of details.
The work is apparently causing noticeable vibrations in the Montlake and perhaps wider neighborhood. This site has more details. Following widespread complaints, the PNSN installed a new seismometer, and verified the noise.
Shown below is a seismogram (vertical component of acceleration) recorded on the foundation of a house on 19th Ave East, just below Interlaken Park. The train signal, which likely results from couplets of pops caused by pairs of wheels of cars in the train crossing track junctions, lasts about 100s as the train passes underneath.
This four-minute signal was recorded at 9:15am (17:15 UTC) yesterday, December 1st. The peak amplitude of 6000 counts corresponds to an acceleration of 0.25% g, above the limit of perceptibility. The shaking is possibly greater higher in the house.
A spectrogram shows the frequency content of the shaking, plotted over the duration of the signal.
This spectrogram ranges from 0 to 80 Hz, and shows most of the energy is in the range 20 to 60 Hz, matching the frequency content reported by a study for Sound Transit. This information allows us to take a look at the source mechanism and to test future recordings to see how loud, common, at what times of days is the disturbance.
[added at the next day:
We've lowered the automatic triggering threshhold so that we don't need to recover data from times of manually reported noises, and now many similar events are being detected. Here are the traces. Clearly, if these signals come from the trains, the issue is their loudness, not whether they can be felt.]
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